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TRANSCRIPT
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Copyright 0 1971 by TAB BOOKS
Printed in the United Statesof America
Reproduction or publication of the content in any manner, with-out express permission of the publisher, is prohibited. No liabilityis assumed with respect to the use of the information herein.Hardbound Edition: International Standard Book No 0-8306-1571-7
V&
RADIO -ELECTRONICS
HOBBY PROJECTS
The projects collected in this book encompass five broadcategories. There are projects for the hi-fi stereo buff, themusician or musically inclined, the auto enthusiast, gadgetsdesigned for use around the home, and eight test andmeasuring devices.
These projects, selected from Radio -Electronicsmagazine, were chosen to appeal to a wide range of interestsand for varying levels of technical ability and constructiondexterity. Some are simple enough to put together in anevening or two; others are more complex and infinitely morechallenging.
In each case, there is a complete parts list and, whereappropriate, detailed construction procedures, augmented byschematics and detailed drawings. Where special componentsare used, a source of supply is given. Should you encounterdifficulty in getting any project to operate properly, be sure tore -read the text and double-check each step to make sure"gremlins" have not crept into your work!
I
t r.
Contents
109
114
114119123127134138
AUDIO, STEREO& HI-FI
IC MULTIPLEX DETECTOR6 -CHANNEL STEREO MIXER PREAMPSTEREO EXPANDER -COMPRESSORFMSTEREOTUNERBOOKSHELF SPEAKER SYSTEM3 -WAY ELECTRONIC CROSSOVERSTEREO HEADPHONE CONTROL CENTERFM STEREO ADAPTER200 -WATT IC STEREO AMPLIFIER
2 MUSICAL DEVICES
DIGISYNTONE MUSIC SYNTHESIZERGUITAR VIBRATORHYTHM LIGHTSELECTRONIC TREMOLO
3
4
PROJECTS FOR THE AUTO 86
TACK -DWELL VOLTMETER 86SOLID-STATE GAUGES 92ELECTRONIC IGNITION 98ROAD ICING ALARM 103AUTOMATIC WINDSHIELD WIPER PAUSECONTROLLER
PROJECTS FOR THE HOME
AUTOMATIC ENLARGER TIMERHALF-HOUR INTERVAL TIMERDANCING CHRISTMAS LIGHTSPHOTOTACHOMETE RTAPE SLIDE SYNCHRONIZERIC SOUND -OPERATED RELAY
1
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144
FM STEREO MULTIPLEX GENERATOR 144
FET DUAL TRACE SCOPE SWITCH 151
SCOPE CAMERA 156
3 -WAY SCOPE CALIBRATOR 162DOT BAR GENERATOR 170LOW -VOLTAGE ELECTROLYTIC TESTER 176AUDIO TONE -BURST GENERATOR 180
3 -WAY WAVEFORM GENERATOR 186
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PART 1
IC MULTIPLEX DETECTOR
fering "birdies" have ever been audi-ble with the device.
Channel balance is within 0.5 dBand total harmonic distortion is lessthan 0.5% at the recommended com-posite -signal input level.
The only drawback found in thedesign is the 19 -kHz and 38 -kHz re-jection figures. Although a typical re-jection is 20 dB at 38 kHz, this is notenough to prevent possible "birdies"when beating against a tape recorderbias oscillator. Therefore a `Twin -T'filter has been added (Fig. 2) to eachoutput to increase rejection another20 dB.Construction & alignment
You can buld this project forunder $21. The actual -size PC patternfor the decoder is included for do-it-yourselfers, and a complete kit ofparts and drilled PC board is available.Please use the components listed, asthe circuit board was laid out forthem
Construction of the project issimple, as is the alignment procedure.A 2.2 x 3 2 -inch PC board providesenough component space withoutcrowding. Recommended coils for thiscircuit are available only in a PCmounting style.
The best technique is to insert theIC and then add. other parts outwardsfrom the IC. Don't bend the leads onR4, IC1, or the transformers. After allparts are properly soldered, add therequired external wires. If you do notplan to use audio muting or mono -
7
by KEN BUEGEL
INTEGRATED CIRCUIT DEVELOPMENTSoften seem to be aimed at exotic ap-plications which are of little interestto the general electronic public. ButMotorola s MC1304 and MC1305 aredifferent: on one chip is a complete,advanced -design multiplex decoder.The MC1305, although identical incost to the MC1304, provides for aseparation adjustment and was used inthis project.
The internal circuitry is formid-able: 10 diodes, 31 transistors and 29resistors. Even the block diagram forthe 1305 (Fig. 1) reveals a level ofcomplexity not often used with dis-crete -component multiplex adapters.
The IC uses the proven balancedtime -switching technique with its in-herent SCA rejection without filtering,and also provides a driver for a stereoindicator. Two separate additional in-puts provide for stereo-mono switch-ing and audio muting. And as if thisweren't enough, a series of diodeswith emitter followers serve as tem-perature -compensated voltage regula-tors for the circuits.
Performance of the chip is phe-nomenal in comparison to the veryconservative Motorola specifications.A separation of 45 dB at 1 kHz istypical; three units constructed re-vealed a separation of 55 to 57 dBat 1 kHz, 44dB at 100 Hz., and 37 to49 dB at 10 kHz These figures are ona par with the very best multiplexadapters of conventional design. SCArejection exceeds 55 dB and no inter-
stereo switching, wires are not con-nected to pins 4 and 5 of ICI.
Although a multiplex generator isthe easiest alignment method, this unitcan be aligned with a broadcast signal.Input level should be about 0 75 voltp-p to achieve maximum channelseparation.
Each output should be connectedto a 22K load to provide the properterminating impedance to the Twin -Tfilters.
Connect an oscilloscope or acvtvm to the junction of pin 1 and C4.Peak LI and L2 for maximum 19 kHzas seen on the scope. This waveformshould be about 1.6 volts p-p with a+15 -volt supply. Move the scopeprobe to the junction of C5 and pin 4of L3 and peak L3 for a maximum38 -kHz trace. This should be about22 volts p-p. Caution: if you do notuse a low -capacitance probe, the cir-cuit may be slightly detuned when the
2
III
121
EF
>-
-7
9
1
L_
19kHzFILTER
38 kHzTANK
0
AMPL 1
1
DETECT
DCAMPL
L
TWIN -TFILTER
RIGHT
MCI305- - INTEGRATED4 6 CIRCUIT ]_
GND FORMONAURAL STERO
INDICATOR
and 38 -kHz components. An elaboratefilter can remove these componentsfor true separation readings, but theseparation will not be increased.
If you do not have access to amultiplex generator, connect the inputto your tuner output. First peak Ll,L2 and L3 for maximum outputwaveforms as described earlier. Con-nect the scope vertical input to theleft output and the horizontal input tothe right output. Tune to a monauralstation and set the scope gains untilthe trace is a straight line at a 45°angle (Fig. 3-a).
19 kHzFILTER
COMPOSITE 19 kHzAMPL AMPL
COMPOSITE 13SIGNAL
INPUT!
1
10
SWITCH
SWITCH
DCAMPL }REGUL-
LATEDVOLTAGES
+15V
Fig. 1-Complex functions performed by 31 -transistor MC-1305 (inside dotted line). Connections to pins 4 and 5 areoptional.
probe is removed. This detuning willbe slight and is corrected in the nextstep.
Connect the scope or vtvm to theright output and set up the generatorfor a left -only output. Set the wiper ofR4 to midposition. Carefully peak Ll,L2, and L3 for a minimum output onthe right channel. Then set R4 for aminimum output.
Now set the generator for aright -only output and read the outputlevel on the right channel. Then setthe generator to a left -only outputThe difference in readings is the chan-nel separation. It will not be as highas the figures given earlier since theresidual reading also includes 19 -kHz
FM station alignment
LEFT
415V
T.-
DOUBLER
5 8_GND TOMUTE
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TWO CONDUCTOR SHIELDED
C8 C9, C12, C13-.001-0 ceramic (CentralabCE102)
C10, C11-002-0 ceramic (Centralab CF202)C14, C15 -0.2 -AF, 10V ceramic (Centralab
UK -10-204)C16-0.1-0, 10V ceramic (Centralab UK -10-
104)C17 -60 -AF, 15V electrolytic (Mallory MTV
60CB151Other partsLl, L2-J. W. Miller type 1361L3-J. W. Miller type 1362IC1-Motorola MC1305PThe following parts may be ordered fromTransitek Co., P.O. Box 98205, Des Moines.Wash. 98016. All prices include postage ICI,$7.20. Ll. L2, L3 (set of three), $5.40. PCboard MPX, $2.95. Complete kit of all listedparts and drilled PC board, $21.00.
PARTS LIST
Use the same -sizeprinted circuit patternabove to makeyourdecoder. Componentsspecified will fit onthe board.Component -sidedrawing_on tne rightshows where to mountthe parts. Some orthe wiring to theunit is optional.
Iq.
All resistors 1/4W, 5%R1, R2-20,000 ohmsR3-4700 ohmsR4 -500 -ohm, 1AW trimmer (CTS type
X201R50113)R5, R6-3900 ohmsR7, R8, R10, R11-4300 ohmsR9, R12-2200 ohmsCapacitorsCl, C3 -5-µF, 50V electrolytic (Mallory MTV
5C850)C2, C4-01 -µF polystyrene (Mallory SX110)C5-.0022-gF polystrene (Mallory SX222)C6, C7-.022 -AF, 100V Mylar (Mallory
PVC11221
aX
CSTEREO
EXCELLENTSEPARATION
Figs. 3-a-c show scope patterns withvertical scope input to left output ofthe decoder and horizontal input con-nected to the decoder right output.
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MONAURALSIGNAL
bSTEREOLIMITED
SEPARATION
Tune to a stereo broadcast andyou will probably see something likeFig. 3-b. This indicates limited separa-tion. Now, while watching the scopeface, slowly tune Ll, L2, L3 and R4until you get a trace most like Fig.3-c. If you can connect your amplifierto the adapter and also hear the out-put-preferably in headphones-somuch the better.
Some stations have stereo pro-grams which feature highly directional
microphone pickup, this type of pro-gram material is the easiest to use foralignment. When the output looks likeFig. 3-c, you must identify the channels. It is possible to tune the unit sothat the output labeled L is actuallythe right channel. Careful tests do notshow any difference in separation orother specifications, however, so ifyou wind up with the channels inter-changed, simply reverse them whenyou plug them into your preamp.
Most tube tuners will have morethan 0.75 volt p-p output. This adapt-er will have decreased separation andincreased distortion at higher inputlevels. The input impedance is around20K so a 100K pot inserted in serieswith the input may be adjusted untilthe input is correct.
An interesting feature of this ICis its 8 -22 -volt supply specification. Ifthe adapter is aligned at 15 volts andthe supply voltage decreased, separa-tion stays almost unchanged. This isnot true if the adapter is aligned at alower voltage which is then increased.In no event should the supply exceed+22 volts. Operation at +15 volts ishighly recommended, since no per-formance characteristic was improvedat higher voltages. As the supply volt-age is not critical, a relatively inex-pensive Zener diode with capacitorfiltering will provide very stable opera-tion.
12
by GEORGE D. HANCHETT
THIS STEREO PREAMPLIFIER AND
mixer is particularly interesting tothose who want to make high -qualitytape recordings. The preamp has fourmicrophone and two line inputs thatcan be switched to left, right, or bothchannels. In addition, two auxiliaryinputs are provided, one for each
channel. The auxiliary inputs cannotbe switched. All inputs that can beswitched are controlled from the frontpanel. The two auxiliary inputs arecontrolled from the rear of the unit.Each output channel has its own VUmeter.
6 -CHANNEL STEREO MIXER PREAMP
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J5 J6
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(FIG.2)
LINE AMPLIF ER 8(FIG.8
PREAMP B2
(FIG.2)2
SK3030 (4)
R470
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R310K
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BLINE AMPLIFIER A
(FIG.8)
R 0
10 II
MULTI INPUT MIXA (FIG.5)
1 2 3 4 5 6 7
D6
UNE INPUTA +20VJ7 -b LINE J7-4
INPUT B
HIGH HIGH HIGHDYNAMIC DYNAMIC DYNAMIC
RANGE RANGE RANGE
PREAMP 81
( FIG.2)
R833R
5
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14
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(FIG.2)
PARTS LIST(Fig. 1)
Cl, C2-1000 25 volts, electrolytic01 through D8-SK3030 (RCA)MI, M2-VU meterR1, R2, R7, R10, R15, R16-potentionmeter,
10,000 ohms (R7, R10 is a dual linear pot)R3, R5, R6, R12, R13, R14--10 000 ohms 1/2
watt, 10%
The stereo preamplifier andmixer is made up of three basic cir-cuits and a minimum of inter-connecting wiring The three circuitsare a high -dynamic -range microphonepreamp, a multi -input mixer, and aheadphone or line amplifier. A blockdiagram of the total unit is in Fig. 1.
The output of each microphonepreamp (see Fig. 1) is fed to a switchwhich can connect it to channel A,the left channel, channel B, the rightchannel, or both channels (A and B)
6
R1610K
R4, R11-47 ohms, '/2 watt. 10%R8, R9-33 ohms, 1/2 watt, 10%SI through S6-rotary switch, 2 poles, 5
positions, shorting typeJ1, J2, J3, J4-microphone jacksJ5, J6-phone jackJ7 -4 -lug terminal strip (screw terminals)J8, J9-phono jack
simultaneously. The output of theseswitches as well as the line input foreach channel is fed into the multi -in-put mixers. A master gain controlcombines or gangs the outputs fromthe mixers installed in each channeland passes the combined signal to theline amplifiers The diode limiting cir-cuit used with each VU meter keepsthe meter from being damaged duringthe charging of the large coupling ca-pacitors in the line -amplifier. Two R -Cpower -supply filters consisting of R8and Cl, and R9 and C2 assure circuit
D7
IC \ II
MULTI -INPUT MIXER8 (P0.5)
2 3 4 5 6 7
AB S4o
B
33II
6 + Cl
TC2 +l's
EACH000/25V
R1410K
AB S5
A° o
Bo
J2 .14
S6
4 °
D8 M2
AB
R410K
10/5V
+C3
C2
/6 +
CIRCUIT BOARDSA complete set of 8 circuit boards needed tobuild this unit are available for $10. Order10708-1. A set of 4 boards for building amono version are $6. Order 1070B-2. Boardsare G-10 glass -epoxy, undrilled. Photo nega-tive for making your own boards containingall board patterns is $1.50. Order fromPhotolume Corp., 118 E. 28 St., New York,N. Y.
IN
ezoaR7
C5I I2V
R3 016.2K SK 3038
R5RI* sea
2
6
stability. Each filter services two mi-crophone preamplifiers.
The stereo preamplifier andmixer is made up of a number of cir-cuits as described above. The descrip-tion of each of the three circuits in-cludes circuit boards and componentplacement diagrams. The individualcircuit boards and the interconnectingwiring required for the stereo preampand mixer may be assembled as de-sired by the builder to form the kindof custom unit he needs.
R2100K
I0/6VCI
PARTS LIST
(Fig. 2)Cl, C5-10 F, 6 volts, electrolyticC2, C6 --300,,F, 6 volts, electrolyticC3-10 F, 15 volts, electrolyticC4-100 j,.F, 25 volts, electrolyticC7-50 15 volts, electrolyticQl, Q2-SK3038 (RCA)Q3-SK3020 (RCA)
R9IOOK
Q3K3020
02
R12684
All Resistors 1/2 watt 10%R1-see table I and textR2, R9-100,000 ohmsR3, R10-6200 ohmsR4, R11, R15-10,000 ohmsR5, R12-68 ohmsR6, R13-470 ohmsR7-820 ohmsR8-potentiometer 10 000 ohms, audio taperR14-1000 ohms
15
R8IOK
10K
-
C6
RIIloK
'1*SEE TEXT AND TABLE I
5R15 OUT10K
4
GN
C4100
/25V +
6470,13
R1347011
The high -dynamic -range micro-phone preamplifier, intended to beused with low -impedance dynamic mi-crophones, will handle loud passagesof music and close talking without ad-verse effect on the output. The ampli-fier has a gain of 1500 to 2000 andcan provide a maximum undistortedoutput voltage of 2 volts rms to a loadimpedance of 500 ohms or more. Themaximum undistorted input is 400mV rms. The frequency response isflat from 20 Hz to 30 kHz.
The circuit for the high -dynamic -range microphone preamp are inFig. 2 The preamplifier consists oftwo stages of current -stabilized ampli-fiers separated by a gain control andan R -C filter, consisting of R7 andC5, that prevents motorboating. Re-sistors R5 and R12, placed in theemitter circuits of transistors Q1 andQ2, improve the frequency responseof the preamplifier by providing some
50/15VC7
R14I K
Microphone Impedance200500
4,000
R1 (ohms)220560R1 not used
I 1\.\\
FIG.3- CIRCUIT BOARD PATTERN for the mike preamp.
TABLE 1
Multi -input mixerThe multi -input mixer is designed
to mix the inputs from up to sevensources, usually microphones, for in-put to an amplifier, recorder, or otherpiece of audio equipment. The mixerhas a gain of approximately unityand, therefore, has no effect on thesystem in which it is installed. If morethan seven inputs are required, asmany as three mixers can be wired inparallel.
16
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-+20V
emitter degeneration. The output ofthe preamplifier is shunted with re-sistor, R15, to make the circuit com-patible with the zero -point switchingcapability used in the master preamp.The output impedance of the preampis low. The table shows the value of R1to use with microphones of various im-pedances.
The printed -circuit -board layoutfor the microphone preamp is in Fig3. A photograph of a completed boardshowing parts placement is in Fig. 4.All ground connections in this circuitmust be made to the same point, asshown in Fig. 3, to avoid formingground loops. This common -groundfeature is built into the printed -circuit
R1 connected across microphone inputjack.
board and must be followed if somemethod of circuit construction otherthan the printed board is used In ad-dition, all preamplifier connections toexternal circuits should be made to thesame ground point.
How it worksThe circuit for the multi -input
audio mixer is in Fig. 5. The resist -
FIG. 4-, PHOTO OF ASSEMBLED BOARDshowing parts placement details.
HIGH DYNAMIC RANGE0 MICROPHONE PRE -AMP
I
10/6
V V V R15C3 C6 100KC4+ +C5 +
R7 R9 RI I RI31K K 1K 1K
R4 RIO
PARTS LIST(Fig. 5)
Cl through C7-10 F, 6 volts, electrolyticC8-50 F, 15 volts, electrolyticQl-SK3020 (RCA)RI, R3, R5, R7, R9, R11 R13-1000 ohms, %
watt, 10%
16K
0
R639K,
o 0 00/6.-10/6 CV6.--
V
91< 39K4 39K
C7
39K
VCI +
RI R3IK IK
ance network shown at the left notonly provides the mixing function butalso to make possible zero -pointswitching of the inputs
In the zero -point switching, asused in this unit, the capacitors at the
R4
RI7*
R2, R4 R6, R8, R10, R12, R14-39,000 ohms,1/2 watt, 10%
R15-100,000 ohms, 1/2 watt, 10%R16-2200 ohms, 1/2 watt, 10%R17, R18-see table II and text
output of the microphone preamps aswell as the input capacitor of themixer are kept charged This is doneby connecting a resistor across theoutput and input. Thus there is nodisturbance, no cracks or pops, when
MULTI- INPUT AUDIO MIXER Gn
tottatto
17
-I -20V
9
C2+R5
IK
R2 Re39K
*SEE TEXT AND TABLE 1E
Q I
SK 3020
RI8*
850/15VC8
I0OUT
II
COM12
FIG. 5-CIRCUIT of multi -input audio mixer. Two are needed to build the unitdescribed
1
92CS15909
FIG. 6-CIRCUIT BOARD PATTERN for the audio mixer.
p8 9 8 8 9 8
RI
8
C7 C6
C2
RIO R6 R3
inputs are switched in or out. The am-plifier portion of the circuit, shown atthe right in the schematic, is currentstabilized by the emitter resistor. Thisresistor is not bypassed, and provides agreater degree of degeneration and re-ducing the overall gain of the mixer tounity.
Some adjustment of resistor val-ues is required if less than seven in-puts are used Table II shows these re-sistor values for from 2 to 7 inputs.When three or more mixers are paral-leled to accommodate more than seveninputs, not only must the outputs ofthe mixers be paralleled but the groundpoints on each circuit board mustbe connected. The gain of the mixerthus connected is somewhat less thanunity
Component placement and cir-cuit board pattern for the multi-inp tmixer are in Figs 6 and 7 along with aphotograph of a completed board
fier is located some distance from themicrophone. If preceded by a micro-phone preamp, the amplifier makes avery useful remote pickup. It is alsovery useful for driving the line inputsof tape recorders.
The headphone or line amplifierhas a voltage gain of 100 and candrive any line impedance of 250 ohmsor more. It has a maximum undis-torted output of 3 volts rms into a500 -ohm line and has a frequency re-sponse flat from 20 Hz to more than25,000 Hz. The input impedance is1,800 ohms.
I
I
RI
COMMON 5
OUTPUT
RI6
R2
FIG. 7-PHOTO OF MIXER shows parts placement on the circuit board.Refer to Fig. 1 for interconnection details of the complete preamp.
TABLE IIRESISTANCE VALUES
No of inputs234567
Headphone or line amplifierThe headphone or line amplifier
is very useful when the power ampli-
R17 R18
8.2k7.5k6.8k6.8k6.2k6.2k
120 ohms110 ohms91 ohms82 ohms75 ohms68 ohms
18
R17
7
C4
R7 4 3
C3
2
RI
C8
RI4 R5
RI
R9 C5
6
CI
+20V
R2
1
IN
2
RII K
5/6
0201 SK3024
CI SK3020 C2
3.9 K
s
+zo
I.,
L
Off
o
6gi+
.
0
FIC. 10 COMPLETED BOARD showingparts placement. See Fig. 1 for circuitboard interconnections.
19
02
R3680K
R44
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PARTS LIST(Fig. 8)
C1-5 ,,F, 6 volts electrolyticC2-100 F, 25 volts, electrolyticQ1-SK3020 (RCA)Q2-SK3024 (RCA)R1-1000 ohms, 1/2 watt, 10%R2-3900 ohms, 1/2 watt, 10%R3-680,000 ohms, 1/2 watt, 10%R4-470 ohms 1/2 watt, 10%
P3
Amplifier operationThe circuit for the headphone or
line amplifier is in Fig. 8. The inter-connection of the transistors in theamplifier makes the operating condi-tion of the amplifier self-adjusting,i.e.; the amplifier can maintain itselfin a stable operating state in spite ofvariations in power -supply voltage andambient temperature. Stability is in-sured by feedback through R3. If Q l's
D2 02
emitter current should increase, thebase voltage of Q2 would decrease be-cause of the additional voltage drop inR3. However, the decreased base volt-age of Q2 results in a drop in Q2'semitter current, a reduction of feed-back voltage to Q1, and hence a de-crease in Q1's collector current. Thisdecreased collector current causes anincrease in Q2's base voltage thatcompensates for the original decrease
FIG. 8- LINE AMPLIFIER circuit usestwo transistors.
303024
0322V ZENER
WATT
0I
FIG. 11-POWER SUPPLY is regulatedcircuit. It is built right on the main
FIG. 9-- CIRCUIT BOARD pattern for chassis of the mixer preamp as you canthe line amplifier is shown actual size. see in the photo
COMMON
OUT
COM
STANCOR TP-3PAl 120 VOLTSSEC SOVOLT5 SK3030
FROM THE REAR OF THE MIXER-PREAMP you can see the auxiliary input and lineinput jacks. Three sets of outputs were connected in parallel to provide connectors forall likely applications.
LOOKING IN FROM THE TOP you can see the power -supply components in the lowerright of the photo. The two VU meters are at the top. Most of the jumble of wiringConsists of leads connecting outputs to the circuit boards.
by W. E. McCORMICK
SPECIFICATIONS
Expansion. To 8dB per channelCompression. To 20 dB per channel.Distortion. NoneInput Impedance. 1000-100,000 ohms
with RCA photo -cell About 250-100,000 ohms with Clairex photo -cells
Output impedance. 47,000-470,000ohms average.
Use with amplifier. 4-8 or 16 -ohm out-put impedance.
Drive voltage required -0 7 volt to initiate dynamic action
Frequency response. I dB throughoutaudio range.
Inputs .... 2 Stereo versionOutputs ... 2Rise time. . . . 10 msec. with RCA
cells; 12-15 msec with Clairex cells.Insertion loss . . . 2 dB on compres-
sion. 6 dB average on expansion.Measured from instant of applied
illumination until cell current reaches63% of its total value. This is a func-tion of distance between lamp and celland illumination intensity. Since themaintaining neon lamp voltage is lessthan the firing voltage, a sufficientlylong decay time is automatically pro-duced. This also tends to produce alight bias on the cells, which keepsthem ready for firing.
the unit. This supply is shown in Fig11. It is assembled directly on thechassis of the unit and is not builtonto a circuit board. Once you havecompleted the power supply, selectedthe desired number of input, mixerand output boards you can proceed toassemble your custom mixer-preamp.We are sure you will enjoy it.
and the amplifier is stabilized. The in-terconnection of transistors just de-scribed also makes possible the lowoutput impedance of the amplifier.
The printed circuit board patternfor this circuit, and a photo of a com-pleted board, are in Figs. 9 and 10.Power supply is last
A simple power supply completes
STEREO EXPANDER -COMPRESSOR
IF YOU PREFER THE FULL, DYNAMICmusic range of original performancesto the electronically compressed mate-rial from tuner, tape or disc, the stereovolume expander -compressor de-scribed here will enable your hi-fisystem to deliver it.
Or, if you want a limited, presetlevel of music or TV audio, the unitwill provide that too. In addition, thevolume expander -compressor will actas a limiter with other reproducingequipment.
It operates with any hi-fi system,and can be driven by almost any signalsource, including carbon microphones.It has two operating modes: expansionand compression. Specifications meetthose of commercial units (see chart),and the compressor -expander can beconstructed for about $20.00.
Amplitude swings produced dur-ing live musical sessions are often toogreat to be broadcast or recorded, andmust be reduced in level for the mediaused to convey them. When these dy-namics are linearly restored, realism isgreatly enhanced.
Various devices for replacing thisdynamic dimension have been mar-keted.
After deciding to add this featureto my hi-fi system, I considered sev-eral methods. Some were merely gim-micks: resistors, varistors, even in-candescent lamps were placed acrossthe amplifier output, where their load
NE4
R7
ing effect varied inversely, but seldomlinearly, with the current throughthem.
Other units with built-in "takecharge" circuits use the variable conduction of back -biased diodes to makethe output of a cathode follower in-crease faster than its input. Theseoperate between fixed points overwhich there is no convenient control.They also require alterations in criti-cal signal circuits, which many audio-philes do not like
The principle employed in somecommercial expanders had an appeal-ing simplicity. Signal -driven, they useneon lamps to activate photoconduc-tive cells and require no other powersupply. Their major component, how-ever, had been especially designed fora specific product.
Special components needed
SI
of the ordinary, but hardly uniqueNeeded were: A transformer with practically flatfrequency response over the audiorange. A high -impedance input winding topermit connecting it across an outputtransformer's secondary without ap-preciable loading. A turns ratio high enough to drive aneon lamp from an audio signal volt-age of 1 volt or so. A neon lamp with a firing voltageand ionization time the same in totaldarkness as in ambient light. A light output that would remainproportional to the voltage appliedand not behave erratically after a fewoverloads. A photoconductive cell with properspectral response. Great sensitivity.Fast rise time. Proper light to -darkconductance ratio. Appropriate resist-ance extremes and extreme linearityat very low light levels.
Such parts were rounded up, andFig. 1 shows a stereo version of the
RI
PC2-NE3HOUSING(RIGHT) PCI -NEI
HOUSING(LEFT)
22
LM2 T2 TI LMICompandor parts arrangement. Note left -right symmetry. Identical parts arrangementwas used for each stereo channel. It makes assembly considerably easier and neater.
With a circuit configuration inmind, however, what was demandedof the components could be assessed.Some of the qualities needed were out
R547K
NEZ RED
LIGHT-TIGHT
0S1-13COM
HOUSING
PRESS
EXPAND
INPUT
RIO C4
47K .47
RU47K
RI2100K
BLUR3330KIW
RS 330K RED YEL
TZN C.
R930K
RT
0 SI-d
RIGHT CHANNEL
Fig. 1-Stereo version of the compandor. Resistance of photoconductive cells is varied
by stepped up signal from amplifier that fires neon lamps. Cell resistance is used in
voltage dividing network to produce desired effect. RI and R7 are .500 ohms, 5 watts.
expander -compressor. If a monophon-ic unit is desired, build only one chan-
nel.
How it worksBriefly, here's how the expander -
compressor works. Since operation ofboth channels is identical, only one is
describedA small signal voltage across po-
tentiometer R1 from the power ampli-fier is stepped up by transformer Tl,
C2 t IR647 100K R2 330K 1W
z
51-7 NEI
NE -2V
LEFT CHANNEL
SI- c
x
_ _ NE4INE-2
GRN
OUTPUT@
7-.
LMtNO.44
TO PWRAMPLOUTPUT
OUTPUT
....... -NE3
" NE -2V---- _ - _
Parts ListCl, C2, C3, C4-0.47 AF, 200V
RI, R7 -500 -ohm, 5 -watt, wire -wound poten
tiometer (Mallory VW500 or equal)
R2, R3, R8, R9-330,000 ohm, 1-watt, 10%
resistorR4 R5, R10, R11
-47,000 -ohm, 1/4 -watt, 10%
R6, R12-100,000ohm, l/2 -watt, 10% resistor
NEI. NE2. NE3, NE4-Neon lamp, Signalite
NE2V or ASA No. A28. Allied Radio part
#K002 122.PC1, PC2-Photo cells, Clairex CL504L or RCA
4425T1, T2-Transformer, primary impedance
500,000 ohms, secondaryimpedance 50
ohms (Argonne AR -142, Lafayette Radio
see text)
R447K
LM1, LM2-fuse, No. 44 lamps, 6.8V, 0 25 amp
S1-4 pole, 3 -position switch (or Lafayette
part No. 99H6170, 9 -pole, 3 -position)
Misc-Terminal strip (Jones 4 -screw, barrier
type or equal), resistor board (for 4, 1 -
watt resistors), solder lug terminal strips
(two with 2 insulated and 1 gnd. lug, one
with 4 insulated and 1 gnd. lug), phono
jacks (4 RCA audio type and plugs as
needed), chassis (BudCB -1628, 11/4" h x
61/." w x 3" d), cabinet (Spectrum Prod-
ucts, No. 488, Bud SC -2130 or CU -465),
metal panel (approx. 3" x 6"), plastic ormetal light housings, panel jewels andthreaded bushing set, lamp -cell housings,
knobs, grommets, wire, etc.
LM2
TO PWRAMPLOUTPUT
BLK
.41
R PWR. AMPLOUT R L
LM2
LOUT IN
RIO
R7
R6
R4
Underchassis closeup shows where the rest of the cornpandor components are located.The circuit is completely passive and, therefore, has no power supply of any sort.
Position 1 (ExP ) places the lightvariable section of the dividing net-work in series with the signal. Onlouder passages the signal has less re-sistance to overcome and expansionoccurs
Position 2, the off position, al-lows the signal, minus the insertion lossof the expander -compressor, to passstraight through. By selecting your lis-tening level with the expander -com-pressor in its off position, compensa-tion is automatic.
Position 3, COMP places the dividing network across the input signal. Onlouder passages more drop takes placeacross series resistor R4 and compres-sion occurs.
Parts placement is not critical.There is no interference between chan-nels with the compact arrangementshown.
CI
R II RI
passed through current -limiting resis-tor R2 and applied to neon lamp NE -1.Lamp LM1 is a fuse to protect thetransformer and lamp NE -1 NE -2,located on the front panel, is a remoteindicator showing NE -1's response. Atsome pot setting, NE -1 and NE -2 willbegin to flicker. The intensity of NE -1will be proportional to the voltageacross it. This light, falling on photo-conductive cadmium selenide (or cad-mium sulphide) cell PC -1, causes itsconductance (resistance) to vary withthe illumination applied. Resistancedecreases as light intensity increases.
Cell PC -1 now acts as a resistorthat varies linearly with amplitudevariations of the power amplifier.
Using the changing cell resistancein the signal -voltage dividing networkconsisting of R4, R5, R6 and blockingcapacitors CI and C2, the switchingarrangement behaves as follows:
R12
C4
R5 SI
24
PLASTIC BOTTLE CAP6-32 X 3/8" BINDERHEAD SCRE
CHASSIS 1/4" APPROX
CEMENTPHOTO CELL
CELL FACEAND ELECTRODES APPROX
PARALLEL 1/8"MAX 1,
Fig. 2 Lamp -cell housings are plastic.Secure lamps and cells with clear cement.
The function selector switch, thetwo operating -level pots and the tworemote -indicator neon lamps aremounted on the front panel. The lampsare held in grommets and jeweledbushings are used for dressup.
On the back of the chassis are fourphono jacks (an input and output pairfor each channel) and a terminal strip How to use itwith four screw terminals (two for When used with an integratedeach drive -signal circuit). Other parts system (preamp and power amplifier
APPROXI/8" 1/2"MAX I
V
15/leAPPROX
[
I245"
13/16'
NEON LAMPCEMENT
I/2u O.D. METALWASHER
15/1e O.D.METALWASHER
3/I6"
ALUMINUMFOILREFLECTOR
are located as shown in the photo-graphs. Grommets are used to protectleads passing through the chassis.
Resistors and capacitors aremounted on solder -lug terminal stripsor supported by their leads. Current -limiting resistors R2, R3 and R8, R9are mounted on a resistor board on topof the chassis near the lamp housings.
Build the lamp -cell combinationsEach lamp -cell combination
should be in a light -tight housing. Thehousing shown in Fig. 2 was madefrom a '3/in" ID plastic bottle. Metal35mm film cans are also suitable. Sawabout 11/2" off the bottom of each bot-tle, and paint the inside black. Cementthe cells and neon lamps in place. Givethe outside of the module a one -turn -plus wrap of black construction paper.Slit the ends of the paper for the neonlamp leads, and slit the center for thoseof the cell. Next, apply cement to thewrap, which is pressed over the cell andlamp leads. Then plug the assemblyinto its cap, which contains a metalstiffening washer, and bolt the assemblyupside down on the chassis.
Pieces of aluminum foil about"he" long x %6" wide, cemented brightside in beneath the neon drive lamps,will increase the unit's dynamic range,sometimes considerably. Cement allcomponents with a clear -drying adhe-sive such as Elmer's Glue or WhiteGlue.
The neon lamp should be notmore than 1/2" from, and perpendicu-lar to, the face of the cell being driven.Lamp anodes should be parallel to theface of the cell.
Alternate lamp -cell housings canbe devised, but must be lightproof. Theflash of a room lamp or the flicker of
fluorescents will affect the sensitivecells. Light from one channel strikingthe cell of the other will disrupt the en-tire system.
6-32 NUTS
25
RIGHT CHAN-.---- OUTPUT
DYNAMICVOLUMEEXPANDER -COMPRESSOR
POWER AMPLOUTPUT
RIGHT CHANDRIVE
LEFTCHANOUTPUT
LEFTCHANINPUT
LEFT CHANDRIVE
LEFT CHANDRIVE
RIGHT CHANINPUT
RIGHT CHANINPUT
POWER AMPLOUTPUT
LEFT CHANINPUT
Enona
otrrpur
Elnnn
INPUT
GC
INTEGRATEDAMPLIFIER
PREAMP INPUT
PHONO
Fig. 3-Hookup for integrated system (a), program material is fed directly to inputs.Drive signals come from speaker terminals and compandor output is fed to preamp.Component system (b) compandor is inserted between preamp and power amplifier.
on one chassis), the expander -com-pressor is inserted between the pro-gram source and the preamp, as shownin Fig. 3-a. Be sure to use the rightpreamp input, since compensation fora record player, for instance, is differ-
ent from that for tape. If the unit isunstable with an integrated system,shield the top of the chassis with per-forated metal and add a metal bottomplate.
RIGHT CHANDRIVE
TAPE
PREAMP
POWERAMPLIFIER
RIGHT CHAN.,..,,.._.--OUTPUT
DYNAMICVOLUMEEXPANDER -COMPRESSOR
TUNER
TI OR T2
RED
BLU
YEL
BL
TO PWRAMPL
N°44 /i
Fig. 4-Transducertoo efficient? Thiscircuit takes care ofthat problem shouldit arise in your unit.
1 jNE-2VI
PCI
1 NE -2V I
1 I
330K1W
GRN
With component systems, p acethe expander -compressor between thepreamp output and the power amplifier input (Fig. 3-b).
In this location, the unit will acton program material from the preamp,allowing you to switch from one pro-gram source to another without chang-ing cables.
Signal voltage to drive the unit isobtained from the output of the poweramplifier, preferably from a 16 -ohmtap. If this impedance is not available,use the 8- or 4 -ohm tap. Never connectthe unit across a 70.7 -volt or other con-stant -voltage line. A small signal swingdrives it through its entire dynamicrange.
Regardless of which terminalsthe drive circuit is connected to, thespeaker system always sees its match-ing impedance.
The volume level at which the unitwill go into action depends largely onspeaker sensitivity. If this is moresound than you customarily use whenloud passages are played, the circuitshown in Fig. 4 will compensate forabove -average transducer efficiency,usually permitting full expansion andcompression at normal listening levels.
To connect the unit to high outputimpedance amplifiers of any kind.place a resistor in series with one sideof the line to the drive circuit. Usea value about 50 times that of the out-put impedance.
Input and output cables for usewith integrated systems can be stan-dard shielded audio cable A length of
3' is recommended although longerones can be used if a capacitance ofabout 100pf is maintained.
With component systems, yourexisting audio cables may be used.Cathode follower preamps usually per-mit the use of much longer cablesLamp cord of any reasonable lengthcan be used between the power ampli-fier output and the drive terminals.
The expander -compressor canperform several limiting jobs. It canbe used when making tape recordingsto prevent overload distortion right inthe concert hall, if you wish.
You can hear soft passages of pro-gram material played in high -ambientnoise locations. By compressing theloud passages and raising the volumelevel of all the material to where theloud passages were, the soft passageswill come through as only the loud onespreviously did.
Even works with TVUsed with a TV receiver, it keeps
those "important messages from thesponsor" at a level commensurate withtheir importance. Connect from thehigh side of the de -emphasis capacitorto ground, and across the volume con-trol. Remove the low side of the de -emphasis capacitor from ground, a 3'cable will substitute its capacitanceclosely enough.
Here's another bonus. In the com-pression mode, try playing those noisyrecords you were about to throw away.Start with the operating level controlsfully clockwise.
+15
+10
-152 6 8 10 12 14
COLLECTOR CURRENT - mAFig. 1-Curve of foward-agc transistor.Low collector currents yield high gain.
28
What happened to those clicksand crackles? Being spike pulses, theywere cut off. The level controls cannow be adjusted to the point whereonly loud clicks are cut off. Too muchcompression will make even a splendidperformance sound singsong.
Now get out your finest tapes orrecords and play them with dynamicexpansion. Set the amount of expan-sion desired for the loudest parts of the
by KENNETH E. BUEGEL
FM TUNERS FOR STEREO RECEPTIONhave steadily improved in quality.Today's audiophile can select a solid-state unit with specifications whichwere never achieved before.
All tube designs, all the oldertransistor designs, and the present FETdesigns use an agc technique knownas reverse bias in which a negativebias is applied to either a grid or baseto reduce the output current (andgain) of the device. While reverse bias
works well with devices such as tubesand certain FET's, which can operateat zero or negative bias, it fails whenused with ordinary transistors whichmust always have some forward biasto maintain operation.
Strictly speaking, the forwardbias on the transistor was only re-duced until the gain decreased atsome very low collector current. Un-fortunately, this meant that the base-emitter junction could be a very goodrectifier for the same strong signalthat was producing the bias. Thusthis biasing technique was a failure inoperation and the setting was ripe forthe introduction of FET's.
The use of a different type oftransistor, however, solves the overload problem. This transistor is knownas the "forward agc" type-one inwhich gain reduction is caused by anincrease in the forward bias appliedto the transistor as shown by the curveof Fig. 1. Note that at collector cur-rents of only 2-3 mA the power gainof the device is maximum, but whenthe current has increased to a highervalue the power gain may even be-come negative, with less output thaninput.
material. As the operating level controll are advanced, the remote indica-tor lamps will begin to flicker, indicat-ing when the compandor takes hold.
The great dimensional changesyou now hear left the recording studioas tiny pips on the signal, some of thembarely got out at all. But now, even atlow listening levels, the full dynamicsof music are present.
FM STEREO TUNER
+20
-5
+5
ccw300.
Of course, since the base -emitterjunction is heavily biased, a strongsignal cannot be rectified at thispoint. Thus, in the FM -1 tuner de-scribed here, we can apply a signal of0.2V rms to the antenna terminalswithout degrading performance.
Although forward-agc transistorscan successfully compete with FET'sunder extremely strong signal condi-tions, they deliver superior perform-ance when handling very weak sig-nals. We get this kind of performanceby close impedance matching betweenthe coupled circuits, which impliesmaximum power transfer. With anantenna impedance of 300 ohms it issimply not possible to transform thisupwards to the higher values requiredfor field-effect transistors. Circuit con-figurations capable of this transforma-tion such as transmission lines andtuned cavities are bulky and of pro-hibitive size.
The transistors used in this de-sign present a much lower input im-pedence and consequently more use-ful power is transferred. The result isa tuner with high sensitivity and an
extremely important side benefit-noshielding is required in the rf section.Whereas a stray coupling capacitanceas low as 0.1 pF (16,000 ohms @100 MHz) can couple a considerableamount of power between two highimpedance circuits, the same stray ca-pacitance effects little power transferbetween impedances nearer 1200ohms.
Remember also that any electro-static shield placed over an inductancemust be much larger than the coildiameter to prevent drastic reduc-tion of Q values and inductance sincethe shield acts as a single shorted turncoupled to the inductor.
The crystal filter impedances inthe FM -1 are closely matched to re-duce phase non -linearity and preservegood stereo separation.
On weak signal reception, for-ward bias to Q1 and Q2 (Fig. 2) issupplied through R22 and R24 fromthe junction of R21 and R25. Adjust-ing R25 sets the bias for highest sensi-tivity. As signal strength increases, thebase voltage on Q5 becomes morepositive until, at a higher signal level,Q5 is providing bias to Q l and Q2.
FM -1 SpecificationsUsing the detailed alignment procedure that appears on these pages, the readershould be able to achieve or improve the following figures.
20 db Quieting Sensitivity .... 1 tcV30 db Quieting Sensitivity . 1.4 AV50 db Quieting Sensitivity . . 3.9 AVCapture Ratio 1 dBAM Suppression 60 dBHarmonic Distortion . 0.5% (measured
at detector output)Hum and Noise .. less than -70dBSpurious Response less than -90dBImage Rejection . . .. less than -90dBHalf i.f. Response less than -80dBAdjacent Channel Selectivity .. -70 dBOverload Sensitivity (for 1%
distortion) 0_2V rms
Multiplex Separation .45 db mid -frequency
35 dB @ 50 Hz30 dB @ 10 kHz
Output Level adjustable up to 1VrmsOutput Impedance approx. 20,000 ohmsDistortion 1%, 1000 Hz, 75 kHz
deviationSCA Suppression 48 dBHum and Noise 45 dB
-60 dB (below reference IVoutput with 19 and 38 kHzcomponents notched out)
29
06osc2N3856
R281000
The resistance in the collectorcircuits of Q1 and Q2 is low to pre-vent saturation at these higher collec-tor currents. The base connections toL2 and L3 provide maximum powertransfer from the tuned circuits. Q3is a dual -gate FET mixer with the rfsignal applied to gate I and the localoscillator signal to gate 2.
Transistor Q6 is arranged to pro-vide a low distortion oscillator signalto the mixer. An additional Zenerregulator prevents frequency variationwhile the R28 -C24 combination re-moves any Zener noise. The base ofQ4 is matched to the mixer output bya tap point formed by the capacitivedivision of C14 and C15.
6324 7K
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6.8pF
CI -d
C20 C25.02 5-12pF
R29ioo2i
RIB -
041ST IF AWLMPS6939
R201800.
R3112K
Transistor Q4 provides the 500 -ohm output impedance for the firstcrystal filter. Each filter requiresabout 5 pF at the IN terminals togive the specified bandpass. For F101(Fig. 3) this capacitance is the lengthof shielded cable connected to Q4. Forthe second filter it is the output ca-pacitance of IC101.
Transistor Q101 amplifies thesignal and applies it to Q103 andIC101. The gain of Q103 and Q104 isjust enough so that the i.f. voltagerectified by D101 -D102 will providefurther bias to the transistors onlywhen the signal is well past the pointof maximum noise reduction, but longbefore the signal level reaches theoverload level. The design gain of the
RI470
R2I106
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R23 05AGC AWLMPS6939
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FROM AGODETECTOR
R27220A
15
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NOTES C1.2-17pFPER SECTION
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Q103 and Q104 stages is centeredwithin this range.
All i.f. transistors are of a specialtype with the emitter lead placed be-tween the base and collector leads.This lead arrangement prevents un-wanted coupling (and phase shift) be-tween output and input of each stage
The ratio detector, T102, has alinear bandwidth of 500 kHz andcontains the diodes within the housing.Because of T1 02's bandwidth, align-ment is simple, and can be done almost as well with off -the -air signalsas with a signal generator.
The tune indicator lights whenthe signal is tuned correctly. The volt-age at R125 will be about 1.1 Vpositive without a signal and rises to
nearly 1.3 volts when even a- weaksignal is tuned in. R126 provides anadjustment for this voltage discrimina-tion. When the voltage at R125 risesin the presence of a signal, Q105 isturned on, in turn saturating Q106 andQ107. LM101 in the collector cir-cuit of Q107 lights to indicate correcttuning. A S202 contact (Fig. 4) iswired to the turn -on bias for Q107. IfS202 is set to the stereo position thisbias will be clamped to ground byQ207 unless the 19 -kHz pilot signalhas resulted in turning on the stereoindicator, LM201. Thus if the MUTEswitch is on, and 5202 is set to thestereo position, only stereo signals willbe heard.
Since Q107 saturates at this time,
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The collector voltage at Q106also saturates Q208 in the stereo indicator circuit, allowing an indicationonly when stereo is tuned in.
The detected composite signalfeeds the high -impedance inputof Q201. T201 and C203 present ahigh impedance in the emitter circuitof Q201 The output voltage appear-ing across R201 is low in 19 -kHz com-ponents. T202 and C202 are tuned to67 -kHz to eliminate SCA interferenceat the stereo outputs. The 19 -kHz pilotsignal appearing across T201 is am-plified by Q203 T203 and T204 aretuned to the 19 -kHz signal. The inputto emitter follower Q204 is a verypure 19 -kHz signal which providessuperior lock -in of the oscillator evenon extremely weak signals. Since thevoltage at the emitter of Q204 is a lowimpedance it is used to drive the pilotdetectors, D201 and D202, as well assynchronizing Q209.
The rectified pilot signal satur-ates Q205, which in turn provides aturn -on bias to Q206, the stereo lampdriver. The emitter of Q206 is in serieswith Q208 Therefore, the indicatorwill not light unless Q208 is alsoturned on. Q208 is driven by thevoltage appearing at the collector ofQ106, and this voltage will bias Q208only when a signal is received. Thestereo indicator lights only when astereo signal is received. There are nofalse indications between stations.
Locked oscillator Q209 can onlyoperate when pin 6 of T205 is re-turned to ground by the saturation ofQ206. T206 in the collector circuit istuned to 38kHz and drives the bal-anced detectors D203 -D205 andD204 -D206. Balanced detection re-duces the 38 -kHz components whichcan appear across the de -emphasisnetworks, R224 -R225 -C219 andR226 -R227 -C220.
The gates of output amplifiersQ210 and Q211 are returned toground potential Source resistorsR230 and R231 are switched toground to turn on and to +12 volts tomute. In muting, the audio output isreduced over 50 db. This switchingvoltage is taken from the collector ofQ107, the TUNE lamp driver. The out-put of each channel may be adjustedto any voltage up to 1V rms by theoutput controls in the drain circuitsof Q210 and Q21 I .
Resistor R204 selects the amountof composite signal, minus 19 -kHz and67 -kHz components, applied to theswitched detectors. This compositeoutput contains the L + R signal at50 -Hz to 15 -kHz, as well as theL - R sidebands at 23 -kHz to 53 -kHz. These sidebands may be attenuated by multi -path reception as wellas phase distortion in the i.f. strip.The detector transformer used in theFM -I has a linear bandwidth of 500 -kHz to minimize this distortion. How-ever the tuner alone is not able tocompensate for all possible causes ofL - R sideband attenuation. There-fore C204 should be installed on thefollowing basis: use the .02-µF unit ifthis multiplex board is not used withthe remainder of the FM -1, or if allyour useable signals originate morethan 30 air miles from your receivinglocation. Use the .05-µF capacitorwhen the multiplex is used with theFM -1 and the stations are both localand distant.
The power supply provides+14.6V for LM101 and LM201 andregulated 12V for all other circuits.Q212 functions as a capacitance mul-tiplier; the +14.6V has less than 5mV p -p ripple. D210 provides a reg-ulated output from Q213. If there isexcessive Zener noise you can add asimple R -C network to eliminate it.
Diode D209 serves only onefunction; to prevent destruction ofQ213 in case of a wiring error or accidental short during testing. With-
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Three circuit boards are used inthe tuner. These boards are the rfhead, i.f. strip, and multiplex andpower supply boards Each board iscompleted according to the sequenceshown. When all boards are wired,they are temporarily mounted in thechassis for alignment. After alignmentis complete, the dial panels aremounted and the unit is ready forfinal wiring and assembly.
Alignment procedureSet dial to 88 MHz. Loosen the
dial drum set screws and slide thedrum off the shaft. Using small piecesof masking tape, tape the dial cablein place so it cannot become tangled.
I.F. STRIP1. Set generator for a sweep
width of about 300 kHz, centeredabout the 10.7 MHz. It possible, usepost marker adding to set two mark-ers, one at 10.6 and the other at 10.8MHz. Set the generator output levelto about 200 p.V. Connect the outputcable ground to the edge of the i.f.strip near the input to F101. Clip the"hot" lead to the insulated cablejacket leading to F101; DO NOT tiethe hot lead directly to the input pinof F101.
2. Set the bottom (primary) slugof T102 for a symmetrical responsewith the top slug tuned off resonance.The scope vertical input should beconnected to the output lead fromR121, and the horizontal input shouldbe fed from the sweep voltage in thegenerator. Follow the generator in-structions for connections and phas-ing procedures.
3. Next adjust the top slug (sec-ondary) of T102 for best linearity
between the 10.6 and 10.8 MHz mark-ers. Do not retune the bottom slug
4. Connect a dc vtvm set on thelowest range between ground and thetop end of R119. Slowly adjust thetop slug of T102 for exactly zero volts.This should be a very small part of aturn.
5. Connect the vtvm to the output lead from R146 and increase thegenerator output until you can readsome voltage above zero. Tune T101for a maximum reading. Reduce thegenerator output and repeak T101.
6. Place a small ceramic -disccapacitor in series with the outputlead of the generator and connect thecapacitor to L4. Leave the vtvm onR146. Increase the generator outputand tune L6 for a maximum reading.Reduce the output and repeak L,6.This completes the i.f. alignment.
RF HEAD
1. Set R25 to mid -position; con-nect a small amplifier and speaker tothe output lead from R121. Set thegenerator output to 88 MHz withenough deviation so that an audio sig-nal can be heard from the speaker.
2. With the plates of Cl fullymeshed adjust L2, L3, L4, and L6 formaximum quieting of the receivedsignal Reduce the generator outputas needed so that the signal is alwaysslightly noisy. Use a small nylon orfiberglass tool to move the coil turns.
3. Set the generator output to107.8 MHz and set Cl so its platesare at minimum capacitance. Peak eachtrimmer capacitor on Cl for maximum quieting, as well as C25.
4. Repeat steps 2 and 3.5. Set generator output fre-
quency to a quiet spot near the upperend of the dial, around 105 to 106MHz. Tune the receiver to this fre-quency and repeak the trimmer on L4for best quieting. "Rock" Cl aboutthis point while adjusting this trimmersince there is some interaction be-tween peaking this trimmer and thelocal oscillator frequency If you do
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not rock Cl you may only succeed in"fine-tuning" the oscillator frequencyand sensitivity will be reduced fromoptimum.
6. Repeat steps 2, 3, and 5.7. Tune Cl to a quiet spot near
the bottom of the dial and adjust R25for maximum quieting on a very weakinput signal. At this time you maywish to check the sensitivity. It shouldbe 20 dB quieting at less than 1 pNinput. LI should be spaced about 1 8
inch below L2. If sensitivity is too low.carefully push Ll closer to L2 andrepeak the antenna trimmer capacitorat 107.8 MHz and adjust the turnson L2 at 88 MHz. Ten sets of tran-sistors were tried in the prototypeunit and sensitivity varied between0.95 to 0.55 p.V for 20 dB quieting.
MULTIPLEXERYou will need an oscilloscope
and a multiplex generator to align thissection. An ac vtvm is also helpfulwhen making separation measure-ments.
1. Connect the scope vertical in-put to the wiper of R204 and inject a67 -kHz input at the input to the multi-plex board at C201 Tune the slug inT202 for a minimum output as seenon the scope.
2. Connect the scope to theQ202 end of C202 Inject a 19 -
kHz pilot signal at a 25 -mV rms leveland tune T201 for maximum traceheight.
3. Connect the scope to the col-lector end of C206 and set S202 tothe MONO position. Tune T203 and
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T204 for maximum trace height. Re-duce the input level of the generatorand carefully repeak the tuning oftransformers T201, T203 and T204.
4. Connect the scope to eitherchannel A or B output and inject a19 -kHz signal at 25 mV rms. Tuneoscillator transformer T205 for zerobeat as observed on the scope. 5202must be in the STEREO position
5. Connect the scope to eitherend of C214 and tune T206 formaximum 38 -kHz trace height.
6. Set up the generator to de-liver a 0.25V rms L -only or R -onlycomposite signal to the input. Connectthe scope vertical input to channel Aand the horizontal input to channel B.Slowly tune T205 until the scope isdisplaying either a vertical line or ahorizontal line. One of the settings ofT205 will be very unstable; use thesetting which is the most stable andconnect the scope so that an L -onlyinput gives a vertical line.
7. Repeak T201 for the mostvertical line and adjust R204 for thebest vertical line. Switching to an R -only input should result in a horizontalline on the scope.
8. Inject an L plus R signal, withpilot, and set R232 and R233 forequal outputs from each channel.
COMPLETE TUNERPARTS LIST
PARTS LIST RF HEADAll resistors are 1% W, 5% unless noted as 2/2W.
R1, R4-47 ohmsR2, R7-330 ohmsR3, R8-4700 ohmsR5, R22, R24-1500 ohmsR6, R9, R14, R29-100 ohmsR10, R16, R23-220 ohmsR13-12,000 ohmsR15-100,000 ohmsR21, R26, R30-10,000 ohmsR25-2500 ohms, Vs -watt trimmerR31-1200 ohmsR32-4700 ohmsR11-470 ohms, 1/2 wattR12-150,000 ohms, 1/2 wattR17-33,000 ohms, 1/2 wattR18-4,700 ohms, 1/2 wattR19-510 ohms, 1/2 wattR20-180 ohms, % wattR27-220 ohms, % wattR28-100 ohms, 1/2 watt
Capacitors
C1-4 gang FM tuning capacitor, (2-17 pFper section. Trimmer range 0.5 to 12 pF.Only three trimmers are used. Local os-cillator section uses separate trimmer.
C2, C17, C21, C22-6.8 pF, NPO ceramicC3-3.3 pF, NPO ceramicC4, C5, C6, C8, C9, C10, C16, C19, C20, C23-
.02 AF, 25V cer.C7, C12-.001 µF, ceramicC11-70 AF, 20V electrolytic (Mallory MTA-
70E20 or equiv.)C13-.05 AF, 25V ceramicC14-68 pF polystyreneC15-200 pF polystyreneC18-1 gF, 50V electrolytic (Mallory MTA
1D50 or equiv.)C24-350 AF 15V electrolytic (Mallory MTA
350 F 15 or equiv.)C25-3-12 pF ceramic trimmer (Centralab
822F2 or equiv.)CT -trimmers on Cl
Semiconductors, coils
Ql, Q2 -2N3690 (Fairchild)Q3 -3N141Q4, Q5 -MPS 6939Q6 -2N3856D1 -10V Zener diode, 1W, 5%L1 -2T, #26 enamel, next to ground end of L2L2 -4T, #18, %" dia, 7/16" longL3 -4T, #18, Va" dia, 7/16" long, tap at 1T
and 3TL4 -same as L3 but tap only at 3TL5 -2T, #18, 5/16" dia, V4" long, tap at 3/4 T
from CldL6-J.W. Miller 9051
IF Strip (101 to 199 series)
C101, C103, C105, C106, C107, C108, C109,C110, C113, C114, C119, C120, C126,C127, C128 C129, C130, C131, C132-.02 ,i<F, 25V ceramic
SPECIAL TUNER PARTSThe following special parts are need-ed for the construction of the tuner.All are available from Transitek, P.O.Box 98205, Des Moines, Washington.Tuning capacitor Cl $7.50Miller coil No. 9051 $2.04Miller coil No. 2072 $2.58Miller coil No. 1354PC . $2.61Miller coil No. 1355PC . .$2.61Miller coil No 1361 .. . $1.80Fairchild 2N3690 $1.25Crystal filter $13.50Rf printed circuit $5.90If printed circuit $7.20Mx and power supply
printed circuit $9.20
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C102-6.8 pF, NPO ceramicC104, C111, C112, C123-.05 0, 25V ceramicC115-5 pF, polystyreneC116, C117, C118-330 pF ceramicC121-5-pF 50V electrolytic (Mallory 5050 or
equiv.)C122-350-0 15V electrolytic (Mallory 350-
F15 or equiv.)C124, C125 -omit due to design change
Resistors all 1/4 watt 5% unless noted
R101-560 ohms, 1/2 wattR102, R112-47,000 ohms, 1/2 wattR103-10,000 ohms, 1/2 wattR104. R111, R120-2200 ohms, 1/2 wattR105, R138, R144 1000 ohmsR106, R110, R117, R139, R140, R145-220
ohmsR107-510 ohms, 1/2 wattR108-4700 ohms, 1/2 wattR109, R121-22 ohms, 1/2 wattR113-6800 ohms, 1/2 wattR114-220 ohms, 1/2 wattR115-470 ohms, IA wattR116-47 ohms, 1/2 wattR118-2700 ohms, 1/2 wattR119-100 ohms, 1/2 wattR122-47,000 ohmsR123-6800 ohmsR124, R125-5100 ohmsR126-1000 ohms, 14 -watt trimmerR127-22 000 ohmsR128, R136, R141-47,000 ohmsR129, R137, R142 R143, R146-10 000 ohmsR130-4700 ohmsF101, F102-XTAL FILTER, $13.75 ea.
Specifications for F101 and F102:
Center frequency -10.7 MHzBandwidth-1.5db, 200 kHz min., 3db, 240
kHz min., 20db, 500 kHz max., 50db, 1100kHz max.
Ultimate attenuation-50dB minimumMaximum insertion loss-5dBPhase linearity -Within plus to minus three
degrees of best straight line over 200kHz minimum bandwidth
Input to filter -500 ohms plus 4.2 pFLoad on filter -500 ohms plus 7.5 pFSize--51mm long, 18mm wide, 33mm highOperating temperature -45 to 110 degrees F.T101 -J. W. Miller 2072T102 -J. W. Miller 1605 PCQ101, Q102, Q103, Q104 -MPS 6939Q105, Q107 -2N3860Q106 -2N5355D101, D102-IN34LM101-12-14V, 25-40 mA
Multiplex and Power Supply (201-299 series)
C202-560 pF polystyreneC203, C206, C207, C211, C212-.01-0 poly-
styreneC204- 02 AF or .05 -AF, disc ceramic, see textC205 -.001 -AF polystyreneC208-.01 0 disc ceramicC210-15-0, 35V, electrolytic (Mallory MTA
15E35 or equiv.)C213 -5 -AF 50V. electrolytic (Mallory MTA
5050 or equiv.)C214-.0015 , polystyreneC215, C216, C217, C218-.002-0 temp. stable
ceramicC219, C220 -.001µF temp. stable ceramicC221, C222-.05 µF, disc ceramicC223-10-0, 35V, electrolytic (Mallory MTA
10035 or equiv.)C226-500-0, 25V, electrolytic (Mallory TC
250513 or equiv.)C227, C228-500.0, 15V, electrolytic (Mal-
lory MTV 5000N15 or equiv )
R201, R206, R212 R213, R214, R215, R216,R217-10,000 ohms
R202, R207-2 megohmsR203-3300 ohmsR204-1000 ohms, 1/4 watt trimmerR205-1000 ohmsR208, R218-6800 ohmsR209-2,200 ohmsR210, R234, R235-100,000 ohmsR211, R230, R231-4700 ohmsR219-560,000 ohmsR220, R221, R222, R223-39,000 ohmsR224, R225, R226, R227-150,000 ohmsR228 R229-1 megohmR232, R233-25 000 ohms, Vs -watt trimmerR236-220 ohms, 1/2 -wattR237-100 ohms, 1/2 -wattR238-56 ohms, 1/2 -wattR239-470 ohmsT201, T203, T204-J.W. Miller 1361T202-J.W. Miller 1362T205-J.W Miller 1354 -PCT206-J.W. Miller 1355 -PCT207 -117V pm, 24V CT secondary, 1A power
xfmr.D201 D202, D203, D204, D205, 0206-1N-
41540207. D208, D209-402670210-13V Zener diode. 1W 5%Q201 -2N3391Q202 Q203, Q204, Q206, Q207, Q208, Q209 -
2N3860Q205 -2N5355Q212-40408Q213-40407S201, 6202 -rocker switchesLM201-12-14V 25-40 mA pilot bulb
C201, C209, C224, C225 -10/50V, electro-lytic (Mallory MTA 1050 or equiv.)
WHAT I WANTED WAS A COMPACTspeaker system with a wide, smoothfrequency response, low distortionand sufficient power -handling capabil-ity to fill at least a small room withrealistic sound.
The requirement for smooth re-sponse and low distortion in a smallcabinet calls for an acoustic suspen-sion system. Although horn -loadedspeakers provide excellent responseand low distortion, the laws of phys-ics being what they are, it is impos-sible to have a "small" horn -loaded
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BOOKSHELF SPEAKER SYSTEM
by ALEXANDER N. RETSOFF
SIDE TOP
SIDE TOP
SIDEMCABINET I
CABINET 2
1
43
24"
11
14"
48"
BOTTOM
9" 1' 14"-1
Fig. 1-Cutting diagram shows you how to cut two speakers from a single board.
hole somewhere in the cabinet allowsthe rear sound wave from the speakerto enter into the room. These systemsboast relatively high efficiency and ex-tended low -frequency response.
In general, every dynamicloudspeaker exhibits a primary reso-nance at the lower end of its fre-quency response. The frequency atwhich the resonance occurs dependsupon the mass of the moving system,i.e. the cone and voice coil, and thecompliance of the suspension.
For a speaker in free space, thesuspension is made up of the flexiblespider, which supports the voice coilend of the cone, and the rim supportat the large end of the cone. The sys-tem is analogous to a weight on theend of a spring. Once the weight is setin motion it bobs up and down at arate or frequency that depends on themass of the weight and the cornpliance of the spring. By increasingthe size of the weight and/or pickinga softer spring, frequency can be low-ered. The converse is also true.
The point is that a speaker's out-put drops off very rapidly below itsresonant frequency. Also, at its reso-nant frequency the cone likes to moveand does so very readily, resulting ina peak in the response curve. Just likethe weight on the spring, once in mo-
tion at its resonant frequency, thespeaker tends to keep moving even af-ter the signal has disappeared. This"hangover" causes poor transient response and muddy sound.
The degree to which the peak ap-pears in the response curve and thetransient response is impaired is deter-mined by the "Q" of the resonant sys-tem. This in turn is controlled by thedamping or friction in the movingsystem. The chief causes of dampingare the air loading on the speakercone, the friction in the suspensionsystem and the damping factor of theamplifier.
If damping is high, there is onlya mild peak in the response curve,since the energy is rapidly dissipatedin the friction; the cone quicklycomes to rest after the excitation isremoved. An underdamped systemwill exhibit a large peak and long"hangover." Obviously this is to beavoided.
What has all this to do with thechoice of a speaker enclosure? Well,the speaker and its enclosure must bedesigned for one another. It is thecombination of the two, the speakerin its enclosure, which must be testedfor resonance, response, etc.
Now, a bass -reflex enclosure is abox with a hole in it, and a box with
SIDE
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A-3-3/491/4"
12-1/2"
14"
3/4 INCH FIR PLYWOODSPEAKER BOARD a REARBOARD 7-1/2" X 12-1/e.SCREW TO GLUE BLOCKSUSING 12- NO.8 FLAT HEADWOOD SCREWS I-1/4" LONG.
Fig. 2-Use good cabinet assembly techniques when constructing your speaker.
a hole in it is a resonant system itself.It is called a Helmholtz resonator. Asoda bottle is a Helmholtz resonatorif you blow across its mouth.
The design philosophy behind abass -reflex enclosure is to have thebox resonate at the same frequency asthe speaker mounted in it. This canbe accomplished by properly con-trolling the volume of the box and theport size and ducting.
When two resonant systems arecoupled together like this an oddthing happens. Instead of gettingtwice as big a resonance as you mightsuspect, you get two resonant peaks,one higher in frequency than the orig-inal and one lower in frequency. Adip appears where the original reso-nance was. The spread between thetwo peaks depends on the degree ofcoupling between speaker and box.This in turn depends upon the size ofthe box and the amount of dampingmaterial in it. In this way, one canextend the low -frequency response ofthe speaker system to the lower of thetwo resonance peaks.
This sounds like a terrific idea,and indeed the bass -reflex system waswidely used several years ago, and stillis in many smaller enclosures. Thereare certain disadvantages to this sys-
tern: (1) increased distortion, espe-cially near the resonant points, sincethe system is somewhat uncontrollednear resonance; (2) less than opti-mum transient response, leading tomuddy sound; (3) irregular responsein the bass region formed by the tworesonances and the trough.
Why an infinite baffle?This leaves us the infinite baffle
The infinite -baffle enclosure includesany type which prevents the soundfrom the rear of the speaker fromgetting into the room. In its simplestform it is an infinitely large wall inwhich the speaker is mounted.
In a more practical form, it is acompletely sealed box filled with fiber-glass or felt mats to absorb the rearsound energy. The walls of the boxare strong and rigid enough so theydo not vibrate in sympathy with thespeaker. The infinite baffle performsthe primary function of the speakerenclosure: preventing the rear soundwave from blending with the frontwave. Since the two waves are out ofphase, they cancel each other ifallowed to meet (The sound emanat-ing from the port of the bass -reflexenclosure is delayed by the enclosuredesign enough to emerge in phase
44
GLUED & MITERED" "
3/4' X 3/4" GLUE BLOCKS3/4 X 3/4 GLUE BLOCKS
CORNERS FRONT & REAR
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45
with the front wave in the bass rein-forcement region At higher fre-quencies all rear energy is absorbedby the damping material in the cabi-net, and in this region the bass reflexacts as a sort of infinite baffle.)
Early infinite -baffle enclosureswere made very large. Since the airtrapped in the cabinet acts as a springbehind the speaker cone, system com-pliance is reduced and the resonantfrequency of the speaker system israised above that of the speaker alone.Since the output of the system dropsbelow the resonant frequency, it ap-peared advantageous to use a verylarge cabinet for minimal increase inthe resonant frequency.
The acoustic -suspension system isbasically an infinite baffle in whichthe compliance of the trapped air iscounted upon to provide some of thespeaker suspension. Such a systemuses a speaker with a very light (com-pliant) suspension that has a very lowfree -air resonance. It is put in a rela-tively small sealed enclosure. Thetrapped air decreases system com-pliance and substantially raises theresonant frequency by as much as anoctave.
Thus one can see the necessityfor starting with a very -low -resonant -frequency speaker. The acoustic sus-pension provided by the trapped airhas one very distinct advantage: itlowers the harmonic distortion of thesystem. The main cause of nonlineardistortion in a speaker system is thenonlinearity in the suspension. Thetypical spider and rim -surround sus-pensions do not provide a linear re-storing force at extreme cone ex-cursions.
The "spring" provided by thetrapped air, however, is extremelylinear. In a well -designed acoustic -sus-pension system, the air cushion pro-vides the majority of the restoringforce. For example, if the resonantfrequency of the system is one octaveabove (twice as high) the free -airresonance of the speaker alone, three-
quarters of the restoring force is pro-vided by the air cushion and onlyone -quarter by the speaker suspension. Thus, the effect of the nonlinearsuspension, that is the distortion, isreduced three times.
In addition, the air cushion isprovided by making the enclosuresmall instead of large Further, the re-sponse is relatively smooth-assuminggood damping to an acceptably lowfrequency-if the original speaker isof the high -compliance type. We payfor all these advantages with a de-crease in overall efficiency.
In choosing a speaker for use inan acoustic -suspension system onemust look for a low resonant fre-quency. Next, you need a good, pow-erful magnet, which implies gooddamping and the capability of longspeaker excursions. The latter, al-though they give rise to Doppler dis-tortion, are unfortunately necessary toget a reasonable sound -pressure levelfrom a small cone.
How to build itProbably many speakers will fill
the bill. I chose to build the systemaround one available for $8.95 fromLafayette Radio Electronics (cata-logue No. 99E01554). It is a 5 -inchunit with a 11/2 -lb magnet and reso-nant frequency of 40 Hz It is ratedat 16 watts (peak). For a tweeter Ichose another Lafayette unit (cata-logue No. 99E01562) at $2.95.
The enclosure is 14 x 9 x 9inches and is constructed from 3/4 -
inch plywood. I chose to construct itfrom single -sided walnut -veneered ply-wood, available from several firms.Following the cutting diagram of Fig.1, two speaker enclosures can bemade from one 24 x 48 -inch sheet ofplywood.
Other veneers are also available.If you like oiled walnut, two or threecoats of boiled linseed oil rubbed inwith cheesecloth and the excess re-moved will give you a better finishthan you can expect on commercial
WHITE
cabinets. Let each coat dry overnightand rub lightly with the grain usingfine steel wool. The recessed front andback of the cabinet will not show andcan be made from 3/4 -inch fir ply-wood available from any lumber yardA two -conductor barrier strip ismounted on the recessed rear wall.
Fig. 2 shows how the cabinet isput together. Be sure to make it air-tight. Seal all cracks with glue, filleror caulking. The inside of the cabinetshould be loosely filled with Tufflex ora similar sound -absorbing material.Such materials are readily availablefrom most parts dealers
The wiring diagram is shown inFig. 3. A 4-µF, 50 -or 100 -volt ca-pacitor is used as a highpass filter tothe tweeter. This capacitor should bea paper or mylar type Aluminum orother electrolytics should not be used.
ANECHOIC CHAMBERMEASUREMENT
100Hz !kHz
FREQUENCY 20kHzFig. 4-Anechoic chamber curve issmoothed by placing unit in corner.
100Hz I kHz I OkHz
FREQUENCY 20kHzFig. 5-Impedance curves for the woofer in free air and damped in the cabinet.
RED
20Hz
WOOFER IN FREE AIRfo = 48 Hz
When wiring the speakers besure to observe polarity If you usethe speakers recommended, you willfind one terminal on each insulatedwith a red fiber washer and the otherwith a white one. Connect the whitestogether as the common return andthe reds with the capacitor.
More exotic crossover networkscould be used with this speaker sys-tem, but they are really unnecessary.The woofer used in this system han-dles itself so gracefully at the higherfrequencies that there is no real rea-son to prevent it from reaching them.
When you're through, connectthem to a good amplifier with at least5 watts per channel capability into 8ohms and listen. As is true for allspeaker systems, bass response isaffected by placement of the units inthe listening room. As much as 9 dBboost in the low end can be achievedfrom corner placement.
Fig. 4 shows the response of thesystem, measured in an anechoicchamber, and the response expected ina corner placement, the recommendedlocation for this system. Placed in acorner, a speaker sees a much reducedsolid angle of radiation at low fre-quencies. Thus, its efficiency is greatlyincreased. This is especially importantfor small -size systems. You can ex-periment with various placements inyour home. You will probably bequite surprised at the difference posi-tions can make.
WOOFER IN DAMPEDCABINET fo = 75Hz
1
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1
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46
WHITE
4 pp50 V
WOOFER
TWEETER
Fig. 3-Wiring diagram for speakers. Doriot use tantalum electrolytic capacitors.
+10
0
-10
PREDICTED CORNERRESPONSE
wz.cr0LaaM
20Hz
10kHz
3 -WAY ELECTRONIC CROSSOVERby NORMAN H. CROWHURST
Impedance curves of the wooferin free air and in the damped cabinetare in Fig 5. You can see that theresonant frequency rose from 48 Hzto 75 Hz, almost an octave. In addi-tion, the height of the peak is sub-stantially reduced. As explained, theacoustic response of a speaker systemfalls off at about 12 dB per octavebelow the resonant frequency. FromFig. 5, you can see the resonant fre-quency is at 75 Hz, and, sure enough,from Fig. 4, the response starts to falloff at just about that point.
The results of this little systemwill amaze you. The sound is sur-
The basic amplifier circuit aroundwhich a 12-dB/octave filter can bebuilt is in Fig. 1. It consists of a centralvoltage -gain stage and two emitter fol-owers, to provide impedance isolation
so performance is not affected by ex-ternal circuits and their impedances.
The output emitter followermakes the output signal voltage devel-oped by the voltage -gain stage avail-able with a low source impedance, andthe base circuit of the input emitterfollower is fed by a 2:1 voltage di-vider. There is an additional 2:1 gainloss due to the 6 dB feedback used tosharpen the response to its correctshape.
So the voltage gain of the middlestage should be precisely 4 (12 dB).This will make the output signal volt-age the same as the input, from eachchannel, within that channel's range.
Using a 12 -volt supply and tran-sistors with a current gain of about100 (this is not critical in the circuitchosen) we calculate values. For the
prisingly clean and live. The in-struments of the orchestra are welldefined and project into the room.This is attributable to the smoothmid -range. Except for a dip and peakin the response at 4 kHz and 6 kHz,the response throughout the criticalmid -range is extremely smooth andwell balanced compared with any sys-tem. Couple this with adequate re-sponse, again without any sharp vari-ations, out to the limit of audibility(16 kHz), and solid bass responsedown to 60 Hz or so, and you have asystem to put many of its larger andmore costly brothers to shame
output emitter follower, we want theemitter de voltage to be 6 volts Thisallows maximum swing with the 12 -volt supply.
Picking 510 ohms as the emitterresistor (assuming a 500 -ohm load asminimum external output termina-tion), the dc load at the base will beabout 100 times this, or 51K. Using abias potentiometer of 5.1K and 5.6Kwill bring the emitter voltage veryclose to 6 -volts on a 12 -volt supply.
Now to calculate the base inputresistance of the output emitter fol-lower: This consists of 5.1K and 5.6K,along with the reflected impedance ofabout 51K (with no external load con-nected) all in parallel. This combina-tion figures to 2.5K. An external loadof 500 ohms would reduce the re-flected part to about 25K, reducingthe combined input impedance to2.25K, which is not a serious change.
T achieve an in -range gain of4:1, we assume a collector resistor ofIK and calculate the collector load,
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which is this 1K paralleled for sig-nal purposes with 2.5K, to make 720ohms. With this collector load, anemitter resistor of 180 ohms will con-trol the stage voltage gain to 4:1.
To get 6 -volts on the collector ofthis stage, its current must be 6 mA, sothe emitter voltage will be 1.08 -voltsThe 180 ohms will reflect to the basecircuit as 18K.
The next step was based on us-ing identical capacitors for controllingturnover in each stage of coupling.For the high-pass filter, the reactanceof the coupling capacitor between col-lector and output emitter followerneeds to be 3.5K (2.5K plus 1K) atroot -2 times turnover frequency.
The input emitter follower willprovide negligible source impedance,so the base input impedance of thevoltage -gain stage should be 3.5K also.
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Fig. 1-Basic 12-dB/octave filter (below) uses two emitter followers. Fig.2 (above)-Voltage relationships thatdetermine feedback resistor value.
Trial and error led to the choice of5.1K and 43K, along with the 18Kreflected through the stage from the180 -ohm emitter resistor, which comesvery close to the 3.5K desired.
This combination of resistors alsocontrols the emitter voltage to 1.08volts, and thus collector current to6 mA and the collector voltage to 6volts.
The input emitter follower alsouses a 510 -ohm emitter resistor. Itsbias will normally be taken throughthe feedback resistor. Its input imped-ance, reflected from the emitter resis-tor, will be about 51 K. Using two 3.6Kresistors across the input signal voltagewill divide the input voltage in half(before feedback is considered).
Feedback will reduce this signalvoltage to one-fourth input and the4:1 gain will bring the output voltageup to equal the original input signalvoltage, as well as reversing its phase.
The source resistance for the in-put signal voltage of 1/2, at the base, isthe parallel combination of two 3.6Kresistors, or 1.8K. The signal voltagewill be dropped, when feedback is ap-plied, to 1/4, while the feedback resis-tor will connect back to a signal volt-age of 1, reversed phase. (Fig. 2).
So the feedback resistor will have1.25 times the original input voltage
GND
43K5%
180
.
NI
NIIf
.053
910n
GND
0.53
Fig. 3-High- and low-pass circuits alone make a useful 2 -way crossover.
INPUT
.063
.063 910R 500-.,2,000Hz
.16
Fig 4- Band pass circuit added for a 3 -way crossover. See tab e for values.
053
3.6
0.5 3
GND
ti
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across it, thus needing to be 5 timesthe impedance that "drops" the 0.25part. Five times 1.8K is 9K, for whicha 9.1K resistor will serve.
Using a blocking capacitor tokeep dc out of the input circuit, thebias of the input emitter follower istaken from a 6 volt point (the outputemitter) through 9.1K and 3.6K,yielding a working voltage of about1.7 volts. As the signal level at thispoint is 1/4 the input and output signallevel, this voltage provides adequatemargin for handling the signal.
The impedance reflected throughthe input emitter follower will be 1.8Kdivided by 100, or 18 ohms. Andthrough the output emitter follower,720 ohms divided by 100, or 7.2 ohms.Thus output loading can have little ef-fect on operation.
Low-pass designThat about sets the picture for
the high-pass configuration. For low-pass, we have the collector -circuitparallel impedance calculated at 720ohms. This is the impedance the shuntcapacitor works with. If we use a seriesresistor in the base circuit to make thatalso 720 ohms, the same value ofshunt capacitor will serve. With the3.5K in the base circuit, a 910 -ohmresistor will bring the shunt combination at the base to 720 ohms. This re-sistor needs dc blocking, unless thesection is bandpass, so the capacitor isneeded for the high-pass function.
Thus the completed two-waycrossover looks like Fig. 3. To makecalculating the capacitors for turnovereasier, we will figure them at whatevercrossover frequency is chosen.
For high pass, they should be fig-ured at root -2 times crossover, so theirreactance will be root -2 times theseries -circuit impedance at crossover,or 5K.
For low pass, they should be figured at crossover divided by root -2,so their reactance will be the parallelcircuit impedance divided by root -2 atcrossover, or 500 ohms,
This makes the figuring very convenient. We have tabulated some val-ues of capacitors for selected cross-over frequencies. If you want to makeyour crossover three-way or more, itis simple to combine the two functionsinto one circuit for any bandpass chan-nels you use (Fig. 4).
If you plan on making several ofthese, in varying combinations, it maybe worth planning an etched circuit(Fig.
5) that can be wired for lowpass, high pass or bandpass. Fig. 6shows a completed circuit for a three-way crossover, using these circuitboards.
Finally, you may need to knowhow to check the performance. Thefirst step is to open the feedback loopThis can most simply be done by lift-ing the 9.1K resistor from the outputemitter and connecting a 12K resistorin series with it to the negative sup-ply Now the unit should show pre-cisely 2:1 gain, and 90° phase shiftwith zero gain at frequencies that areroot -2 times crossover for high pass,and crossover divided by root -2 forlow pass.
This 90° phase shift patternshould look like a circle on the scopewhen horizontal is connected to inputand vertical to output with both inputsset to the same sensitivity. This can beset by paralleling them on the inputand adjusting to give a 45° straightline. Check this at the appropriateroot -2 frequencies. If the result is in-correct, find out where it goes wrong.
To find an error in a high-passfilter, or the high-pass function of abandpass, use an electrolytic capacitorto bypass one of the turnover capaci-tors in turn; first one, then the other.Each of these should yield the 45°trace at root -2 times crossover fre-quency (Fig. 7).
If one of them doesn't, the ca-pacitor has the wrong value and shouldbe corrected. Finally, reconnecting thefeedback should give the ellipticaltrace at crossover, instead of the circleat root -2 times frequency
50
;t
51
POS
0I
01
SUPPLY
2,000 HZUP
To find an error in a low-passfilter, or the low-pass function of abandpass, merely disconnect one endof the turnover capacitors, each ofthem in turn Each of these condi-
tions should yield the 45° trace atcrossover divided by root -2. Again, ifone of them doesn't, the capacitor re-maining in circuit is off value andshould be corrected.
20 -200 HZ
NEG
IN-PUT
INPUT(UVESIDE,TOGROUNDSUPPLY+)
Fig. 5-Copper side ofcircuit pattern used forcrossovers. Actual sizeof the PC board used is3 x 6 inches Draw or en-large optically. Fig. 6 be-low shows the com-pleted and wired 3 -waycrossover. Use table forcrossover values.
SUPPLY
200-2,000)4Z
Fig. 7-Scope pattern of the 45° phase -
shift frequency, identifying 8 points onthe ellipse. Here the 45° line is set ata mid -band frequency, and the ellipseshould appear at the root -2 frequency.If the 45° line is set by paralleling ver-tical and horizontal, the ellipse will betwice as high for the same width. Fig.8-Vanous power supply circuits: a is a12V winding, one side grounded, b is12V winding with center tap grounded(or 6V, one side grounded) and c is 6Vwith the center tap grounded.
0
3311
.707 b.5
0ill.
.5 CT63V
500/l5V+
5000/15V
5252
+
...
+
1
1
C.' II 12V 500/+15V
I .707 .5 .5.7037 I
.707
A little practice with this will findit quite simple to do, and the circuitsobey the rules nicely.
Almost any pnp transistors wilserve in this circuit. If you want touse npn transistors, merely reverse thesupply polarity and the polarity of anyelectrolytics. With pnp, positive supplyis also signal ground for input and out-put. With npn, negative supply is alsosignal ground.
The lowest low pass can incor-porate a rumble filter, if desired, "atno extra cost," by using identical elec-trolytics with a value to give a 12-dB/octave rolloff at the desiredfrequency, say 20 Hz (as shown inFig. 6).
The supply requirement for eachunit of the crossover is about 21.5 mA.Thus a two-way crossover requires43 mA; a three-way, 65 mA. This caneasily be obtained from a filamenttransformer with small diode recti-fier(s) and really large electrolyticsfor final smoothing (Fig. 8). The
500/15V
0
500/I5V
15V
5000/I5V
MN* of Capacitor ValuesCrossoverFrequency Capacitor Values (eF)
Hz Low Pass High Pass20 16 1.625 13 1.330 10.5 1.9540 8 0.850 6.3 0.6360 5.3 0.5380 4 0.4100 3.2 0.32120 2.7 0.27150 2.1 0.21200 1.6 0.16250 1.3 0.13300 1.05 0.105400 0.8 0 08500 0.63 0.063600 0.53 0.053800 0.4 0.041,000 0.32 0.0321,200 0.27 0.0271,500 0.21 0.0212,000 0.16 0.0162,500 0.13 0.0133,000 0.105 0.01054,000 0.08 0.0085,000 0.063 0.00636,000 0.053 0.00538,000 0.04 0.004
10,000 0.032 0.003212,000 0.027 0.002715.000 0.021 0.002120,000 0.016 0.0016
5000/I5V+
500/I5V
0
+500/I5V
3311
cause nothing is at all critical, exceptthat it needs adequate filtering toavoid hum.
STEREO HEADPHONES are great-theyprovide the ultimate in separation andyou aren't distracted by room noise.Only trouble is they usually don'tmatch your amplifier output. This ste-reo headphone control center takescare of the matching; moreover, itgives you full control of volume, bass,blend and balance. And you don't haveto disconnect it to use the speakers.
The control center can be usedwith any stereo headset and any stereoamplifier-unless you have a very un-usual headset or an amplifier.
For controls, it has a three -posi-tion selector switch: You can choosespeakers only, phones only, or both atthe same time. (The third choice mayseem a bit strange, but it is much ap-preciated where one listener is hard ofhearing. He can wear the phones andenjoy the music at a suitable volumelevel. The others can listen at a corn-
fortable volume through the speak-ers.) The switch is of the shorting(make -before -break) type to reduceswitching clicks.
There is, of course, a volume con-trol for the phones ganged so that bothchannels are controlled simultaneous-ly A balance control compensates foruneven tracking between the two sec-tions of the volume control, and forindividual differences in hearing sensi-tivity between the listener's ears.
The blend control corrects for thesometimes excessive stereo separationin headphone listening. At its extremecounterclockwise position, the blend -control resistance is switched out ofthe circuit, allowing full separation. Atthe other extreme, the two channelsare connected together, producingmono sound.
The bass control provides up toapproximately 6 dB of boost at 40 Hz(referred to 1 kHz) to compensate forheadsets whose air seal against thesides of the head is not good.
ConnectionsLeaving aside the convenience this
box provides-which may or may notinterest you-it might seem thatthere's a simpler way of connectingphones to an amplifier. The most ob-vious would be simply to connect thephones across the speaker terminalswith a switch in series to turn thespeakers on or off. This works, but hasa couple of very serious disadvantages.First, earphones require only between
STEREO HEADPHONE CONTROL CENTER
KY®
configuration used depends on thefilament supply. You can probably usesomething from the shelf for this, be -
Controls
53
47f2 -
RI
SISELECTOR
R3
1541
R6 -bRIO -b
502
5012
50012RI2
BLEND
54
R4
R5 1511
'TOSPKR
L FROMAMPLOUTAMPLCOM
FROMAMPL
R OUTTOSPKR
X
+ CI100 7.3V
C2VOLUME
R6 -a
RIO -o
251110 V/
(21
C31003V
R7
R8
k-4741-0
Fig. 1-Unique switching arrangement makes it possible to switch in speaker orphones only, or both. Amplifier is loaded, either by the speakers or RI and R2.
PARTS LISTCl, C2, C3, C4 -100-µF, 3 -volt electrolytic
capacitorJ1, J2-Three-contact phone lackR1, R2 -25 -ohm, 10 watt wirewound resis-
torR3, R4, R8, R9 -47 -ohm, 1/4 -watt resistorR5, R7 -15 -ohm, 1/2 -watt resistorR6-Dual 1000 -ohm potentiometer, log
taper (Centralab, F5-1000 & R5-1000)
10 and 1.00 milliwatts for quite loudvolume levels. This is in the neighbor-hood of one -thousandth of the poweroutput capability of typical amplifiers.
Not only is the full amplifier pow-er unnecessary for phones, but it candestroy them in a fraction of a secondby burning out the voice coils or rup-turing the diaphragms. As a result, thevolume control on the amplifier can beonly just barely cracked open. Therehas to be some way of cutting thepower fed to the phones.
Furthermore, the high sensitivityof the phones results in a good bit ofnoise along with the music. The normalamplifier hiss and hum, usually inaudi-ble a few inches from a speaker system,become definitely audible in high -sen-sitivity headphones pressed close toyour ears.
RIO-Dual 50 -ohm wirewound potenti-ometer
R11 -100 -ohm wirewound potentiometerR12 -500 -ohm potentiometer (with S2)S1-4 pole, 3 -position shorting type (make -
before -break) rotary switchS2-S.p.s.t. switch (on R12)Misc.-Plastic case with aluminum panel
(Lafayette 99 H 6272 or similar); bar-rier -type terminal strip (5 terminals)
The usual way of solving bothproblems is simple and quite satisfac-tory: Stick a resistance in series witheach "hot" earphone wire. The valuemost commonly used is around 300ohms. It cuts down the power to thephones and, of course, cuts back thenoise at the same time. It also reducesthe damping factor to nearly zero. But,this seems to have little audible effecton phones, which have very small, low -mass cones or diaphragms with littleinertia. They are usually pretty welldamped by internal absorbents and byclose coupling to the ear chamber.
But a load in the vicinity of 300ohms is almost an open circuit as faras the amplifier output is concerned. Itbecomes necessary then to provide adummy load for the amplifier when thespeakers are switched out. The value of
R2
R9
X
SPKRS
PHONES
i
SI; R IIBAL
10041
JI
0
0
0
55
C
TO R SPKR
T. FLOATING TIE POINT (SOLDER -LUGSTRIP W TN 2 INSULATED LUGS,ETCI
CONTROLCENTER
Fig. 2-To avoid undesirable power loss in the speaker wiring, don't use a cablelonger than approximately 25 feet. Use impedance taps to match your speakers.
STEREO AMPL TO L SPKR
5 - COND CABL
R8
16
CHASSISSCREW
e e
e0
RI, 82
R
16
l_ 84
C
happens to be unstable with no load.Occasionally a defective or poorly de-signed amplifier will oscillate with noload and damage its output transistors.
Construction
Few things could be simpler orless critical to wire than this switchbox.The low impedances and relativelyhigh signal levels make it unnecessaryto observe any precautions about wirelength, shielding, routing, and so forth.Be sure, though, to use wire no thinnerthan No. 22 for the leads that willcarry the full speaker current (heavylines drawn on the schematic).
The photos show all the detailsfor wiring the control center. Thesolder -lug strip mounting is solderedto the back of the balance control.
Shorted output circuits in a solid-stateamplifier can be destructive-use a bar-rier type terminal strip. Note: speakerand amplifier output terminals are shownin reversed order-follow the schematic.
The Stereo Headphone Control Centerworks as described. It is important to tryto obtain log -taper (audio taper) controlsfor R6, otherwise the change from flat re-sponse to full boost occurs all in the firstfew degrees of rotation from the counter-clockwise stop.
Be sure to wire the control exactly asshown. If you don't, the taper is in effectreversed and the rate of change will beeven more extreme than with a linear -tapercontrol.
56
the dummy load is not critical. It isn'tnecessary, or even desirable, to matchthe dummy to the amplifier output.
A considerable upward mismatch(for instance, 8 -ohm output loaded by25 -ohm resistor) means much lesspower wasted in the dummy load asheat, since the amplifier will not usual-ly develop nearly as much power intoa 25 -ohm load as it will into an 8 -ohmload. This headphone box switches ina pair of 25 -ohm 10 -watt resistors (onefor each channel) in the PHONES posi-tion of the selector only
It might be worth while to digressfor a moment to explain this matter ofprotecting amplifiers with a dummyload. In general, the dummy is mostimportant with tube amplifiers, prin-cipally because the high inductance ofan unloaded output transformer candevelop peak voltages during loud sig-nals or instability high enough to breakdown transformer insulation or causearcs in tubes or tube sockets. Almostany value of load resistance less than 5or 10 times the nominal output impe-dance of the amplifier will load thetransformer enough to prevent thistype of trouble.
In transistor amplifiers, shortsacross the output, rather than opens,most often cause trouble. However, itcan do no harm to load the output ofeven a transistor amplifier in the sameway. Dummy -loading may even savethe output transistors if the amplifier
If the electrolytic capacitors haveuninsulated metal cases, as the ones inthis model did, insulate them with tapeto prevent their touching the panel.The panel must be electrically commonto the common side of the circuit un-less you want to provide insulatingbushings for the phone jacks.
Modifications
Of course you can provide onlyone jack if you wish, or three or more.
57
Fig. 2 shows how the control cen-ter is to be connected to an amplifier.Note that the common terminal on thecontrol center goes to the amplifierchassis, not to either of the speakerterminals often labeled "common" or
By KENNETH F. BUEGEL
Volume will diminish as you parallelmore sets of phones. If you like, youcan provide a separate volume controlfor each headset. You can, in fact,duplicate the portion of the circuitafter the switching (to the right of thedashed line X-X on the schematic) sothat each headset has full, separatecontrol facilities.
If you like your headphone musicextremely loud, you may have to forgothe bass -boost circuit (R5, R6 and R7and Cl, C2, C3 and C4). To provide6 dB of bass boost, the circuit mustintroduce 6 dB of loss everywhere elsein the spectrum, except at the lowfrequencies. In a passive equalizer,there is no way around this fact oflife. So if you need loudness, discon-nect or omit the bass boost circuit.
"C". The reason is that in many ampli-fiers the so-called "common" terminalsare not common to each other or tothe chassis, and cannot be connectedtogether without disrupting some circuit function.
This connection is for headphoneoperation only. The normal connectionto the speakers is in no way affectedby the addition of the box if you followthe scheme given here. The speakerscan be turned off or on by the selectorswitch in the control center, but this isdone by interrupting the high (4-, 8 -or 16 -ohm) side of the wiring, not bydisturbing the connection to the ampli-fier's low side.
Flat, five -conductor antenna -ro-tator cable is handy for making theconnections between control centerand amplifier because it can be slippedunder a rug or run alongside of base-board molding.
The wire should be No. 22 orheavier. Avoid running more thanabout 50 feet to prevent a sizableportion of your expensive ampli-fier power to be dissipated in thewiring.
FM STEREO ADAPTER
THIS STEREO FM DEMODULATOR IS DE -signed around recent silicon planar tran-sistors and uses high -stability polysty-rene capacitors. It is compatible withany detector output level between 0.3and 5 volts peak to peak. Separation ona properly constructed unit will exceed30 dB from 50 Hz to 14 KHz, and 40dB from 100 Hz to 10 KHz
Many stereo listeners are puzzledwhen a good stereo adapter added to apreviously satisfactory tuner fails toproduce adequate separation. Often the
trouble lies in unexpected areas. Whenyou've been intelligent and careful inconnecting the adapter to the tuner, andthe antenna installation isn't faulty, whatelse could be wrong?
In almost every instance I have in-vestigated, the problem was caused byrestricted tuner bandwidth, which causesa distorted response in the 23 -53 -kHzsubcarrier range. Many otherwise finetuners may have this defect. It is possibleto compensate for it.
Connecting the thing
1111
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TO STEREO INDICATORAUDIO OUT
58
Q21-VcC
This decoder incorporates just thatkind of compensation. The schematic isin Fig. 1. Input stage Q I has adjustablegain to match different tuner output lev-els Because of heavy feedback the in-put impedance is high enough to allowdirect connection to a vacuum -tubetuner without heavy loading. The valueof R2 is selected during alignment.
L is the SCA (Subsidiary Communications Authorization) rejection filter,which, with the well-balanced output detectors, keeps SCA program matter in-audible. Emitter follower Q2 sends com-posite audio at low impedance to thecenter tap of T3, a 38 -kHz doubler inthe collector of Q4. The pilot signal de-veloped across TI is amplified by Q3and synchronizes the 19 kHz locked os-cillator, Q4.
Q5 selects and amplifies the 19 -kHzpilot signal. R22 is the trigger adjust-ment, which sets the level of pilot signalrequired to switch the adapter to stereoreception. D5 and D6 charge C24 withrectified pilot signal until Q6, a dc amplifier, is forward -biased.
Q3
Q6 drives Q7 into saturation, whichlights lamp PL and brings the lower endof T2 to ground potential. Under theseconditions Q4 oscillates and T3 pro-vides the switching signals to the output detectors. Diode pairs DI D3 andD2 D4 will conduct alternately at the38 -kHz switching rate and channel leftand right components of the signal ap-plied to T3 centertap to correct outputs.
During monaural reception, whenno pilot signal is present, Q7 will be cutoff and its collector at supply potential.A slight reverse bias will prevent oscil-lation in Q4. There is now a paththrough D1, D2, RI 3, and R14; thusthe monaural signal applied to the cen-ter tap of T3 appears at output A. Asimilar path exists for the same signalto appear at output B. The de -emphasisnetworks precede output jacks.
The decoder circuits draw about 8mA from a 14 -volt supply. PL's currentis 20 mA from a slightly higher poten-tial. If it is not convenient to draw thismuch current from an available source,
R3 SEPARATION
R22 TRIGGER Q5INPUT FROM TUNER
06
19 kHz
.411!TRIGGER
( ow19 kHzts
TRIGGERrs
IS k Hz
I OT1LOCKED OSb
10K4V
INPUTAMPL
R639K
R149K
RI9150K
STEREOIND
I
Cl, C21-0./ AF, 30V,ceramic
C2, C5, C6, C7, C8,C20, C22 - .01jcF, polystyrene
C3, C23-.02 µF, 30V,ceramic
C4, C9, C14, C18-5/IF, 15 V, electro-lytic
C11, C12, C15, C16-.002 AF (Sprague192P or equiva-lent)
C13, C17-.001 pr(Sprague 192P orequivalent)
C10-.0012 µF, poly-styrene
C19-.0015, itF, discceramic
C24-1 IS, 15V, elec-trolytic
C25 - 10 ttF, 15V,electrolytic
Capacitors specifiedas polystyrene areMallory type SX,listed on p 293 ofAllied's catalog 260(1967 general cata-log)
Ceramic capacitorscan be Sprague'slow -voltage type, al-though higher -volt-age capacitors areof course equallyusable.
101-06-1N4446 (G E)(1N914A is alsousable)
Q1 -2N3391 (G -E)
01-06-1N4446Ql- 2N339102,3,4,5,7-2N3860as -2N508
59
R4
01
RI
I.8MEG
INPUT
I/30V R3SEPARATION
R2 IKSEE TEXT
R5 II(
.01 Q4LOCKED OSCa DOUBLER
R12470K
R94.7K
12
R8 T2
t.
C19 ---C25-10/15V,0015
STEREO TRIGGER
AN05 R22
19kHzAMPL
L8 MEG
T4 C204.01
R23 10K
C45/I5V
T3Vrt
C8RII SEE TEXT
.01
RIO
4.7 K
C9-5/I5V
1N914 -A
10K C2I
PARTS LIST
D5
R2422K
D6C24
I5V
Q6
1
5
39K
00
.0012
RI3
D2
5T.0022
D4 -
C17
M001
Q2, Q3, Q4, Q5, Q7 -2N3860 (G -E)
Q6 -2N508 (G -E)PL -16 -volt indicator
lamp (15-20 mA)(Sylvania 16CSB-p. 316 of Alliedcatalog 260)
R1, R6, R21-1.8megohm
R2, R11-see textR3-trimmer resis-
tor, 1,000 ohms(Mallory typeMTC-4)
R4, R7, R23, R26 -10K
R5, R8-1,000 ohmsR9, R10-4,700 ohmsR12 -470KR13. R14, R15, R16 -
39K, 5%R17, R18, R19, R20 -
150K, 5%
R22-trimmer reststor, 10K (Mallorytype MTC-4)
R24 -22KR25 -100KR27-220 ohmsAll resistors 1/2 watt,10% unless other-wise specifiedT1, T2, T4, T5 -19 -
kHz tank (.J. W.Miller 1354 -PC)
T3 -38 -kHz outputtransformer (.1.
W. Miller 1355 -PC)
1-series bandpassfilter W. Mil-ler 1352 -PC)
For construction onchassis instead ofetched circuit board,order Miller unitswithout PC suffix
C II.0022
CI2170022D3 C13 +C14 A
_L--) (_...._,,,.001
R15 15" 5) OUTPUTSRI8 R20 +I 5V/ 44./ (
150K13C18
sv..R7
07STEREOSWITCH
R 26
10K
R 25100K
R27
2zon
V2PL (SEE FIG.2-0)
Fig. 1-Follow this wiring diagram carefullyfor best operation of the FM stereo adapter.
2
21
VB+0
DI
D3
R3.03
POWER RATING'R .001 WATTS
+V2
1N3754
E 8 + -15
C
1.250p.F25V
15V
a
IN3754
T
CI 250pF 350µF25V 5V
the regulated supply shown in Fig 2will power the decoder.
When constructed on an etched cir-cuit board, the complete decoder, in-cluding required bottom clearance, fitsin a space 33/4 x 6 x 2 inches. Since theunit is extremely stable and won't needconstant adjustment, it can be tuckedaway in an unused cabinet corner. Ifyou decide on chassis construction, arecommended size is 7 by 9 inches. Thisprovides room for the power supply,terminal strips, transistor sockets, etc.If you plan to build on a chassis insteadof on a circuit board, be certain to orderthe transformers without the PC(printed circuit) suffix.
In mounting parts to the wiringboard, first install the transformers,then R3, R22, all other resistors andcapacitors (except R2 and RI I), andfinally the transistors and polystyrenecapacitors.
The first step in alignment is tomeasure the multiplex output level ofthe tuner with an oscilloscope. Use thehighest peak to -peak reading as a refer-ence. Use an input level, from an audiogenerator to the decoder, of 25% higher
470t1
C2
2N3414
B
C3 .22µF/50V
than this reference level. (Use any con-venient frequency in the audio range.)Choose a value for R2 between 3.9K and12K (larger input, larger value) whichallows undistorted reproduction of theinput signal as seen on a scope at theemitter of Q2.
Next insert a 67 -kHz signal (thisshould be accurate) and tune L for min-imum signal at Q2's emitter. Insert a19 -kHz signal at one-fourth referencelevel and tune T1 for maximum voltageacross its winding. Reduce the input sig-nal and tune T4 and T5, with R22 atmaximum, for maximum voltage acrossR24. During tuning reduce the inputlevel until the voltage across R24 starts
2
R2
Parts list for Fig. 2
C1-250 aF, 25Velectrolytic
C2-350 pc, 15Velectrolytic
C3-0.22 µF, 50V,paper, Mylar or
60
2200 I4V
B
Fig. 2-a-Regulated power takeoff for using the demodulator with a tube -type tuner.b-If you don't wish to draw power from a tuner, this power supply will perform well.
ceramicD1, D2 - 1N3754
(RCA) or equiva-lent IOW siliconrectifier
D3 -Zener diode,15V, % or 1
watt (Motorola
1N4744, Interna-tional Rectifier12F15110 orequivalent)
Q -2N3414 (G -E)R1-220 ohmsR2-470 ohmsT-filament trans-
former, 117 -voltprimary, 26.5volt ct secon-dary, 0.6 amp(Knight 54 A1476, Triad F -40Xor equivalent)
Fig. 3-a-Poor stereo separation; b-good separation; scope at adapter output.
to decrease; at this point the tuning ef-fect is most pronounced.
Tack in a 470 -ohm resistor for R11and temporarily ground the collector ofQ7. Tune T3 for maximum amplitudeof the 38 -kHz waveform at Q4's col-lector Then tune T2 until any jitter inthe waveform disappears, indicatingsynchronization with the input signalContinue to retune T1, T2, and T3while reducing the input level, until theoscillator is synchronized with a 19 -kHzinput signal at least 26 dB (one -twenti-eth) below reference.
Select a value for RI I between 200and 1,000 ohms that provides maximumundistorted 38 -kHz output. Too low avalue will give lower -amplitude switch-ing voltages and reduced separationAdjacent cycles of the switching wave-form will not have identical height (thisis normal) but should be within 20%.
If you have access to a stereo mul-tiplex generator, remove the ground atQ7's collector and inject a composite
signal at reference level. Set R22 toplace Q7 in saturation and adjust 121for maximum separation.
If such a generator is not availableyou can run through the procedure justdescribed with the decoder connectedto a tuner receiving a stereo transmis-sion. Connect a potentiometer (about100K) to the tuner output to allow theinput to the decoder to be reduced dur-ing alignment.
After preliminary adjustment re-move the potentiometer from the cir-cuit. Connect outputs A and B to thevertical and horizontal inputs of ascope Tune to a station known to betransmitting a stereophonic broadcast.Adjust R22 until PL lights. The scopepattern will probably resemble Fig. 3 a.Adjust R3 until the scope display lookslike Fig. 3-b. A slight touchup of T1and resetting of R3 will result in bestseparation. If the separation seemspoor. try another stereo station orbroadcast
200 -WATT IC STEREO AMPLIFIER
TABLE IData taken with split supply at±37 5 volts, 4 -ohm load, 1 -kHzsignal frequency and feedback restsfor = 820 ohms.
Zero signal dissipationwatts'
Input voltage = 1 00 @ 100wattsoutput
THD 0 38% @ 100 watts out-put
Input voltage -0 62 @output
THD 0.14% @ 35 watts output* Of this, 585 mW is dissipation inresistor R , R and R-, leaving adevice dissipation of 1 14 watts.
ADVANCES IN HIGH -POWER THICK -
film technology combined with aunique circuit have resulted in a hy-brid IC power amplifier with universalapplicability. The amplifier was de-signed for the industrial and commer-cial market where a high -power, audio-range, reliable and compact linearamplifier was needed.
Although not fabricated specifi-cally for high-fidelity applications, theperformance characteristics of theTA7625 hybrid power module (seeTable I and Graphs I-III) suggest itis quite adequate for this purpose. Thephoto snowing an encapsulatedand unencapsulated module mounted
1 725
35 watts
61
on a heat -sink type chassis indicatesthe extreme compactness possible fora 200 -watt stereo amplifier. The pow-er supply for the modules is mountedon a separate chassis, although single -chassis construction is possible if pre-ferred. The power modules are avail-able for $80 each.
The protective circuitry of themodules insures they cannot be inter-nally damaged by a sustained short atthe output or faulty power -supplyvoltages. The design permits use of themodule with capacitive loads (elec-trostatic speakers).
The internal structure of the hy-brid amplifier is shown in Fig. 1. Ithas a rugged steel base plate on whichboth the thick -film circuit and the out-put assembly are mounted. This out-put -transistor assembly is an all leadsolder mount system with a lead framefor easy fabrication. A ceramic sub-strate is used under each pellet toisolate it from the base plate. The pel-let is mounted on a heat spreaderwhich reduces the heat flux density bythe time it gets to the ceramic.
Structure & circuits
62
-----:1,
i
= 7N,
SPLIT SUPPLY: +375V4 -OHM LOAD820 -OHM FEEDBACK
RESISTANCE
0.1 I 10 100OUTPUT POWER -WATTS
SPLIT PWR SUPPLY:
SINGLE PWR SUPPLY: ---
40W
0I-
I
I- 0.5cn
02O 0.12rr 0.05
0.02
0 003I- 0.1 0.2 0.5 I 2 5 10 20 50 100
POWER OUTPUT WATTS
The Fig. 1 photo shows the interior of the hybrid module
63
FREQUENCY.IKHzSPLIT PWR SUPPLY -SINGLE PWR SUPPLY.---
2-0
0(r) 0wcrw -1
< -2wcc -3
10Hz SO AIN IKHz r 100KHzFREQUENCY
Graphs I-III aboveshow performancecurves for the IC.
The schematic for the circuit ofthe hybrid power amplifier is shownin Fig. 2. A differential bridge (QIQ2) is employed at the input of theamplifier to provide good control onthe quiescent operating point zero -output voltage. Imbalances of thebridge circuit with supply variationsare solved by using constant -currentsource Q3 in the emitter circuit. Thisapproach is almost universally usedin integrated circuits to function over awide range of supply voltages.
The voltage at pin 8 dictates theoutput voltage at pin 3. A pair of
09
DT
5 -37 5V
ance source (a current source). Thecurrent transfer characteristics of atypical output stage do not exhibit theoffset steps as do the transconduc-tance characteristics. The high -imped-ance source consists of a second cur-rent source, Q4, direct -coupled to theclass -A predriver, Q5. In this case, theclass -A stage is a pnp device since thebridge devices are now npn type.This arrangement also allows the cur-rent source to be an npn type as welland can share the same reference di-odes, DI and D2.
This approach is a simple col-lector -to -collector connection of com-plementary devices providing an in-herently high collector impedancesource; together with the differentialbridge, the system performs as a "con-trolled bidirectional current sou' cf.."Its operation is completely indepen-dent of supply voltage variations.
Versatility was achieved by hiing-ing the feedback terminal out (pin
,
.100
D2
RIODI 1000
D6
R4'ax
C7R2I 02.270
R23220
RI560
17
R718K
ReIK
R98K
812270
0900
CI.05
R2OK
011
RI9: 1.8K
5/25V
08
LOAD
R20270
-Miller No 5220 or SO turns of No. 20 copper wire (3 layers) over a 2W carbon resistor (1000 -1
RIO1000
02
Q5
50
ibkaAA
.im
on
[f
...
11'4 m
,0 W
3
3031.13
CS
onin
003
110111
alla
..--.-4,
4M
f .-r104
1
=
R310K
O
R62TO
matched resistors (R2 and R3) con-trol the output voltage to half thesupply voltage. Pin 8 is grounded di-rectly for the split supply case (Fig.3-a) and ac -grounded with a 100 -AFcapacitor for the single supply case(Fig. 3-b). The constant -currentsources (Q3 and Q4) allow the am-plifier to be biased properly from atotal supply voltage of much less than30 volts to the 75 -volt maximum.
The true class -B quasi -comple-mentary output/driver stage is drivenby the bidirectional current source.The output devices (Q10 and Q11)are chips from the 2N3055 family andthe pnp/npn drivers are from the2N5322/2N5320 families. The bidi-rectional current source npn/pnp'sare from the 2N2101/2N4036 fam-ilies.
A technique for eliminating cross-over distortion in class -B output stagesis to operate them from a high-imped-
64
HI T )1-''"
10µH
INPUT
5ILFI2V
9
470S1
= _50µFI2V
+37V
Fig. 3-a-This power supply configuration is a split supply (pm 8 grounded)
C I -C2
TA7625
65
-37V
C2
-v +VPower supply c rcuit for stereo ampl.
TA -7625 Power Supplies
Pou, DI -D4120 watts 5000 /IF 1N1202A 7A
(60W than.) 50V40 watts 3500 p.F 1N1614 2A
(20W/chan.) 50V
7). Maximum gain is limited by the100 -ohm internal resistor, RIO.
The load -line limiting circuit(shaded area) protects the amplifierfrom a sustained short at the output.This unique limiting circuit places re-strictions on the allowable operatingarea of the drivers and outputs. Itfunctions as a voltage and currentsampling circuit to provide operating
T
INPUT
I 5µF25V
50µF12V
4T- 1 0 0 F50V
7
-48V5
10µH
8
7
6 2000µF47011 +=50V
I I7V6 OHZ
SW
(SLO- BLO)
FULL WAVEBRIDGE
Fig. 3-b-The single supply arrangementwith pin 8 ac grounded through 100 F.
I
DZ
D4
CI
limitations on the output devices. Thelimits chosen do not inhibit normaloperation, but any overload conditionincluding a short circuit will remainwithin the safe area of operation ofthe device. Diodes D7 and D8 andthe load -line limiting circuit protectagainst a faulty output transformersecondary that may reflect voltages tothe amplifier output termination, ex-ceeding the power supply voltages.The diodes clamp this reflected volt-age to the power supply, while theload line limiting circuit limits thedrive.
Controlling the gain bandwidthproduct of the output stage in relationto the preceding stages and adding the10-1.1.H inductor and R23-C7 for ca-pacitive loads are other design factorsfor added versatility.
D3
4i1LOAD
8TA7625
3
2
8IILOAD
IF
r.
II
2
III NM
IIII
I
II V
II=
II
66
PART 2
by F. B. MAYNARD
Musical Devices
DIGISYNTONE MUSIC SYNTHESIZER
THE DIGISYNTONE IS A RADICALLY NEWapproach to a build -it -yourself musicsynthesizer. Basically, the Digisyntonesystem is a special-purpose IC digitalcomputer. It reduces the musical scale tonumbers. and generates tonal effects bydigital operations with these numbers.
The Digisyntone system gives you awide range of voices, whose quality canbe controlled by mixing pure and accu-rate harmonics. Digisyntone is the onlyelectronic musical instrument you canbuild that never needs tuning-it's auto-matically tuned when you build it.
The basic project uses 21 IC's, andcosts from $75 to more than $100.
Although Digisyntone has a stan-dard organ keyboard, it is monatonic.That is, it will play only one note at atime, and hence is strictly a solo in-strument like the trumpet, clarinet, saxo-phone, etc. Pure harmonic mixing re-sults in many tonal effects-the instru-ment's major musical capability.Numerical musical relations
There are several basic numericalrelations in music. Octaves are relatedby a factor of 2. For any note, middle C,for example, there are other C's above2x, 4X, 8x, etc the middle C fre-quency, and some below 1/2, 1/4, etc. thefrequency. Hence, a simple way of generating octaves is multiplying or dividingexactly by 2.
The musical scale has 12 notes ineach octave. The frequency relation herebetween any one note and its neighbor isa 12\/2 factor That is, the frequency ofC#, and this applies to any C# in thescale, is the frequency of any Cl2VT.The numerical value of 1:=V2 is irrational: like it never comes out even
A close value is 1.0595. The series ofnumbers listed in Fig. 3 have this rela-tionship Other series of numbers couldbe used, but they would be no better, ifas good as these. The derived intervalsare not perfect, but are very close. Theworst case error is less than 5% of asemitone.
A third numerical relation concernsharmonics. By definition a second har-monic is 2x the frequency of any start-ing or fundamental frequency, or simplya frequency 1 octave above the funda-mental. The fourth harmonic is 4X thefundamental, or 2 octaves above, and soon with the even harmonics.
A third harmonic is 3X the funda-mental, and octaves of this give thesixth, twelfth, twenty-fourth, etc. Thefifth harmonic is of course 5X, and thisgives the octaves as 10X, 20X, 40X,etc. harmonics.
Much more could be written aboutthe theory of musical tones but, for thisproject, the above is quite adequate.
The basic digital systemUsing electronic circuits, it is much
easier to divide frequencies than to mul-tiply them. With integrated circuits, afrequency can easily be divided by anypositive whole number, and this is thebasis for the digital music system to bedescribed. Fig. 1 shows the basic systemin functional block form.
The oscillator operates at a rela-tively high frequency. The precise fre-quency can be controlled with RI. andthe frequency can be modulated with avibrato signal through R3.
The oscillator drives a divisorcounter which is in turn controlled by a
67
logic control or decoder and keyboardswitches. The logic controller controls afeedback to the counter and the com-bination divides the oscillator frequencyby any one of 12 whole numbers. Thesenumbers, listed in Fig. 3, have ratiosclose to 12V2.
Hence the output from the logiccontrol feedback circuit will present thefactors of a musical scale which is inde-pendent of actual frequency. This outputis now split into two paths. Path PA isdivided by 2 twice, providing two oc-taves of the third harmonic. Path PB isdivided by 3 and by 2 three times. Thisprovides four octaves of the fundamen-tal. (Note that since the path PB is di-vided by 3, the frequencies will be lowerby this factor than from path A; i.e., Ais a third harmonic of B.)
The six outputs are routed to at-tenuator controls and into a system offilters which change the output wave-forms from square to sinusoidal. Fourbypass switches are provided to pass thesquare waves for bright -tone effects.
Circuit details are shown in Fig. 2,3 and 4. Figs. 2 and 3 consist almostentirely of integrated circuits. Two in-tegrated circuits are called for theMC724P quad 2 -input gate, and theNIC790P dual JK flip-flop. Also suitableis the HEP 570 quad 2 -input gate, andthe HEP 572 dual JK flip-flop.
In Fig. 2, ICI is the oscillator, withCl and RI and R2. This simple RC os-cillator can be set up to run at any fre-quency to about 4 MHz. In this case, itsfrequency range. variable with R1, isabout 1.2-2.5 MHz. ICI also provides asingle -input, two -output buffer.
The oscillator output drives the in-put (pin 2 of IC2) of the divisorcounter, an eight -stage binary counter(IC2 to IC5). There are two binaries ineach IC. These are cascaded to countstraight binary. Each binary has twooutputs: Q outputs, pins 13 and 9 (forthe two flip-flops in each package), andQ (not Q), pins 14 and 8. These out-puts, except for pin 9 in IC5, go to thedecoder logic (ICIO-IC21). These outputs are designated in Figs. 2 and 3 as 1,
OSCILLATOR1/2 ICIFIG. 2
10 FG 2
L
L ..e. /...........
V
FEEDBACKRESET
LOWER OCTAVE
KEYBOARD FIG.2
1/2 106 1C7 1C9+2 -3 +2
FILTERSFIG.4
68
14
1, 2,j., etc. from the flip-flops down thecascaded chain.
The decoder logic control is shownin Fig. 3. This shows the flip-flop inputsacross the top of the figure, and horizon-tal lines indicating connections to thelogic control IC inputs.
This decoder logic system consistsof 12 MC724P gates connected as eight -input NOR eates. One of these IC's is de-tailed in Fig. 3. Pins 3, 5, 8, 11 and 14are connected to a common -output cir-
cuit with external load resistors R5 toR16. The load resistors go to a 2.3VB+ source obtained by decoupling fromthe main 3.3V source through R17 andC3. When the final circuit tests aremade, if there are any conditions underwhich operation appears fuzzy, they canoften be corrected by adjusting this voltage. Making R17 smaller raises the volt-age, and vice versa.
The input pins are 1,2,6,7,9,10,12and 13. These are all the same, and any
VIBRATOGEN VG.
FIG. 4
R3 DIVISOR COUNTERIC2 TO 105
FIG 2
DIVISORLOGIC CONTROL
ICIO TO IC21FIG 3
3RD HARMONIC
+21/2 IC8FIG. 2
PREAMPFIG.4
OUTPUT
FIG. 1-BLOCK DIAGRAM of the sys-tem. The three main sections, oscilla-tor/counter, logic control and filters aredetailed for wiring in Figs. 2, 3 & 4.
UPPER OCTAVE
IC9+2
R2 RI
BUFFERFIG. 2
1/2 ICI
1/2 106+2
O=
FUNDAMENTAL
PA
FIG.2
PB
ME
NIM
MIIM
IPIN
MIO
NM
Ehr
IMM
ON
I111
01i1
A 1
101E
m
69
input can be connected to any flip-flopoutput These connections are shown asdots on the 12 horizontal lines whichrepresent the inputs and outputs of the12 IC gates. IC 1 0-IC21. The IC outputsare labeled to notes of the scale, B, A#,A, G# etc., indicating the key switchesto which they are connected. The col-umn of divisor numbers indicates the setof numbers which closely approximatethe ratios of 12\/2. The logic connections(inputs to the gates) are derived fromthe equivalent binary numbers shown tothe right in Fig. 3. For convenience,these binary numbers are shown back-ward from conventional-the least sig-nificant digits are to the left.
These NOR gates function as fol-lows. IC 10, for example, divides by thenumber 131, and connections are madesuch that when there are l's in thebinary numbers, the Q, or 1, 2, etc flip-flop outputs are connected to gate in-puts; where there are 0's, the Q outputsare connected. During its cycle of 256counts, when and only when the counterhas accumulated 131 counts, all the flipflop outputs to IC10 will be 0's, and thisis the only tune when ICIO can producean output
The logic control system is shownin Fig 2 as the lower dashed -line box,with flip-flop inputs T, 1, 2, 2, etc. andthe gate outputs.
Gate outputs B, A#, A, etc. areswitched by key -switch contacts on key-board KS I. This has two key -switch con-tact sets (that is, two make contacts) andbuses on each key The upper contactsare wired with all B keys, all A# keys,etc. on common circuits, making 12 key -switch inputs from the 12 gate outputs.
When a key is pressed, the output istransferred to bus A which goes to ICIinput pin 12. Part of IC1 serves as theoscillator already described, as well asthe buffer function. The buffer has twooutputs. The reset output (pin 8) goesto pins 10 and 12 of counter binariesIC2-1C5. This completes the feedbackloop, causing a reset of the counter tozero at the number of counts designatedin Fig. 3 for any key pressed.
The trigger output from the buffer(pins 9, 10, 14 of ICI) goes to the sec-ond set of key switches on the keyboard.This set is split into buses B and C, each1 octave long. When a key in the upper
octave is pressed, the trigger from IC1 istransferred to bus B. This connects topin 2 input of IC6 and pins 2 and 6 in-puts of 1C7. The input frequency forthese will be the same as the trigger fre-quency, or the oscillator frequency di-vided by the divisor number for thatkey.
When a lower octave key is pressed.the trigger goes to bus C, and to the pin2 input of IC8. The pin 13 output of thefirst flip-flop in IC8 is coupled throughC2 to the same inputs connected to busA This operation divides the trigger fre-quency by 2, lowering it by I octavewhenever a lower octave note is played.If the manual or keyboard used has 3octaves of keys, a third trigger bus un-der the lower octave routes the signal toa divide -by -4 (two cascaded binaries) orone MC790 connected in cascade, andthe output of this is connected through a0.00 15-gF capacitor to the bus B inputline. This lowers the trigger frequency 2octaves. This connection (not shown inFig. 2) is sketched in Fig. 5.
IC6 divides its input frequency by 2and again by 2, providing two third -har-monic outputs, 9 and 10. IC7 dividesthis same output by 3 (note the specialfeedback connections on IC7). The pin9 output from IC7 goes to the pin 6 in-put of IC8 and output 11. The output ofIC8 pin 9 divides by 2 on output 12,and IC9 divides by 2 twice more, pro-viding outputs 13 and 14.
These six outputs go to Fig. 4. At-tenuator pots R18-R23 insure that anyof these outputs, in any combination,can be gated from zero to maximuminto the filter system of Fig. 4. Note 1Kpots are called for, but any values be-tween 1K and 10K are suitable here.
The filters are four low-pass RCnetworks which convert the squarewaves from the IC circuits into sinewaves. These are desirable for the bestharmonic mixing effects. Some squarewaves are also desirable, and two eachof the fundamentals and harmonics arebypassed through switches S2 to SS. Thefilter and square -wave outputs are col-lected on a common line into a pre-amplifier with volume control Q2, andpatched into any medium- or high -gainexternal amplifier through C21, whichshould be a 200 -volt or more paper orMylar unit for safety in patching intovacuum -tube amplifiers.
1-maz
0ZC0
12
3 4
5 6
7IC
6 H
EP5
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RM
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13
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10 9
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T
12
3 4
5 6
72
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71
24
56
1C7
IC2
1C4
14 1
312
11
10 9
814
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FIG
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. 3).
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, the
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FIG. 3-DECODER LOGIC section uses 12 IC's connected aseight -input NOR gates as shown in the lower drawing. Dots onthe horizontal lines indicate where the NOR gate inputs shouldbe connected to the divisor counter (IC's 2-5) Outputs tothe switches are the scale notes (vertical column)
72
2 2 3 3 4 4 5 5 6 6 7 7
R5 -R1662011
OUTPUT
I 2 4 8 16 32 64 128
ConstructionDespite its complexity the Digisyn-
tone is surprisingly easy to build, thanksto the integrated circuits. Make sure all
1 2 ICIO8 131 11000001
IC11A# 139 11010001A 147 110010016* 156 00111001
IC14165 10100101
IC15 F# 175 I I 1101 01IC16
F 185 I 0 0 I I I 0 IIC17 196 00100011ICI8
DO 208 00001011IC 9
D 220 00111011IC20 C* 233 1 0 0 10 1 1
IC21C 247 II10IIII
connections are correctly and securelymade, and that no, shorts occur fromstray bits of solder and incorrectly ori-ented connecting wires.
There are several good ways ofmounting IC's including the use of sock-ets, which, hovever, are expensive. Thebest way is probably on etched circuitboards. The mounting used for the pro-totype is on pattern G Vector boardwith T-28 push -in terminals. The IC'sare mounted upside down and the power
Transistor Q1 provides a 6 -Hz twin -T oscillator for vibrato. This outputgoes to resistor R4 on the main os-cillator, IC1. In the event the vibrato istoo deep, R4 can be made larger, andvice versa. The vibrato is stopped byopening switch S I.
1 2 3 4 5 6 7IC1O TO IC21 INCL
HEP570 ORMC724P
14 13 12 II 10 9 8
B+ 2.3V
C31001.,F
+ 3V
El+ 2.3V
4 5 6 7 8 9 10 H 12 13 14
1C12
1C13
B+ 3.3V
3
©023 ®R22 "5N
55
032 og R353K 3K
Lcf
gt NS. V
FIG. 4-FILTER SYSTEM receives the sin outputs oftheharmonic
and fundamental divider IC s (6-9) RC networkscOnvert square wares int° sinewaves. and Q1 Q2 serve asladed 1.2vibrato and
toprR49pleamifier respectively (Note change R44
K
ORVIBRAT
OFFO
PREASIls
ass
d+l
2006 hOAMPt
R49-1,200 ohmsR50-68,000 ohmsR52-20,000 ohmsR53-200 ohmsR54-13,000 ohmsR55-5 ohms, lOWR56-100 ohms. 2WR57-120 ohms, 2WSemiconductorsD1 -3.6V, 1W Zener diode (HEP 102 or equiv)Q1, Q2-MPS2926 or HEP50 transistorQ3-npn power transistor (HEP 247 or equiv)RECT 1-full wave bridge rectifier (HEP 175
or equiv)ICI 1C10-1C21-Quad 2 -input gates (MC724F
or HEP570)1C2-1C9-Dual JK flip-flop (MC790P or HER
572)Other partsT1 -6.3V, 2A filament transformerS1-55-spst switchS1-spst toggle or slide switchS2 through S5-spdt toggle or slide switchKeyboard (see text)Pattern G vector boardT-28 push in terminalsMiscellaneous wire and hardware
PARTS LIST
and in and out leads are made eitherdirectly from the terminals or to ex-tended leads to terminals. Fig. 5 showsenlarged views of typical mountings forboth the MC724P gates and theMC790P flip-flops. A suggested switchand control arrangement is shown inFig. 6.
The circuit requires a supply volt-age of 3.3 to 3.7 volts at about 180 mA.A suitable power supply to furnish thisregulated voltage is simple to constructand is shown in Fig. 7.
The mounting of the assembled cir-cuits, keyboard and controls is not criti-
Using the DigisyntoneThe several controls which govern
the musical capability are describedhere. Refer to Fig 6 for a pectoral lay-out of these controls The instrumentpitch is under PITCH control R1. Sincethe system automatically delivers a eor-
cal. It is probably advisable to mountthe oscillator, or at least control pot R1,in a shield, since it has a frequency inthe radio broadcast range. Also, keepingthe interconnecting leads as short as pos-sible will reduce tendencies for cross-talk.
Off10 11 52V
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4,,, ti
ru
CapacitorsC1-200 pFC2-.0015 /LEC3 -100-#F, 3V electrolyticC4-C6-0.01 µFC7-C9--0.033 pFC10-C12-O.1 pcC13-C15-0.25C16-2 lOyC17, C18-1 gF, lOyC19, C20-10 µF, 10yC21-4 µF, 200VC22 -2,000-µF, 6V electrolyticC23 -5,000-µF, 6V electrolyticAll resistors 1 W, 10% unless notedRI, R18-R234, R5I-5,000-ohm linear poten-
tiometersR2-1,500 ohmsR3-680 ohmsR4, R4I-R44-10,000 ohmsR5-R16-620 ohmsR17-33 ohms, 1WR24-R36-13,000 ohmsR37-R40-390,000 ohmsR45-6,800 ohmsR46-R48-47 000 ohms
73
MOUNTINGMC790P
VOL
aR5I
D B
GND B--IN-PUT- - 1- - - POWER LEADS
I AND CASCADE0 'I 0 CROSSOVER
UNDER BOARDo \
0
MOUNTINGM C7 24 P
FOR DECODER
LOGIC LOGIC LOGICINPUTS GND
INPUTS
1111 1111
13 12 11 10 9
14 131 8
C
10 TO PIN 2,o .4.- NEXT
BINARY
PITCH
LO v HIRI
Q
OUTPUT
SQUARES3 S2 S5 S4
6666ONSI
R19 RI8 R23 R22 R2I R20
74
1 2
0112
0o _,
0 oI °
Q
11
n1 Q
B+ 3.3V
FIG. 5-IC MOUNTING technique willnsure a neat and functional layout.
LOADRESISTOR \
B+ SIM. NEXT_STAGE/
G
FIG. 6-KEYBOARD and control arrangement suitable for the Digisyntone.
B
RESETLIN
rect musical scale independent of pitch,the pitch can be adjusted to tune to anyconditions. A point can be found wherethe middle A key, for example, will tuneto 440 Hz. in which case the entire instrument is in normal tune to the stan-dard scale. 121 can also tune to anyother frequency. This provides, amongother things, a key -shift feature in whichthe player may play on the keyboard,for example, in a simple key such as C,but obtain sound in any other key. Thiscapability should be a welcome featureto the many "favorite key" musicians.This same control (R1) permits dy-namic sliding or gliding tone effects. Trya Hawaiian melody using RI to swoop
Other special effectsThe voL knob is simply a volume
control which can be manipulated as adynamic swell or loudness control forspecial effects. The four harmonic mixercontrols (1, 2, 4 and 8) provide the fun-damental, and 3, 6 controls the third -harmonic system. When off, to the left,these controls do not permit any signaladmittance to the output system. In the
A
the tone upward or downward in realIsland style
E
0
D
4
G
3
I 2 3 4 5 i 6 7, - -/11 9 8
F
03HEP247
R56100.D.
2W
RECT
+
R55511
IOW
+
Cl2000AF
6VA note on keyboards
A suitable keyboard for the Digi-syntone is the one thing a builder can-not readily buy over the counter. Thereare a number of possibilities for salvaged keyboards from a piano, toy pi -
A* A*
117 VAC
TI6.3 FILTRANS2 AMP
R57120S12W
DIHEP102
3.6VIW 50tifP
6V
+3V
A A
75
HEP175
FIG. 7-POWER SUPPLY for the syn-thesizer provides well -regulated 3 volts.
full clockwise position, all the signal isinjected. These controls may be adjustedof course to any level in between. Many
ADDED +4 FOR3 OCTAVE 37 NOTEKEYBOARD
FROM TOOUTPUTS (FIG.3) PIN 12
C A A# B ICI
C A C AP` B C
BUS A
BUS D BUS C BUS B
TO IC8 PIN 2 TO IC6 ,IC7INPUT INPUTS
B-
I 2 3 4 5 6 7MC79OP
14131211 10 9 8
O
.0015p.FTO ICS , IC7,1
INPUTS - FIG. 8-HIGH C wiring for suggestedB+3.6V keyboard will play an octave low.
combinations are possible with these sixcontrols, and most of them will sounddistinctive.
The harmonic mixing will be mosteffective on the sine filters (S2-S5 up).Other tonal effects can be obtained withsquare waves (S2-S5 down). The vibra-to is turned on or off with Si.
It has been said that the instrumentis monotonic; i.e., only one note isplayed at one time. Note that if twokeys are played at once, except for oc-taves, nothing sounds. Inherent in the di-gital tone system, this characteristic maybe used to provide interesting chopped -tone effects by playing on a key andrapidly keying another to chop the tone.
-TO S2
0 0
BOTTOM PLATE
76
by JACK JAQUES
ADD THIS VIBRATO TO THE GUITARand you'll have given that amplifiernew life. In simple terms the vibratomakes the sound pulsate-come andgo at a controlled rate. This pulsatingrhythm adds real life to otherwise"flat" or "dead" music. Also, with alittle educated knob twisting, you cancreate some unusual sound effects.
While vibratos are most com-monly used with guitars and accordi-
The three series -connected 9 -voltbatteries must besecured so there isno possibility oftheir coming looseand damaging othercomponents. Too,replacement shouldbe simple and easy.Use battery clipsand holders boltedto the bottom coverplate as shown.
ans, the sound of many other musicalInstruments can be greatly improved.For example, the reed type electricchord organ can be greatly enhancedby adding a vibrato. For instrumentsthat do not lend themselves to a spe-cial pickup, try using a standard high-impedance microphone
If you take a look at the vibratocircuit you'll see that it consists ofone transistor, one FET, 2 potentiom-
BLACK
BATTERIES
SPLICE(SOLDER AND TAPE)
TO GROUND LUG+
RED
ano, accordion or reed organ. Most anyof these can be fitted with key switchesto provide a suitable playing medium.
The best bet, however is a com-mercial organ keyboard. These can beobtained from 3 to 5 octaves, with dualkey switches and buses, for approxi-mately one dollar per note, from PrattRead & Co., Inc. Ivoryton, Conn.06442 You should specify "3 octave, 37
note, manual with two key buses pernote, no resistors," and ask for quota-tions.
This keyboard has a high C whichwill be dead in the system shown. Butthis can be doubled or connected in par-allel with the next lower C key. It willsound an octave low, but this is a fairlycommon way of handling an end note insome organs. This is shown in the dia-gram of Fig. 8.
GUITAR VIBRATO
Complete construction detailscan be found in the diagrams, al-though parts layout is not critical. Cutthe perforated board to size and care-fully drill the three mounting holes.Next insert the push -in terminalswhere indicated. Then wire in allparts. Watch out for battery and ca-pacitor polarity and you're home.
C70.1
phone or pickup device to input jackJ1. Connect output jack J2 to thehigh -impedance input of any ampli-fier system. Place the VIBRATOswitch and the POWER switch in theoN position Adjust potentiometer R4and R11 to approximately midrange.Then play single notes or chords onthe musical ins rument being usedand adjust the RATE and DEPTH controts to obtain the desired effect.When operated this way, Q2's gainis effectively used, and you'll find ithandy if you are using a low -poweramplifier.
-1E-
7 .
Q2HEP801
S
R547K
VIBRATO
77
eters. a battery power supply, inputand output jacks, and the variousresistors and capacitors needed tocomplete the circuit. The unit deliversan overall gain of 6 to 8 dB, and itshigh -impedance output is ample fordriving most power amplifiers to fulloutput.
In the circuit transistor Q1 isconnected as a twin "T" oscillatorthat produces a sine -wave of approx-imately 6 Hz. This signal is appliedto the source (S) of FET Q2, whereit amplitude modulates any incomingsignal applied to the gate (G) of Q2through input jack J1. The ampli-tude of this modulation can be var-ied by the DEPTH control R4, to cre-ate either a light or a deep vibratoeffect. The vibrato rate (frequency)is adjusted with RATE control R11, ormay be turned off with switch SI fornormal sound.
Operating the vibrato is easy.Connect any high -impedance micro -
R3I MEG
INPUTJI
C3D
RIO15K
R450K
DEPTH
J2OUTPUT SI o-
RII3K
RATEAN't
R9IK
A simple, yet effective vibrato circuit. Vibrato effect is produced by a 6 -Hztwin T oscillator that modulates the amplitude of the signal in the sourcecircuit of the input transistor. Input and output impedances are high.
R747K
POWER
DI
HEP25I
+ C42p.F
S2
LoBi 9V
B2 9V-0B 3 9V
R8C5 47K C6
Tle.F ATIILF
CI5/:F
R6
RI4.7K
2.2K
R25.6K
C2licF
z
PARTS LIST
+ C3 -
SI
78
J1 J2-phono lacks
C4
RU
Resistors -1/2 -watt 10 unless noted
R1-4,700 ohms
R2-5,600 ohms
R3-1 megohm
R4-50,000 ohm potentiometer (Mallory U35or equiv.)
R5, R7, R8-47,00 ohms
R6-2,200 ohms
6p
1
TOBATTERY-.
E
S2
3
Ql-HEP251 pnp transistor
Q2-HEP801 n -channel FET131, B2, B3 -9-V mercury battery (Eveready
E146X or equiv.)
Capacitors -25 volts or more
C -5 uF, electrolytic
C2, C3, C5, C6-1 uF, electrolytic
C4-2 uF, electrolytic
C7-0 1 uF
BLK
3
POWER
C7
RC5
+
Physical layout of parts in the vibrato Observe polarity of the electrolytics. Thenumbered circles represent the push -in terminals used as tie -points. Thin insulatedhook-up wire is used throughout. The perforated board chassis is suspended fromthe top of the case on three screws. These are visible in the head photo. Controlsand switches are mounted on the front of the case; jacks are on the rear.
R6
DEPTH
I N PUT
JI RED
S
9
R4
R9
RATE VIBRATO
TOBATTERY -I-
5
5
OUTPUT
J2
0
R3
+ C2
R2
14o
o
. .O 0
00
o cCOU 0 .0
O o IIO 0
O 0O 0
O 0 0 ...O 0 0O 000O 0 0
O 0 0 0O 0 0 C
C 0 00 0 C
00 ol
10
0 C 0 00 0 0 0 0
0 0 0 0 0 D
0 0 0 0 00 0 0 0 0 n
0 0 01
Q1
C6
02
0 °
0 0 0 00 0
00 0 00 0
0000 0 0 0 00
00
0 o020
0 0 00 C
o cG,0 o o 0 0o D 4 0 0 0 0 0
0 e o 0
00
Knobs (2)
51, 52-spst toggle switchMiscellaneous Parts
Chassis, 5" x 7" x 2"
Chassis cover, 5" x 7"
Perforated terminal board 25/8" x 33/" (Key
stone 1753 or equiv )
tor Q1 (Fig. 1), this transistor is agrounded -emitter stage with feedbackfrom collector to base for stability. A100-pF capacitor (C12) decouples thepower supply from this low-level firststage The signal next goes to transistorQ2. Resistor R3 established Q2's oper-ating point and capacitor C4 reduceshigh -frequency signals. Final amplifi-cation takes place at transistor Q3. Volume is controlled by 10,000 -ohm potR11, which acts as part of the bias net-work for Q3. Diode D1 compensatesfor temperature effects on Q3.
Transformer T1 isolates the low-voltage amplifier stages from the high -voltage, lamp -driving stage. The signalis rectified by diode D7. Capacitors C8and C9 limit the frequency responseto about 500 Hz. Trigger diode D13,and R12-BRIGHTNESS--Set the levelat which the SCR (Q4) conducts. TheSCR drives the lamps to various bright-ness levels and is powered throughbridge rectifier circuit D9-D12.
Neon lamp PL1 indicates poweris on. Transformer T2 supplies 12 voltsfor the amplifier.
Machine screws 440 x 3/16" (6) (for battery
equiv )
!nlr
'a411
'4P.
F
Ir
Circuit operation
79
R9-1,000 ohms
R10-1.5,000 ohms
R11-3,000 ohm potentiometer
TODAY'S POP MUSIC IS WILD AND Excrr-ing. Tie into it with a light -and musicshow! Put Rhythm Lights near livemusical instruments-or radio or hi-fi-and flood lamps will fill the roomwith color in rhythm with the music.This sound -to -light converter is allsolid-state for trouble -free operation.No special wiring is necessary-justplace the unit near the sound and thelights flash dynamically filling the roomwith a brillant color array.
Assembly and wiring of RhythmLights is easy. All the components arelaid out on a printed -circuit boardwhich is enclosed in a simple, one-piecechassis. All components used are avail-able at most electronic distributors.
A crystal microphone picks upsound from radio, hi-fi or musical in-struments. The high -impedance micro-phone is directly coupled to transis-
Battery clips (3) ( Keyston 72 or eq
Battery ho ders (3) (Keystone 79 or equiv
Mach ne screws, 6-32 x 3/4" (3) (for fermi (
board mount ng)
holder mounting)
Nuts, 6-32 (9)
Nuts, 4--a0 (6)
Rubber feet gar
Transistor sockets (2)
Term nal ug
Push n terminals (18) (Keystone 1499 FT or
(Mallory U8 or equiv.)
LIGHTS
na
By R T. MONTAN'E
4.1K
80
TI
:2MEG C5
i0/12V022N339I
A
Cl, C4-500-pF, 150 -volt ceramic capacitorC2, C3, C5 -10-,./F, 12 volt, electrolytic capac-
itorC6, C12-100 F, 12 -volt, electrolytic capaci
torC7 -2000-g, 20 -volt, electrolytic capacitorC8, C11 -0.1-µF, 150 -volt, paper capacitorC9-0.047-,LF, 150 -volt, paper capacitorC10-0.008-0, 150 -volt, paper capacitorD1 -1N34 -A diode
RI356K
SI
R6 ,
22K R, 7L CSIW27
tcomv 2°MRF20 V
BOTTOM YI(W rm.1 Tmatdanatva mime the ..gnat and power aerhans Ian. and high rrxhag..OF
2n3391 Transotom 01 and Q2 may hr tuhshheled with dmiiar types, as 'loge dengn ;In
PARTS LIST D2, D3, D4, D5-Diode, at least 500 mA, 100piv (Lafayette 19H5001 or similar)
D6, D8 -1N4001 diodeD7 -1N4003 diodeD9, D10, D11, D12-Diode, at least 1.5 amps,
200 piv (RCA 40267 or similar)D13-Pnpn trigger diode, T1-42 or similarFl -Fuse, 5 amps, and holderQ1,Q2-2N3391 transistorQ3 -2N669 transistorQ4 -2N3528 silicon controlled rectifier
If you use the printed board lay-out shown in Fig. 3 you should have notrouble with assembly. Parts placementis shown in Fig. 2. I put the assembledboard in an aluminum chassis coveredwith contact paper on which I letteredthe names of the controls. I punched ahole in one end of the chassis for themicrophone.
All controls and the flood -lampreceptacle are mounted on the frontcover. Use a three -wire power cord forsafety-the ground wire should be con-nected internally to the chassis and goto a grounded receptacle. Keep allwires insulated and cover all line -volt-age terminals with electrical tape toreduce possible accidents. A neat wir-ing job can keep you out of trouble.
The microphone can be mountedexternally or you can mount it insidethe chassis as I did. Sensitivity of theamplifier is good; therefore, mike lo-cation isn't critical. Transistors Q1
81
Sl. S2-S.p.s t switchesTl-Interstage audio transformer, primary
10,000 ohms, secondary 200 ohms (AlliedRadio 54E2386 or similar)
T2-Filament transformer, primary 117 volts,secondary 12 volts, at least 1 amp (Al-lied Radio 54134136 or similar)
PL1 Neon lamp with series resistor for 117Vac operation
MISC-High-impedance crystal microphone;
5" x 10" x 3" aluminum chassis (BudAC -404 or similar) with bottom plate (BudBPA-1591 or similar; 4-2" standoffs; 3 -con-ductor line cord; ac receptacle; hardwireand wire.
and Q2 are soldered directly into theprinted circuit board-be sure to heatsink the leads when soldering. A low -wattage soldering iron is recommend-ed. Use a heat sink with SCR Q4;this precaution will extend its life.
Four standoffs are used to mountthe printed board to the cover. Thecover in turn is screwed to the alumi-num chassis with sheet -metal screws.Transformer T1 and T2 are mountedon the printed board. Very little hard-ware is needed for the construction,wiring is simple, so assembly time is notlong. If good quality components areused, your Rhythm Lights should oper-ate satisfactorily for many years.
Using Rhythm Lights
Apply power to the unit and turnon S2. Turn BRIGHTNESS and VOLUMEcontrols fully counterclockwise. Thenplug a flood light or spotlight into thereceptacle. Advance the BRIGHTNESScontrol until the light barely glows.
DI}T142
oeIN4OO
A
R1-2.2-megohm resistorR2 -10,000 -ohm resistorR3, R5-1-megohm resistorR4 -4700 -ohm resistorR6 -2200 -ohm resistorR7 -100 -ohm resistorR8 -68,000 -ohm resistorR9 -39,000 -ohm resistorR10 -5000 -ohm 8 -watt resistorR11 -10,000 -ohm potentiometerR12 -250,000 -ohm potentiometerR13 -5600 -ohm resistor
Construction
SZ
5.11,,
D4
II] VAC
D9
02
aOM
2N3528 moea
T2
D2
23G, BRIGHTNESS
AC RECEPT FORFLOOD LAMP
7.7
DUOS!ONLY
D6
C; I,,F
C 10
.
.
sow
)
.EIP
as MSTIPPI'vac
-3--"cra
R2
Fig. 2-Lay out parts on the board and solder carefully before mounting boardto chassis. Watch electrolytic and diode polarities.
present in the circuit. This switch opensthe emitter circuit of the final amplifierstage (Q3 ), disconnecting the input sig-nal. Lamp intensity can still be con-trolled, however, with the BRIGHTNESScontrol. Another effect you might wantto try: Hook up a string of Christmas -tree lights to the unit. Use your imagi-nation; you'll find many interesting ap-plications for Rhythm Lights.
B 3/4*
Fig. 3-Use this pattern to make the printed board for RhythmLights. You will need a copper -clad board 91/2" x 51/2", a pintof etching liquid, 1/16" resist tape, 3/16" resist circles, and aplastic container in which to immerse the board for etching.
TO
,
ca ----0
0 4 4 (;;174j
oo
R3
TOFLOODLAMPRECEPT
TO SI
TOPL I
C7
D9
C4
Furnish music at a normal level andturn up VOLUME until the music turnson the light. For best results, two 150 -watt flood lamps of different colorsshould be used in a semi -dark room.The results are dramatic.
Switch S1 is a remote on-off de-vice. You can use lamp cord of almostany length, since only 12 volts dc is
D
R6T2
4--.. TO CHASSIS ONLY
OND
I" D7
PRI
D8
TO
+ C3
3
De 08 34
ELECTRONIC TREMOLO
2N497
82
81
2N2646
7VRZ5K
By R H. KEENAN
AN EFFECT COMMON TO MOST KINDSof classical and popular music is tremolo: periodic, fairly rapid variation inloudness. It is particularly common inwind instruments, even including thepipe organ. What it amounts to isamplitude modulation of the musicalnote by a low -frequency, subaudiblesignal (usually around 5 to 8 cycles persecond). In conventional musical in-struments it can be produced by varying the wind pressure applied to them.
Tremolo is not the same thing asvibrato, which is slow frequencymodulation (FM) and sounds quitedifferent. Pipe organs never have vi -
RIaR
QILEADSIAN2leNGThi
brato The two words are often con-fused.
Tremolo (AM) is easy to add toan existing music source. The simpledevice described here can be used withelectronic guitars, organs or other in-struments, or with recorded music ornoise to produce special effects forelectronic music or sound -and -lightshows. The circuit is simple and can beadded quickly to any amplifier or taperecorder.
How it worksThe subaudible tremolo signal is
produced by Ql, a unijunction transis-
ELIGHT-PROOF SHIELD
r ILM I
R6100K SEE F10.2
Fig. 1-Two 9 -volt transistor batteries in series will power the tremolo nicely.You can install the lamp and photocell any convenient e from controls.
PARTS LIST
C1 -5 -AF, 50 -volt electrolytic capacitorRI -220 ohm resistorR2 -15,000 -ohm resistorR3 -10,000 -ohm potentiometer (RATE)R4 -470 -ohm resistorR5 -1000 -ohm potentiometer (DEPTH)R6 -100,000 -ohm resistor for shunting PC1
(you may need to select best value from47,000 to 470,000 ohms)
PC1-Photocell (Clairex CL607)LM1-Incandescent lamp, 28 volts, 40 mA
(G -E 327 or similar)Q1-Unijunction transistor (2N2646)Q2-Large.signal npn transistor (2N497 or
similar)S1-S.p.s.t. switch of any conven ent kindCase and perforated board or other means for
mounting Q1-Q2 circuitry.
tor, used in a simple relaxation oscil-lator (Fig. 1). Q2 amplifies the signaland drives lamp LM1, which flickersat the rate set by the oscillator. Thevarying light falls on cadmium -sulfidephotocell PC1, causing its resistance tovary accordingly. The photocell is partof the series arm of a voltage divider(attenuator), so the audio level variesperiodically as the oscillator swings.R3 is the tremolo RATE control; R5controls the depth of modulation.
The photocell and lamp (seeparts list) are placed end to end androlled together into a strip of black
(MI R6
EXISTINGBLOCKINGCAP BREAKx_ TO GRID
OR BASE
IPOt
-- -1LIGHT -PROOFSHIELD FROM Q2
MI
ADD BLOCKING CAP INTRANSISTOR CKTSIF NEEDED TO PREVENTDISTURBING DC BIAS.(VALUE 10 TIMES C)
Fig. 2-Insert the photocell betweenamplifier stages as shown. Be sure touse blocking capacitors to avoid dethrough the photocell and changing bias.
plastic electrician's tape, making asingle, lightproof unit with leads com-ing out the ends. Total current drainfor the circuit shown, including thelamp, is about 20 mA. The circuit willprobably work with other transistors;Q1 is a 2N2646, a common and inex-pensive unijunction, and Q2 is a 4 -wattnpn 2N497, used because it washandy. The circuit could be rearrangedfor use with a pnp transistor as Q2.This transistor should have fairly highbeta (hrE) and be capable of dissi-pating at least 200 mW.
Circuit connection
Connect the tremolo device be-tween amplifier stages, either tube or
transistor, as in Fig. 2 You may wantto trim the 100,000 -ohm resistor,depending on the values of the resist-ors in the circuit. Keep the leads onthe photocell short, else they may pickup hum. The leads to the lamp may beany length: this means that the unitcontaining Q1 and Q2 can be put inany convenient place.
When the tremolo is used with anelectronic organ, it is usually desirableto keep tremolo off the pedal tones(tremolo in low bass notes just doesn'tsound right). Thus you may want toinsert the photocell in the lead thatcarries signals from one or moremanuals only, before the pedal tonesare mixed in (Fig. 3). For more spe-cific information, consult the diagramof your organ.
MANUALSON WHICHTREMOLOIS WANTED
INSERTPC FORTREMOLOHERE
EXP. PEDAL
PEDAL TONES
Fig. 3-You may want to split -feed anelectronic organ, adding tremolo to onlycertain manuals, and not to others. Seeorgan manual before attempting this.
Pipe organs generally do not havetremolo on all manuals. The greatalmost never has it, the swell doesfrequently. You may want to arrangeyour electronic organ similarly.
Potentiometer R3 is for rate, R5for depth. Although electrically thetwo controls are quite independent,psychologically there seems to be someinteraction between rate and depthadjustments. You may want to rig upa switch with two or three values offixed resistance so that you can select
MINIM
Controls
aged by heat, so use heat -sink clipswhen soldering, and solder quickly. Itscase is connected internally to base 2,so be sure the case doesn't come intocontact with any alien wires. A one -turn wrap of tape or a tight -fitting plastic tubing will prevent mishaps.
I
I I I - I ak I
I fa r.1
Ali - 521r 'IP I R.
T 1 11
-a
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- - INA
11, 1111WLe-- - -I MI
411 01" I- I. IN I m 1111.111
111 I
I". I - 111-1,f
J
quickly, while playing, different ratesor depths of tremo'o.
To check out the circuit, connectan ohmmeter across the leads of PC1.If the circuit is oscillating and the lampis glowing, the needle should fluctuate.
The 2N1646 is fairly easily dam-
Projects for the Auto
86
TACH-DWELL VOLTMETER
Fig. 1 -Meter scale for the TDVM is re-
produced here actual size. Drawing can
be traced or photostated.
By J. COLT and L. M. BOGGS
system voltage measurements. It costsapproximately $10.
The TDVM s (Tach-Dwell-Volt-meter ) low cost and professional ap-pearance come from using low-costcomponents and a predrawn meterface and from combining circuit func-tions (Fig. 1). Many of the electronicparts come from discount -house "2 -for" or "5 -for" buys-for example, theswitches, Zener diodes and the 0.1-pFcapacitors three of which are paral-leled to make Cl Even if you decide tobuild the TDVM with name -brandcomponents, the price of the complet-ed unit should be well under what
THE IDEAL AUTOMOTIVE TEST IN-strument should be able to measureeverything from headlamp candlepow-er to the water content of exhaust gases.It should be self -powered and thinenough to carry in your wallet. Costshould be less than 50 cents.
You may have to wait a few yearsfor that instrument. For the time be-ing, you might like an instrument thatmeasures rpm and dwell (4. 6 or 8 cyl-inders) and includes a 3 -volt and a15 -volt range for individual cell and
µ SI X .1 Ify
0,4 NI.
RI12V 2700
61-0 6V
R2680
DI/4.7VZENER
war._ 2N2924CL2TO BATT
CL3(PROBE) TO POINTS ORVOLTAGE CHECK POINTS
RI3 R 2
V
3900 33KIW
6V1001iRI4
1N4001R7 47011 04
R9OK
RIO
IK
2 5K RIIDWELL SET
C220/10V
02
FLAT SIDE
...01,02 LEAD ARRANGEMENTmy LEADS)
87
CLITO BATT +
R3 R42700 33K
C30l
RR3.310
2N2924
IN34D2
Fig. 2-One-shot multivibrator generates pulses for TDVM's tachometer section.
you'd expect to pay for a comparablecommercial unit.
Try to use the meter specified. Ifyou have a different 1 mA movementyou want to use instead, you may haveto make a new meter face and choosecorrect multiplying resistors for thevoltmeter ranges. Substitutions fortransistors Q1 and Q2 are not recom-mended unless you're sure your substi-tutes have an hr. of at least 100.
For clarity, the description will bedivided into three parts, each sectiondescribing one of the three function -
R55.6K
WOORPM
F
QI
R65K
RIR ISV27K
3 47VZENER- T
ally separate subcircuits (see Fig. 2).The tachometer consists of a
monostable (one-shot) multivibratorwhose output is a constant -width pulse.Since the output is of constant width,the dc component of the output pulsesvaries linearly with the repetition rateof the pulses fed into the one-shotmultivibrator. These pulses are takenfrom the engine breaker points.
Speed range is increased byswitching the timing resistors; R4 is forlow speed, and R5-R6 for high speedsThis changes the discharge time of Cl.
6"
5/16" le-, DWELL SET POT
GROMMET HOLEFOR 3 TEST LEADS
5/16"e 3 9/16"
Fig. 3-Author's front -panel dimensions. Parts and controls layout is not critical.
To make both 6- and 8- (or 4-)cylinder measurements, the output ofthe one-shot is fed to the meterthrough two different resistors, R15and R16, which are adjusted to giveaccurate readings Both are current -limiting resistors. At 600 rpm, a 6 -cyl-inder, 4 -stroke engine generates(across the points) 30 pulses per sec-ond, and an 8 -cylinder engine generates40 pps-two different rates whichmust give identical currents throughthe meter. Since the dc output of theone-shot is going to vary, the obviousway to obtain identical readings is to
provide two different current paths forthe two conditions.
D4 is included because transistorQ2 has a small saturation voltageacross it when it is fully on This volt-age tends to bias the meter in erroreverywhere except an original calibra-tion point. Q2's saturation voltage isapproximately 0.2 volt, and the turn -on voltage of D4 is approximately 0.6volt, so that the meter reads zero whenQ2 is saturated. D4 can be any goodsilicon diode.
Power for the multivibrator issupplied from the car battery via the
I
5/8" 2 17/32"-*-+ 0
1 -0 -1 I S2 SPDT SLIDE SWITCH I
29/32" (0-1000,0-5000 RPM)
2-17/37"
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4 HOLES, 1/8" DON 2-17/32"SQUARE
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SI DPDT SLIDESWITCH(6-12V)
1/4"
5/16"
I-1/8"1/2"
5/16"13/16"
R3
R8
SI
C2
R9
C3
D3
R 4
RI7
89
DI
Construction is not critical. Components are mounted on push -through pins. Theperforated board is held in place by the two terminals on the meter. Observe polarityof the diodes. Avoid excessive heating of the leads from the diodes and the transistors.
D2
R6
R5
R7
S2
shunt regulator combination R1 or R2and D I . R1 and R2, as well as R13and R14, are switched to provide es-sentially constant voltage across the
Q I
CI
R4
04
Zener diodes used in the tachometerand dwell -meter sections.
Note that the clip lead marked( + ) need be connected to the car
R10
RI 2
RI
RI3
RI 5
RI8RI9
R2
IIIIIII 11///
IGNITION COIL
TO IGN SWITCH \4AND BATTERY
BALLAST
'CONDENSER'
1 ROTATING CAM
BREAKER POINTS
Fig. 4-Connect TDVM probe to con-ventional ignition system as shown, withground clip to a convenient point.
battery only when using the TDVMtachometer function.
The dwell -meter section is madeup of R13, R14, D3, R12, C2 and R1I.It operates on the principle that, for aparticular dwell angle, the duty cycle(ratio of "on" to "off") of the pulsesacross the car's points is constant.Since constant amplitude is assured byD3, we have a train of constant -ampli-tude, constant -duty -cycle pulses whichhas a certain dc component. The onlyway to change this dc component is tochange the duty cycle, i.e., change thedwell. The dc component is read at themeter as dwell angle.
To calibrate the dwell meter ap-ply full battery voltage (engine run-ning) to PROBE and adjust R11 so thatthe pointer reads full scale (0° dwell).
A short comment on dwell set-tings is in order. Most auto manufac-turers design their ignition systems for40° (or thereabouts) dwell for 6 -cyl-inder cars, 30° for 8 -cylinder and 60°for 4 -cylinder cars. This amounts topoints open one-third, points closedtwo-thirds of the time or a duty cycleof 331/2 %. This is also the reason thatthe 40 and 30 -degree marks coincideon the 6- and 8 -cylinder dwell scales.If you're ever caught without specs fora particular auto, you will almost al-
ways be within a few degrees of cor-rectness by setting dwell to the 40°-30°mark
The dwell -meter section is com-mon to the tachometer portion in thatinput pulses are provided to the one-shot through a high-pass filter (differ-entiator), R10 -C3. D2 eliminates neg-ative spikes from the one-shot. R9 isan isolating resistor. The differentiatoris required because at low engine speed-or 4 -cylinder operation-inputpulses from across the auto's pointsare longer than the output of the one-shot. Without the differentiator, thiswould lead to inaccurate readings,since the input would tend to hold themultivibrator on for the duration ofthe input pulse.
The voltmeter is a basic series -resistance ammeter configuration. Inthe interests of economy, the voltme-ter input is provided from the probecommon to all other tests The onlydrawback of this scheme is that whenmeasurements are made of voltageshigher than the Zener voltage of D3,D3 conducts. Dissipation in D3 is noproblem for input voltages up to 15.The main disadvantage is that whenV. is reached, the voltmeter is nolonger a basic 1000 -ohms volt device,because the voltage source being meas-ured must supply current to D3 aswell as to the meter. If you expectthis to be a problem, a separate volt-age -test lead could be provided. Butthe circuit as it stands has proved morethan adequate for everyday automo-bile system checks.
When used with the meter move-ment specified, the values of R17 andR18 -R I 9 provide acceptable accu-racy. These are not precision resistors,just good name -brand 10% resistors.
Diasssemble the meter and re-move the original face. Be very care-ful. The movement can be ruined byexcessive force on the pointer. A goodrule of thumb is that any externalforce is too much. It might be a goodidea to engage the help of a calm,nearsighted friend for this step. Also,
PROBE FOR
RPM & DWELLMEASUREMENTS
_L.
I
90
Calibration
turn off fans and air conditioners,avoid drafts and try not to breathe intothe works.
Copy the meter face shown inFig. 1. Using the white dots as guidesfor the meter -face screw holes, rubber-cement the new scale to the back ofthe old one. (You might want to usethe 1 mA scale again some day.) Toprevent wrinkles, press the new scaleuntil it is dry, then tnm the new scaleto the size of the meter face. A goodidea is to make a couple of copies orphotostats of Fig. 1 in case the firstattempt flops, or in case you want tobuild another unit later.
Construction
The dimensions given in Fig. 3should make layout easy. Of course,you may wish to "human engineer" tosuit your particular needs or desires.
Circuit wiring was done on a 3"x 334" piece of perforated phenolicboard with push through terminals.Drill two holes in one end of the board,and mount a cantilever style with themeter posts as support This arrangement has proved more than adequate;if you decide that you need addedrigidity, the board can be supported atother points
As part of the usual precautions,observe correct meter, diode and elec-trolytic -capacitor polarities. Pay par-ticular attention to the basing diagramof transistors QI and Q2, shown inFig. 2 along with the schematic.
Calibration of the dwell meter hasbeen described; it should be re-emphasized that the meter should be set to 0°dwell with the engine running. Systemvoltages are higher with the enginerunning, and this is, of course, whenthe dwell meter will be used.
Tach calibration should need bedone only once, barring unforeseencircumstances such as the calibrationresistors being jarred out of position.
If you have a friend with an accu-rate tachometer, you've got it made,as far as tach calibration goes. If youdon't, another method will be de-scribed shortly, but we'll assume fornow that you do have access to antachometer and an 8 -cylinder auto.
Start by setting R6, R15 and R16o mid -range. Set S1 to either 6 or 12
volts, depending on the car; S3 toTACH 8 and S2 to 1000 RPM. AttachCL1 and CL2 to battery positive andground, respectively, and CL3 to thelow -voltage wire connecting the dis-tributor and coil. (See Fig. 4 for atypical auto ignition schematic.) Ad-just the engine idle to 600 rpm (asmeasured by the reference tach), andset R16 so that the meter reads 600.Now switch S3 to TACH 6 and adjustR15 so that the meter reads 800. (Thereason for this is that, for a givenrpm, a 6 -cylinder engine generates 3/4as many pulses across the points as an8 -cylinder engine; conversely, for agiven point rep rate, the 6 -cylinderengine is turning at 413 the speed of an8 -cylinder engine.) Switch S2 to 5000rpm and set the idle screw so that theengine is running at say 2000 rpmAdjust R6 so that the meter reads2000. The tachometer is calibrated.
II7V1-*
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BATT (-)
Fig. 5-Bench calibration requires onlya dc supply and 6.3 -volt filament trans-former for 60-pulse/sec-signal source.
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If you have a 6 -cylinder car, cali-bration for the low end can be carriedout as above, but with S3 set to TACH6. Then switch to TACH 8 and adjustR16 so that the meter reading is 44of the TACH 6 reading.
You can calibrate your instillment on the bench by using a 6.3 -voltfilament transformer and a 6 -volt lan-tern battery, or other 6 -volt dc supply,as shown in Fig. 5. For this calibrationscheme, short out C3. Set S1 to 6 volts,and then plug in the transformer andturn the tach on, having preset R6,R15 and R16 to mid -range. Set S2 to1000 RPM, S3 to TACH 8 and adjustR16 for a meter reading of 900 rpm.Then switch S2 to 5000 RPM, and ad-just R6 so that the meter reads 900rpm in the high scale. Now switch S3to TACH 6 and adjust R15 so that themeter reads 1200 rpm. The tach is nowcalibrated, having never left the bench.
If you're concerned about the ac-curacy of the voltmeter with the 10%components specified, you can substi-tute appropriate adjustable resistors ofthe type used for R6, R15 and R16,and calibrate the voltmeter to greater
accuracy, using an appropriate refer-ence scheme. The cost of two moreadjustable resistors of this type willnot increase total cost of the instrumentby more than about 60 cents.
SOLID-STATE GAUGES
by JACK SADDLER
a plug-in socket behind the instru-ment panel. This regulator is an elec-tromechanical unit and can be inac-curate.
Use
HAS YOUR CAR EVER OVERHEATED ANDleft you stranded? Or how about thetime you ran out of gas? Both timesthe dash indicators read o k. Probablyneither the sending units nor thegauges were wrong. Most likely, theculprit was the regulator used to sup-ply voltage to the gauges.
When something is wrong withthe gauging system, usually both gasand temperature gauges will be in-fluenced. The instrument voltage regulator is a small device mounted in
The instrument is designed for useon negative -ground systems, since al-most every auto made today is wiredthis way. The dwell meter, however,can be used on positive -ground systemsby simply reversing the roles of PROBEand NEG (ground) leads.
If you own an auto with positiveground and want to build this instru-ment to help in tuning it up, all is notlost. Simply reverse all diodes, the me-ter and the electrolytic capacitor. Sub-stitute pnp transistors for Q1 and Q2,but be sure they have an hFF of 100 ormore. Be sure you remember that, inthis configuration, PROBE is connect-ed to negative when making voltagemeasurements.
In use, the PROBE and GROUNDleads are used for all tests, and the( ) lead is used only for tachometermeasurements.
How most gauges workA typical setup for indicating
temperature and gas tank contents ina modern automobile is shown in Fig.1. The sending unit in the gas tankis a potentiometer with its wiper armattached to a float. Usually an emptytank will provide a 60 -65 -ohm re-
REGULATOR,11.11.11.0---IP
,i, TEMP GASGAUGE GAUGE
IGNITIONSWITCH
I t.7V7 4 TO 154V
I / -THER MISTOR
Fig. I Arrangement for gas and tem-perature gauges in most cars today.
voltage of this regulator. Auto man-ufacturers' service manuals usuallyrecommend checking with known -good gauges and sending units.
Input voltages to the instrumentregulator vary widely. With the en-gine turning over at 2000 rpm on acold morning the output from the alternator may be as high as 15 4 volts.The voltage may drop to 11.7 on ahot summer day.
Each panel gauge has a resis-tance of about 11.5 ohms. The high-est load on the regulator occurs whenthe engine is hot and the tank is full.The current drains from the fuel -gauging and temperature -sensing sys-tem total about 500 mA.
When engine water temperatureis low and the gas tank almost empty,the regulator consumes the most power internally but has to put out theleast. Total drain for both systems inthis case is 127 mA.
Zener-resistorThe simplest solid-state replace-
ment for the electromechanical regu-lator is shown in Fig. 2. It consistsof a shunt regulator made from aresistor and a Zener diode.
The resistor-Zener combinationis correct for most U.S.-made autosexcept those made by General Mo-tors. It was specifically designed forAmerican Motors cars made from1961 to 1967. Formulas for calcu-lating values for other voltages andcurrent requirements are given later
The first step is to open the oldregulator. The metal case is crimpedaround a phenolic board (Fig. 3).The best tool to use for uncrimping
sistance reading, while the full readingis 10-11 ohms.
The temperature sensor in Fig. 1
is a thermistor (temperature -depen-dent resistor) mounted in a brasscylinder. It is located in the waterjacket of the engine close to the ther-mostat. A cold resistance readingwould be 70-75 ohms, and the re-sistance just before the water boilswould be about 9 ohms.
The gauges are hot-wire milliam-meters. This meter type consists ofa bimetallic strip connected to thepointer. The bimetallic element iswrapped with insulated heater wire.Voltage applied to the heater causesthe bimetallic strip to bend; theamount of bending is proportional tothe current.
The regulator contains a bime-tallic strip heated by the passage ofcurrent through a separate heaterwire wrapped around it. As the bi-metallic strip heats, it bends becausethe two metals have different expan-sion rates. When a predeterminedamount of current has passed throughthe winding, the contact is broken.When the bimetal strip cools, it makescontact again and the cycle is re-peated.
The ratio of the time on to thetime off determines the effective valueof the resulting voltage. It is impos-sible with any easily available testinstrument to determine the output
i2.51a 7W 1N5338117VTO15 4V
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5.IVREGULATED0 127A TO 0.5A
93
Fig. 2 Simple shunt regulator replace-ment for electromechanical devices.
Fig 3 Typical cases for bimetallic -strip regulators with components.
1
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It1N4733
Fig. 5 Transistor-Zener circuit.
R23611 5V
0.127ATO0.5A
is a pair of diagonal cutters. The met-al lip is loosened all around. Afteryou open the case, bend the bimetallicelement up and clip it off even withthe terminal. Remove the silver -tippedbrass contact blade similarly. Thenback out the adjusting screw. Allthree of these parts can be discarded.
Now solder a 12.5 -ohm, 5 -7 -wattresistor between the two terminals(Fig. 4). The input from the ignitionswitch is marked tot4. The output terurinal is unmarked.
Drill a small hole in the metalcover near the spot-welded ground
Fig. 4 Photo shows positioning ofresistor-Zener regulator combination
94
strap. The negative end of the Zenerdiode is threaded through this hole.Check to see that neither of the re-sistor leads nor the positive Zenerlead touches the case.
Complete the assembly by re -crimping the can with pliers. Bendthe Zener diode lead parallel to thecan and solder it. The original can istin-plated and easy to solder. For thefinal test, connect the positive termi-nal of a 12 -volt battery to the IGNterminal and the negative lead to thecase. A voltmeter connected betweenthe unmarked terminal and caseshould read 5.1 V.
After assembly and test, merelyplug the solid-state regulator in placeof the electromechanical unit. No al-terations are required in the wiring
7 5 nR 2N492
engine -temperature and fuel -tank con-ditions. it is designed to deliver theregulated voltage under all conditions.
This means some power will bewasted under most conditions TheZener unit consumes about 8 wattsin the worst case. When things aregoing well with the car there s noproblem. But there are times wheneven the power consumption of thetransistor radio is bad For this rea-son the second circuit in Fig. 5 isshown.
While this system is slightly morecomplex, it is a more efficient regu-lator. It consumes only about 2 wattswhen the input voltage is low andoutput current demands are at a mini-mum.
How does it work'? The transis-tor is in series with the positive supplyline. It acts as a variable resistor.The Zener diode and R2 together actlike the sliding contact on a poten-tiometer. As the supply voltage rises,more current flows through R2. TheZener diode and the transistor base
Hut
2N4921
CASE
Transistor-Zener regulatorThe simple resistor-Zener unit
has a great advantage over electro-mechanical unit: with no movingparts, it is quite reliable. Its maindisadvantage is that it consumes morepower than the bimetallic unit. Andsince it must regulate for all possible
SILICONEGREASE
Fig. 6 (photo)-Layout for transistor reg-ulator in tape -insulated mounting case.
Figs. 7 a,b-Drawing shows transistor iso-lation and connections for the compo-nents.
SOLDERLUG
4-40SCREW
360RES STOR
MICAWASHER
Series resistance -min. battery voltage - regulator design voltage
1.1 (max. output voltage)
Zener power rating - regulator voltage (1.1 max. current - min. current)
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MM. output current -regulator voltage regulator voltage
- (empty tank resistance (cold thermistor resistance± gauge resistance) ± gauge resistance)
Max. Zener current = 1.1 (max. output current - min. output current)
Zener voltage = regulator design voltage
split this current to maintain the out-put voltage at 5 volts. The ability ofthis circuit to maintain a constantoutput voltage is even better than theZener alone. The control effect of theZener is transferred to the transistorso that output voltage is rock -solidregardless of changes in input voltageor output current.
As in the Zener regulator, firstopen the case and remove the oldbimetallic element. Drill a 7/64 -inchhole in the case along the center lineat a convenient distance from the end.This hole will be used for mountingthe transistor.
Before mounting the transistor,its leads must be bent away from itsheat sink at about a 30° angle.
Cover the inside of the can withplastic electrical tape except for thearea where the transistor will bemounted (Fig. 6).
The collector of the transistor isconnected to the bare metal spot onits case. In this circuit, it must beelectrically isolated from the heatsink (the case of the plug-in unit).For this reason the transistor ismounted with a mica insulator be-tween the transistor and case. The
insulator is supplied by the manufac-turer. Use silicone grease on bothsides of the insulator to provide agood heat path to the case.
The mounting is completed witha solder lug atop the transistor asshown in Fig. 7-a. The connectionsare shown in Fig. 7-b. When solder-ing the leads and wire to the tran-sistor, make sure the leads are freshlytinned Clamp the transistor lead withdiagonal pliers or an aluminum clip -on heat sink. The silicon transistor isquite rugged, but it's best not to takea chance of damaging it by overheat-ing. The 7.5 -ohm resistor is an IRCtype BWH. It is 1 -watt compositionsize but is capable of dissipating 2watts.
After soldering all parts togeth-er, close the case. If the unit is cor-rectly assembled, an ohmmeter shouldread infinity with its negative leadconnected to ION and positive leadto case. Reversing the ohmmeterleads will give a low reading-some-thing between 100 and 350 ohms.Similar readings taken between theunmarked output terminal and caseshould be identical to input readings.
Zener regulator with other earsThe resistor-Zener combination
shown in Fig. 2 is designed specific-ally for 1961 through 1967 Ramblercars. It is also useful with Fords andChryslers. Some cars measure oilpressure electrically and the regulatormust supply the current. It is best toreview wiring diagrams to determinewhat circuits are connected to theregulator. If your service manualshows different resistance values forthe sending units or the meters, jacketand gas tank, here are the measure-ments needed:
1. Normal voltage output frominstrument regulator 2. Empty -gas -tank sending -unit resistance. 3. Full -gas -tank sending -unit resistance. 4.Cold -engine sending -unit resistance.5. Hot -engine sending -unit resistance.6. Gauge resistance.
And here are the calculations:(The 1.1 is an empirical safety
factor used to assure correct per-formance of the Zener diode whenit draws minimum current.)
Generally the Zener diode can bemounted by its leads. In my systemthe diode is capable of dissipating 5watts at all temperatures to 167° F
*This factor combines several design factors and is useful only with the 2N4921transistor. The complete design calculations are in the Motorola Zener Diode Hand-book.
with the case 3/8 inch from the solderjoint. The 1N5333 series is availablewith voltages from 3.3 to 200 volts.Should more dissipation be requiredfrom the Zener, the 1N3993R seriescan be used. In this series, the1N3996R is the 5.1 -volt Zener. Theseare stud -mounted units and can befitted inside the case. It's snug -fit withthe resistor, which should be as smallas possible.Transistor-Zener calculations
The 2N4921 transistor is quiteadequate for any system requiring 8volts or less. All calculations beloware based on the use of this transistorMeasurements in the car or from aservice manual are the same as forthe Zener unit. Minimum and maximum currents required by the send-ing units are also the same. To makecalculations easy some elements ofthe equations are combinedZener voltage = required output volt-age ± 0.6 V
The 0.6 volt is the maximum volt-age drop from collector to emitter forthe transistor used.R1E,Ez - 2.6
0.56
Table I-Zeners for Output Voltages
Desired outputvoltage Zener voltage Type
Max. currentamps
Resistanceohms Factor*
3.0 3.6 1N4729 0.252 10.0 1 383.3 3.9 1N4730 0.234 9.0 1.283.7 4.3 1N4731 0.217 9.0 1.164.1 4.7 1N4732 0.193 8.0 1.064.5 5.1 1N4733 0.178 7.0 0.985.0 5.6 1N4734 0.162 5.0 0.905.6 6.2 1N4735 0.146 2.0 0.826.2 6.8 1N4736 0.133 3.5 0.746.9 7.5 1N4737 0.121 4.0 0.687.6 8.2 1N4738 0.110 4.5 0.628.5 9.5 1N4739 0.100 5.0
I
LudirZWINM
98
lz = R2 ± Rzwhere E., is minimum input voltage
is maximum input voltageEE is Zener voltageI,z is maximum output currentI,, ,, is maximum Zener currentRz is Zener resistance (see Table
1)
The maximum current throughthe Zener diode must be calculated
The 2.6 volts is composed of 0.6volt for the collector-emitter voltagedrop, and the 2.0 volts is a collector-base voltage that will keep the tran-sistor out of saturation.
52R2 - 1,, + factor from chart
Em,. - Ez
DURING RECENT YEARS A NUMBERof transistorized automobile ignitionsystems have appeared. The majorityof these systems (manufacturedunits and construction projects) hadmalfunctions after a few thousandmiles of engine use due toexceeding germanium power transis-tor capabilities and poorly designedcircuits.
The detrimental effects on ger-manium transistors occur with con-tinuous operation at junction temper-atures of 100°C. Temperatures in theengine compartment can range from+70° to +110°C or higher on hotsummer days.
to .nsure its power rating is not exceeded. The maximum currents shownin Table I limit dissipation of the Zener to about 1 watt. The 1N4728 seriesis capable of handling twice the cur-rent shown if the leads are less than3/13 inch long and the heat sink isadequate.
The only remaining calculationsnecessary are the power ratings ofthe two resistors:Power rating, R1 -(1R1Power rating, R2 -
Either of these regulators inyour automobile will give you moreconfidence in the gauge readings. Youmight save a long walk to the gas sta-tion or a costly repair bill.
Silicon transistorsTo beat the temperature prob-
lem, two transistor ignition systemswere developed: a negative -groundsystem for American automobiles,and a positive -ground system for someforeign automobiles. A reliable high -voltage silicon power -switching tran-sistor, having excellent beta withlarge collector currents at cold andhot temperature extremes, was se-lected. Silicon transistors have fastswitching times and a junction tem-perature range in excess of +125°Cwith the proper heat sink.
These units were "worst case"designed for high reliability and lab-
R4
99
oratory -tested at ambient tempera-tures -55° to +125°C.
The negative -ground system hasbeen in operation in a 1963 6 -cylinder85-h.p. Ford Falcon and a 1965 8cylinder 325-h.p. Chevrolet. The sys-tems have exceeded 10,000 mileswithout malfunction.
Circuit operationOn the negative -ground system
(Fig. 1), current flows through R3 tothe base of Q1 when the ignitionbreaker points are closed. This satu-rates the transistor and current flowsthrough R4 to the base of Q2. Tran-sistor Q2 then saturates and its col-lector current flows from the +12 -volt battery terminal through ballastresistor RI and transformer T1. En-ergy equal to 1/2 LI2 (where L is theinductance of the transformer pri-mary and I is the current flowing inthe transformer primary) is storedin the Ti's primary.
When the points open, Q1 nolonger receives base drive, turning itoff, and Q2 is cut off when it losesbase drive from R4. Energy is stored
Ri
Inside the ignition system there's plenty ofroom to fit all parts without cramming.
in TI's primary until it reaches thebreakdown voltage of Zener diodeD2. This flux change in the trans-former builds up a voltage in the sec-ondary. The secondary voltage is theignition pulse which jumps the sparkplug. Ballast resistor R1 sets the pri-mary current to the required amount.Zener diode D2 limits the voltageacross transistor Q2 to a safe 100volts.
During normal running, the bat-tery is fully charged to 14-15 volts.Under these conditions, the ballast re-sistor is set to give a peak coil currentof approximately 12. amps. When thecar is starting, the battery voltagedrops to 8-10 volts because of thestarter load. This reduced batteryvoltage could mean less coil currentand therefore a weak spark.
Therefore diode D1 is added tobypass the ballast resistor during en-gine cranking. This maintains the de-sired peak coil current and a fullspark.
On the positive -ground system(Fig. 2), when the points are closedcurrent flows through R3 to the baseof Q 1. This saturates the transistor,and current flows through R1, Q1 R4,R5, R6 at the primary of the trans-former Tl. When the points open thetransistor no longer receives basedrive. Current flow through the tran-sistor is interrupted, and the opera-tion is the same as the negative -groundsystem, including RI, D1 and D2.
Construction
The unit is built in a 21/2 x 21/2 x5 -inch aluminum box. The heat sinkfor D1, D2, Q1 and Q2 is the outsidetop of the box (see photo), D1, D2,Q I and Q2 must be electrically insu-lated from the heat sink and ground.This is done by mounting the compo-nents with their insulating hardwareand applying silicone grease on themetal surface. The grease aids in con-ducting the heat away. Arrangementof the components inside the box is
R5 RI
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SI
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IGNITIONSWITCH(RUN POSITION+12 VOLTS DC
IGNITIONBREAKERPOINTS
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SI MODEPOSITION 1 - TRANSISTOR SYSTEMPOSITION 2-REGULAR SYSTEM
- GROUND
REGULARIGNITIONCOIL
NOTES.**HEAT SINK
*DIST CAP
IGN. SWDIST CAP
Fig. I-This circuit is the one to use if you are building an ignition system to use witha negative -ground car. This includes almost all American made automobiles made inrecent years. If you have a car with a positive -ground system try the circuit in Fig. 2.
PARTS LIST (Negative Ground)R1 -1 -ohm 100 -watt adjustable resistor Claro- C1-22 AF 600 -volt ignition -point capacitor
stat VKlOONA or equal (see text)R2 -680 -ohm 1/2 -watt ±10% CR1-1N1199 50 -volt, 12 -amp silicon diodeR3 -100 -ohm 5 -watt ±5% Clarostat VPR5F or CR2-1N3005 100 -volt, 10 -watt Zener diode
equal Q1 -2N4901 pnp silicon transistor (Motorola)R4 -10 -ohm 20 -watt Clarostat VPR-20H Q2 -2N3772 npn silicon transistor (RCA)
or equal T1, Sl, MISC-See positive groundR5 -47 -ohm 1/2 -watt -2:10% TO -3 Power Transistors, 2 req.
not critical except D2 should bemounted near Q2.
Capacitor Cl (negative -ground sys-tem) should be the same value as thecapacitor in the distributor next to the
ignition breaker points. It may beplaced inside the box or at terminalTB -6. The original breaker -point ca-pacitor may be used later after thetransistor ballast -resistor RI is ad -
100
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justed. This adjustment will be dis-cussed in more detail later.
Do not remove the capacitor inthe positive -ground system. The timeconstant of R3 and the capacitor is
IGNITIONSWITCH(STARTERPOSITION)
RI11).
100W(SET TO0 25-0.5n)
R37.505W
small and has little effect on the inputtrigger pulse.
Ignition coil T1 is mounted onthe engine firewall as close to the distributor as possible. Number 14 wire
R2470
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TITRANSISTOR COILFIZT
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1:100REGULARIGNITIONCO ILIGNITION 3
BREAKER 4POINTS
C(00 NOT REMOVE)SI MODE TO TB 3-4-00
IGNITIONPOSITION 1- TRANSISTORPOSITION 2 -REGULAR IGNITION
BREAKER POINTS*HEAT SINK
Fig. 2-Here's a circuit for an ignition system for a positive -ground car. This coversmany European makes and some older U.S. models. Note that in this version, circuitground floats. If your car has a positive ground, the positive battery terminal will beconnected to the frame of the car. A glance under the hood is all you need.
PARTS LIST (Positive Ground)
R1-1 ohm 100 -watt adjustableVKlOONA or equal
R2 -47 -ohm 1 -watt -±-10%R3-7 5 -ohm 5 -watt -±5% Clarostat VPR5F or
equalR4, R5, R6 -1 -ohm 10 -watt ±5% Clarostat
VPR1OF or equalDRI-1N1199, 50 -volt, 12 amp silicon diode
CR2-IN3005 100 -volt, 10 -watt Zener diodeQ1 -2N3772 npn silicon transistor (RCA)TI-Transistor ignition coil, 1:250T, Mallory
F12TSl-dpdt 15 -amp 125 -volt toggle switch, Ar-
row -Hart 82609MISC-Barrier terminal block, Cinch Jones
6-142. Mini Box 2% x 2% x 5", Bud CU -
3004A. transistor mounting kit, MotorolaMK -15. (For TO -3 power transistor. 1 req.)
101
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will set R1 to the correct value,which should be 0.5-0.75 ohm. Stop the engine, removejumpers, etc., and connect wiresto all terminals; remove the dis-tributor capacitor (negative sys-tem only), and place it in thetransistor box or at TB -6. Place the center high -voltagelead from the distributor to thetransistor coil Make sure switchSi is in position 1. Your car willnow start and run in the tran-sistor mode.
This is the adjustment procedurefor positive -ground systems:
Bolt the transistor ignitionsystem to the firewall. Connect awire from TB5 to the negativeterminal of transistor coil T1 andthe positive terminal to frame (noother wires to terminals). Place Si in position 1 (transistor ignition mode). Start and run engine with nor-mal ignition system until the battery is fully charged With the engine running, con-nect a jumper wire in series withan ammeter to TB2 and the nega-tive terminal of the battery.(Set resistor R1 to 1 ohm.) Connect TB3 to the positiveterminal of the battery or autoframe, and adjust ballast resistorR1 for a current of 12 amps. Thevalue of R1 should now be be-tween 0.25 and 0.5 ohm. Stop the engine and removethe jumpers, meter, etc., and con-nect wires to all terminals asshown in Fig. 2. (Do not removethe distributor capacitor.) Place the center high -voltagelead from the distributor to thecenter hole of the transistor coil.Switch S1 to position 1.
was used in all external wiring to thecoil, ignition switch and starter. It isimportant that good grounds be usedand all connections are secure.
The ignition -system package wasmounted on the engine firewall. Itcould be mounted in another conven-ient location, preferably away fromthe exhaust manifolds. Tests had indi-cated it was not necessary to mountthe package near the radiator or fan,thus simplifying installation.
The change -over arrangementfrom a conventional system to thetransistor ignition is simple. To oper-ate in the conventional mode, Si isplaced in position 2 and the centerhigh -voltage distributor lead is pluggedinto the conventional coil. To operatein the transistor mode, place switchS1 to position 1 and plug the centerhigh -voltage distributor lead into thetransistor coil.
Here is a seven -step sequence foradjusting your negative ground transistor ignition system:
With the transistor ignition sys-tem bolted to the firewall, connectwires to the positive and negativeterminals of the transistor coil(no wires to other terminals). Place Si in position 1. Start and run engine with nor-mal ignition system until batteryis fully charged. (Note: No con-nections have yet been made be-tween the car's electrical systemand the terminal strip.) With the engine running, con-nect a jumper wire with a 15 -amp dc ammeter and ballast -re-sistor R1 (set to 1 ohm) in seriesbetween TB -4 and the positiveterminal of the battery. Connect TB -5 to ground andadjust ballast -resistor R1 for acurrent reading of 12 amps. This
102
103
How it worksHumidity sensing element R3
(see schematic) is a sensitive resistiveelement (humistor) that increases inresistance as the ambient moisture in-creases. It controls the collector cur-rent in common -emitter dc amplifierQ1 by controlling the base -to -emitterresistance. As the humidity increases,R3's resistance increases, Q1 base -to -
emitter voltage increases, Q1 collec-tor current increases, and indicatorlamp LM1 lights.
Potentiometer R2 (LEVEL it) setsthe lamp brightness to a suitable levelduring critical humidity conditions,and diode D1 maintains a constant Q1amplification over a wide ambienttemperature range.
Inverse feedback is provided byconnecting the positive end of the basebias network to the collector of Qi.
30
Relative humidity near 100%combined with a temperature nearfreezing or below can make driving orflying very dangerous, since these arethe conditions that can cause unex-pected formation of ice. Anyone cansee ice forming on his windshield whenconditions are beyond the criticalrange, but when both humidity andtemperature are near the criticalpoint, one may not realize that ice isforming just a short distance beyond.
The icing -condition indicator(ICI) described here is intended tokeep the driver alerted to critical mois-ture and temperature conditions, in-dividually and in combination. Con-struction is simple and easy, the costis reasonable, and once installed andadjusted, it is stable over a wide tem-perature range and should require nofurther attention.
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PARTS LISTCl. c3-10AF, 6 -volt electrolytic capacitorC2, C4 -50µF, 6 -volt electrolytic capacitorD1, 132-5A4 silicon diode, International Recti-
fier Corp03-1N1518 Zener diode, 3.9 -volt, 1 -watt, IRCLM1, LM2 Type 49 lampQl, Q2 -2N3242 -A silicon transistor, NPNR1, R7 -1.5k ohm, '/2 -watt resistorR2, R8 -3.0k ohm potentiometer, linear taperR3-Humistor, DeVry Hygropak HA -26R4, R10-680 ohm, 1/4 -watt resistorR5, R11-7.5 ohm, wire wound resistor, IRC
type BWHR6 -1.0k ohm potentiometer, linear taperR9-Thermistor, 200 ohm @ 25° C, Fenwal
KB 22J1
This improves the overall circuit sta-bility and, combined with a silicontransistor, decreases the transistorwarmup time to a egligible interval.Resistor R4 is used for testing the hu-midity portion of the indicator.
The temperature sensing portionof the circuit is the same as the humid-ity section, except that thermistor R9senses temperature. Since the thermis-or resistance increases as the temper-ature drops, LM2 lights when thetemperature drops to the criticalrange. Thus, LM1 lights steadily whenthe humidity nears 100%, and LM2lights steadily when the temperaturenears freezing.
In addition, to provide a distinc-tive indication when both conditionsare nearing the critical point, capaci-tors C2 and C4 were added to forman astable multivibrator circuit. TheGAIN control, R6, sets the multivibra-tor operating point to cause alternateflashings of LM1 and LM2 when thehumidity reaches approximately 95%any time that the temperature is about35° or below. Resistor R12 is madeapproximately equal to R6 at the start-ing point of the multivibrator. Thisgives approximately equal lamp intensity when they are flashing.
A Zener diode, D3, is used tomaintain the operating potential ap-proximately constant at 3.9 volts withnormal variations in battery voltage.The input voltage can be any dc supply
R12-470 ohm, 1/4 -watt resistorR13-15 ohm, 2 -watt wire wound resistor
(6 -volt operation)50 ohm, 5 -watt wire wound resistor (12 -volt operation)
81 -3 -pole, 6 -position rotary switch, shorting,Mallory 3136J
2-miniature lampholders, Leecraft 7-202-lens, 11/32" diameter, Dialco 5141-knob, 1/4" shaft1-4 x 2 x 2374 aluminum box, PMC 10151-21/4 x 2% x 1% aluminum box, PMC 10001 -4 -wire cable, 22 AWG stranded, 7 x 30,
Belden 84442 -6 -terminal miniature tie pointswire, grommets, solder, plastic strip, tube
socket decals, etc.
desired, provided that a suitable drop-ping resistor (R13) is used. The 15ohm value of R13 given is for a 6 -voltsupply voltage, If a 12 -volt battery isused, increase R13 to 50 ohms (5watts) and mount it outside the case.
As shown in the photos, bolt thehumistor inside the top section of thebox with an intervening block of foamrubber. Make the top clamp for asection of thin plastic, in which a 5/8"x 3/4" window is cut to permit unre-stricted air flow to the humistor.Fasten clips removed from a 7- or 9 -pin miniature tube socket to the plasticto connect to the humistor pins. Solder 2 -inch flexible leads to each clipbefore mounting to prevent meltingthe plastic clamp. Use a 4 -terminal tiepoint to mount the thermistor and tomake cable connections to the controlunit. Solder the thermistor to the tiepoint last to minimize the chance ofheat damage.
Drill about 20 small holes (about3/46" diameter) in both sides of thebottom section of the box to permit afree flow of air to the sensing elements.If the box is to be mounted where rainwill strike it, make a small metal plateto mount above the vents on the topside. This will allow air to flow in thebottom vent and out the rear slotformed by the box top and the plate.
A machine screw can be mountedin the back of the box, or in any othersuitable location, for mounting the
-
sensing unit to the vehicle. After con-struction is completed, cover all open-ings with plastic tape. A rubber gasketcan be used between the top plate andthe outer edge of the box. Mount theunit where air can flow over it freely,but preferably where water cannotstrike it directly. Also, if dusty roadsare encountered frequently, mount theunit in a sheltered place to minimizecontamination of the humistor.
Make the 4 -wire cable connectingthe sensing unit to the control unit longenough to reach the control unitmounting position easily. Allow plentyof extra length for routing around ob-stacles in the engine compartment.Make sure that the cable is locatedwhere it won't interfere with any main-tenance or repair work on the engine.Wrap the cable with thin cord just in-side the sensing unit box to preventstrain on the tie point.
All parts except the sensing ele-ments are mounted in the control boxwhich will normally be located insidethe vehicle. The metal box listed is
SCREWS FORMOUNT!HUMISTORTERMINACLIP
4TERMINATIE POIN
PLASTICCLAMP
CONTROL UNIT
Q2
R3
D3 D2
SI
DI
R9
106
suitable where mounting space is lim-ited, but if you have plenty of space, alarger box can be used to make wiringeasier. After drilling all holes andmounting the switch, lamp holders,and potentiometers, make two subas-semblies on 6 -terminal tie points, usingthe parts shown. Q1, Q2, C4, R5, andR11 can be soldered to unused switchcontacts. Point-to-point wire all otherparts, and install the subassemblies. Besure to insulate all bare leads.
After mounting the control unitin the vehicle, connect the 4 -wire cablefrom the sensing unit to the terminalsprovided on the top tie point. Thenconnect the positiVe and negative leadsto the vehicle battery via the chassisand the ignition switch. Make surethat the "hot" lead from the controlunit is connected to the coil side ofthe ignition switch as shown.
An "OFF" position is provided onthe control unit switch, but it does notdisconnect the supply voltage. It sim-ply provides an idle position bygrounding the bases of both transistors
SENSINGUNIT
Take a close look at the inside of the sensing and control units. Match this partsplacement as closely as possible and you will have little trouble building the alarm.
to reduce the collector current to theminimum. Wire potentiometers (R2,R6, and R8), so resistance decreasewhen the shaft of these controls is ro-tated clockwise. This causes lampbrightness to increase and flashing tostart with clockwise rotation, makingadjustments easier.
107
R5
R4
I EL HADJUST
RCI
RI
C2.
Another view of the control unit. This time we're looking at it from the humiditycircuit end. Obviously, if you fit a larger case in your car, it will make the construc-tion lob considerably easier as it will leave more room for parts and avoid squeezing.
First make a coarse adjustmentas follows: Set the three potentiomet-ers to their maximum resistance posi-tion (fully counterclockwise). Setcontrol switch SI to TEST S (steady)and then rotate the LEVEL H and LEVELT controls clockwise until the lampsare visible in a slightly darkened area.(A vtvm or a 20,000 ohms volt vomconnected across LMI or LM2 shouldindicate about 0.85 volt at a suitablelamp intensity.) Now set the switch toTEST F (flash) and carefully rotate theGAIN control until the lamps just startto flash.
Next make a fine adjustment during critical humidity and temperatureconditions. This is best done by actualmeasurement of these conditions. Withthe switch at the OP/ADJ position.make the adjustment for even lamp in-tensity and then for flashing at a hu-midity of about 95% and a tempera-ture of about 35° F. Any hygrometerand thermometer of moderate accu-racy will be suitable for making themeasurements, since the lamp indica-tion of humidity and temperature isnot a precise indication. It is intendedonly to indicate the range where dan-gerous conditions may exist. I used aninexpensive slide hygrometer that isavailable at Lafayette Radio Electron-ics (Stock No. 99C9006) for $2.69.
After making the fine adjustment,check that the lamp intensity is ap-proximately the same at the TEST Fand OP ADJ positions during criticaltemperature and humidity conditions.
CAINADJUST
Here's that control unit again. This time from the opposite end where the temperaturecircuit is located. Parts placement is not critical, but if you follow the arrangementhere your assembly problems will be eased and you can reduce assembly time.
If there is more than a moderate dif-ference, change the value of R4 and/or R10 as required to make lamp in-tensity equal at both switch positions.
In use, as either the critical hu-midity or temperature range is neared,the corresponding lamp will becomebrighter. When both conditions reachthe critical point simultaneously, thelamps will start flashing at a greater in-tensity and will continue to do so until
either or both conditions drop belowthe critcal range.
Note: Since it is not always pos-sible to simultaneously obtain humid-ity in the range of 95 to 100% andtemperature in the range of 32° to35°, set the two LEVEL controls sep-arately. Then when the weather issuitable, the GAIN control can be setfor flashing. If a suitable hygrometeris not available, a satisfactory adjust-
-RI2
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02
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ment of the humidity indicator can be however, because the humidity ismade when driving through a light fog. often considerably less than 100% atDo not try this during rain or snow, such times.
PAUSE CONTROLLER
15 seconds . . . It's the pause thatmakes the difference. However, eachsweep occurs at normal speeds.
The unit will not operate withvacuum -operated wipers. Electric wip-ers have been in vogue for the past 15years, and unless you have an oldercar the chances are you can use APC.Federal regulations require that allcars built after 1967 have two -speedwiper systems.
All windshield -wiper motors havea built-in cam -operated "park' switch(S3 in Fig. I) to keep the wiper motorrunning until the wiper blades returnto their park position; even after thewiper switch on the dashboard isturned off. It's this cam -activatedswitch that makes the APC possible.
Two silicon semiconductors, aunijunction transistor (Q1) and a sill-
By S. B. GRYNKEWICZ
LIVES THERE A DRIVER WHO HASN'Tencountered this: A light mist hangs inthe air. Traffic is just heavy enough sothat those @ #!90 -drivers -in -a -hurryperiodically whizzing by you leave athin coating of dirt on your windshield.
If you're like me, you hate to seethose wiper blades flicking back andforth, wearing away, and giving a gritpolish to an expensive windshield.Turning the wipers on and off andreaching for the knob every few minutes didn't appeal to me, so I decidedto let electronics do the job
With this Automatic Pause Con-troller (APC), you can operate thewindshield wiper at its normal speedsin a normal manner (no pause be-tween sweeps) or set the control knobto obtain a desired pause betweensweeps . . 1 second, 2 seconds . .
109
Construction
110
Mechanical layout of the automatic wiper -pause controller is not critical. See varia-tion above. Build it to fit your car.
con -controlled rectifier (SCR1) areused in the APC. The SCR, whichswitches the wiper motor on, is trig-gered by preset pulses from relaxa-tion oscillator Ql. Once the SCR con-ducts, it stays on until its anode volt-age is zero with respect to its cathode.
Operation of the relaxation oscil-lator is simple. When power is appliedto the APC through S1 (S2 off), Clbegins to charge to the supply voltagethrough RI and R2. The charge acrossCl also appears across the emitter andbase 1 of Q1. When the charge reachesa critical level, it drains off throughQl's emitter, base 1 and R3 until theemitter drops enough to stop conduct-rng. (The critical level depends uponthe transistor's characteristics.) Capaci-tor Cl recharges and the cycle is re-peated. Value of R1 and Cl and thesetting of R2 determine the chargingtime. The greater the value of resistance and capacitance, the longer ittakes to charge the capacitor and thelonger the pause between wipersweeps.
When Q1's emitter voltage goespositive enough current flows throughR3, Ql's base 1 and base 2, and R4 Thegreater the current flow, the greaterthe voltage drop across R4. This volt-age is also across the SCR's gate andcathode. When the voltage drop acrossR4 reaches a sufficient level it causesthe SCR to conduct and turn on thewindshield wiper motor.
Once the motor starts to operate,the cam -activated switch closes andkeeps the motor running even thoughit shorts out the SCR and halts itsconduction. The motor will continueto operate until the cam opens S3When S3 opens, the motor will remainat rest until the SCR is fired onceagain by another pulse.
To operate the wipers in a normalmanner simply close S2. When S2 isclosed it overtakes the APC regardlessof the setting of S1 and R2. However,both SI and S2 must be off for thewipers to remain at rest. Note: Onmany cars S2 is actually a 3 -positionswitch (off, low, high). If you are for-tunate enough to own a Cadillac youhave still another position (medium)at your disposal.
Capacitors C2 and C3 and diodeD1 prevent the SCR from "false" firingdue to inductive kickback from themotor when it is shut off, and preventother undesirable effects brought onby switching transients.
A 11/2" x 2" printed circuit boardwas used, but a plain perforated boardand push -in terminals can be substi-tuted, wiring and parts layout is notcritical. However, you must observepolarity of the capacitors and musthook up D1, Q1 and SCR1 properly.
A minor modification of the SCRis necessary to allow insertion intothe board. Carefully cut off the solderlugs brazed to the end of the gate andcathode terminals. The SCR is notmounted flush with the board, but in-serted only enough to make a good
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Fig. 1-Uniiunction transistor controls the wiper by firing the SCR at regular intervals.
111
* TO ACCESSORIES TERMINALON IGNITION SWITCH.
R4IK
WIPER MOTOR_ _ _ _ 1A/ I1
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FOR 6 -VOLT SYSTEM, REPLACE R4 WITHJUMPER WIRE, REDUCE RI TO 15,000 OHMS
-z- =
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a....... --ANODE
CATHODE
MCR2604
The APC can be built on a PC board or a perforated board for point-to-point wiring.
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112
PARTS LISTC1 -50 -AF, 15 -volt electrolytic capaci
torC2-0.1-gF, 25 -volt disc capacitorC3 -1-1P, 25 -volt disc capacitorR1-33,000 ohms (15,000 ohms for
6 -volt systems)R2-Linear potentiometer, 50 000
ohms with switch S1R3-51 ohms, 5%R4-1000 ohms (replace with jumper
for 6 -volt systems)
All resistors 1/2 wattQl-Unijunction transistor (2N2646,
GE, Motorola or similar)SCR1-Silicon controlled rectifier
(MCR2604-4, Motorola or similar)Ill-Silicon rectifier, 200 -volt or higher
PIV (1N2069 or similar)S1-spst, (on R2)S2-Standard wiper switch (see text)MISC-small piece of perforated
board, aluminum sheet, knob,wire, solder
I Cl150µF+
I R2150K
0I2N2646
E
R3
R4 C31K IµF
S2
I
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Fig. 2-For cars with 12 -volt positive -ground system. See text for 6 -volt operation.
InstallationMount the APC within easy reach
of the driver Since the APC circuitis grounded through its metal case, asare most automobile electrical devices,it is necessary for the case to makegood electrical contact with the dash-board. If for any reason a good groundconnection cannot be made in thismanner, run a wire (about No. 20)from the APC's ground bus to a goodground on the car. Connect the B leadfrom S1 to the accessories terminal ofthe ignition switch, or to a lead alreadyattached to this terminal, if you can'tget to the ignition switch. Connect theA lead from the anode of the SCR tothe windshield wiper motor switch S2
solder connection to the shorter gateterminal. You can shorten the longerterminal after soldering it to the board.
Mounting brackets can be madefrom M2" aluminum sheet, cut and bentas shown. The mounting bracket canbe custom fitted for installation on thebottom edge of your dashboard if youdon't want to drill any new holes onthe dashboard's face. The sides of theunit should be left open to help dissi-pate heat. Insulation paper or plastictape should be used where necessaryto prevent short circuits. A coat ofpaint will dress up the mountingbracket. Install a suitable pointer knobto match the car's decor to completethe construction.
82
APC CIRCUIT
s,
I RII 33K
131SCR]
MCR2604-4
C2
WIPER MOTOR_
A
I
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A schematic for constructing anAPC for a 12 volt positive -ground sys-tem is shown in Fig. 2. For 6 voltoperation, reduce RI to about 15,000ohms and replace R4 with a jumperwire.
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Use No. 16 stranded wire for the Alead. A smaller gauge wire (about No.20) can be used for the B lead.
If the wiper -motor switch is notaccessible in your car or if you prefernot to work under the dash, connectthe number 16 wire directly to thewiper motor wire going to the dash-
113
PART 4
Projects for the HomeU
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AN AUTOMATIC PHOTOGRAPHIC EN -larger timer is an invaluable darkroomaccessory. The timer leaves the pho-tographer's hands and mind free toconcentrate entirely on the photo-graphic creation.
The schematic in Fig. 1 is a solid-state, automatically resetting timer.The advantages of a solid-state de-vice over a mechanical timer are theelimination of noise, vibration, manualresetting, and increased reliability.The device is capable of timing intervals from 1 second to 69 seconds in1 -second intervals
The time interval is set by tworotary switches. Switch S3 sets inter-vals of 10 seconds and switch S2 setsunit intervals additively. The timer isactivated by momentarily pressingSTART switch S4. When the timing cy-cle terminates, the device is automatically reset and is ready to be activatedagain by pressing the momentarySTART switch. STOP switch S5 providesan abort capability before the end ofthe timing period if a timing intervalhas been erroneously set. The unit'srecovery time is in the millisecondrange, which is advantageous whenmaking test strips.
Timing accuracy is better than2% up to 30 seconds, after which ac-curacy falls off slightly due to the leak-age current of tantalum capacitor Cl.The unit provides exact time intervalrepeatability, which is important forconsistent print results. Time interval
resettability is of more importancethan the slight loss of accuracy (2-5%) at intervals above 30 seconds.Most enlarging situations call for lessthan 30 second exposures.How it works
Relay RY1 is energized duringtiming and applies 117 Vac to the en-larger via the ac receptacle. The pow-er supply is a half -wave type consist-ing of R21, D1, C2, R22, D3 and R24,and supplies an unregulated dc volt-age to operate relay RY1 and a +27volts dc, regulated by Zener diode D3.The regulated voltage provides an ac-curate reference voltage to the timingsection of the device.
The heart of the timer is unijunc-tion transistor Q1, used as a single -cycle relaxation oscillator. The timinginterval is varied by switching resis-tance values (R1-R16 additively) inthe unijunction emitter circuit to varythe RC time constant.
At the end of the timi g cycle(when Cl charges to the proper stand-off voltage at the unijunction emitter),a pulse is generated at Base 1 of theunijunction. The pulse is applied tothe gate of silicon controlled rectifier(SCR1), which then conducts currentaround RY1 causing the relay to re-lease and end the timing cycle.
Here is an outline of the timingcycle:
1. Momentarily depressing START
switch S4 applies the unregulated
R522K
R422K
R12220K
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PARTS LISTC1-47-uF, 20V, TO electrolytic tantalum
capacitor (Mallory typeC$R13- E476KL orequal; see note below under relay RY1)
C2-100-uF,150V electrolyticcapacitorC3 -0.005-u F, 600V ceramic capacitorAll resistors 1/2W, 10% or better unless notedRI 2000 ohms, 5%R2-20,000 ohms, 5%R3 -R10-22,000 ohmsR11 -R16-220,000 ohmsR17-270 ohmsR18-47 ohms
115
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R15220K
R322K 9
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TIMECALIBRATE
RI 70 27011
2N2647E
1382
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S4START R23
15KN.O. 2W
SCR( ANODE
45
6
12
13
S5 5STOP
6
GATE
C2 +03
27VI00µF ZENER150V I W
7 S6
Fig. 1-Complete timer circuit. Diode Dl (unlabeled) is to the right of R21
RI9-10,000-ohm, 1/4W linear potent ometerR20-150 ohmsR21-100 ohms, 2WR22-12,000 ohms, 2WR23-15,000 ohms, 2WR24-68,000 ohmsR25-10,000 ohmsDl, D2-750-mA,400PIV, diodeD3-27-volt,1W, 5 Zener diode (Motorola
1N4750A or equal)Q1 -2N2647 uni iunction transistor (G.6.1SCR1-C106F2 silicon controlled rectifier
(G E.)
R6X SECONDS X10
22K R722K
R622K
R13220K
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AC COMMON*
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117VAC
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RECEPT FORENLARGER LAMP(100 WATTS MAX
FOR RY USED
* NOTEAC COMMON SIDE OF17vAC SHOULD BEFLOATING(COMPLETELYISOLATED FROMANY EXTERNALMETAL SURFACE 1.
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R25 9OK
R2I100n2W
R22I2K2W
52, 53 -1 -pole, 12 -position rotary switch(Mallory 32112J or equal)
S4-N.O., momentary pushbutton switchS5-N.C., momentary pushbutton switchS6-rocker switchMISC -ac socket, 6% x 3 3 16 x 17/4 -inch
Bakelite case with aluminum panel(Lafayette 99T 6272), knobs, perf boardtransistor socket
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Other partsRY1 -4pdt, 125-mW, 2500 -ohms, 8.4-mA dc
relay (Allied Control TS154-4C. Availablefrom Newark Electronics Corp., 500 NPulaski Rd., Chicago, Ill. 60624 for $3.96plus 2 oz. postage ppd. Stock No. 59F512,1969 catalog). Capacitor Cl is $2.82,Stock No. 17F1542.
St-slide switch
voltage through R23 to RY1,which puts the four relay contactsin the activated position.
2. When the START switch is released,the relay remains activated as theholding current is supplied throughthe normally closed STOP switch S5and through the closed relay con-tacts 6 and 7. If it's desired to ter-minate a timing cycle before the in-terval set, the STOP switch can bemomentarily pressed, interruptingthe relay holding current and re-leasing the relay.
3. With RY1 activated, the enlargerlamp receives 117 volts ac throughcontacts 15 and 16, +27 volts issupplied to the unijunction circuitthrough contacts 12 and 13, andR20 is removed (contacts 8 and 9are open) from across Cl.
4. When Q1 fires at the end of thetiming interval, the positive pulsegenerated at Base 1 triggers theSCR, which .then bypasses currentaround the relay coil, causing therelay to release.
5. Since the SCR anode voltage issupplied through relay contacts 6and 7, the SCR anode voltage isremoved when the relay releases.Therefore, the SCR is restored to anonconducting state and the timeris automatically ready for a newtiming operation initiated by theSTART (S4) button.
6. When the relay releases, contacts15 and 16 open, the enlarger lampis turned off, the regulated +27volts is removed from the unijunc-tion circuit, and R20 is placedacross Cl. R20 completely dis-
charges capacitor Cl within milliseconds to assure that when a newtiming cycle is started the initialvoltage on Cl will always be zero.This insures an accurate timing in-terval.
Capacitor C3 is across relay contacts 15 and 16 for arc suppression.Diode D2 is across the coil of RY1 toeliminate transient spikes during relayturnoff, which could damage the SCRand/or the relay coil. The 1 -secondtiming resistance is divided into tworesistors, R1 and R2. R1 is always inthe timing circuit and provides protec-tion for Q1 if the timer is started withswitches S2 and S3 accidentally left inthe 0 second position R1 contributesonly a small error (less than 1%)when S2 is set to 0 and the S3 (x10)resistances are in use.
Provision is made for accuratecalibration using potentiometer R19.By means of R19, the interbase volt-age can be varied, thus controlling thefiring point of the unijunction transis-tor (the firing point voltage at Qt'semitter is equal to the stand-off ratio(n) x the voltage across Ql, Base 1
to Base 2). The ±10% tolerance ofCl and the variation in the stand-offratio of Q1 (0.68-0.82) are balancedout using R19. Although the timingresistors are ±5% tolerance, it canbe shown that the timing accuracy isbetter than 2% (if the leakage cur-rent in Cl is considered negligible).
ConstructionThe cabinet is a 6% x 33/6 x
1% -inch bakelite utility case. A 117volt isolation transformer was not used.
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Top view of mounting perf board. Make sure S2 and S3 have adequate clearance.
Bottom view of component board. An external relay extends wattage capability.
11.
I III
II I
The components were mountedon a phenolic perforated board whichwas fastened to the bottom of thebakelite case using spacers (legs) toallow clearance for the componentsmounted on the bottom side of theboard. Rotary switches S2 and S3were mounted on the front panel sothat the space left between them wassufficient for the relay clearance. FO-CUS -TIME switch S6 is a "see -saw" (or"rocker") type which adds to the con-venience and simplicity of operation.The relay was selected because of itslow operating power (175 mw). Thecontact rating of the relay will allowit to switch an enlarger lamp of up to100 watts (within the range of mosthome enlargers).
If you wish to use the timer witha higher -wattage lamp, use the unit toswitch another external high -contact-current relay Capacitor Cl is a tantalum capacitor and no other typeshould be used since a low -current-leakage capacitor is required. The accommon points on the schematic inFig. 1 should be joined together andconnected to the indicated side of thepower line; do not ground this sideof the power line to the front panel.
Calibration and use
As with all electronic gear, thepower supply should be checked outfirst before connecting the circuitryLeave the connection to relay pin 13open until the voltage is checked.Double check the power supply wiringand then turn power on The voltageon the open lead should be +27 voltsdc.
Set potentiometer R19 to mid-range and the FOCUS -TIME switch S6to TIME. Set the timer for 5 seconds
118
(S2 to 5 and S3 to 0).Sta t the timerby momentarily pressing the STARTswitch S4. Adjust potentiometer R19for a 5 -second cycle. You will hear therelay drop out at the end of the cycle.Check the timing interval at 10, 15,20, and 30 seconds and touch up theadjustment of R19 if necessary. Alltime intervals up to 30 seconds shouldbe accurate to 2%. Check the timingat 40, 50, and 60 seconds. The accu-racy should be from 2 to 5%. A 5%error at these settings will not at allbe noticeable in the finished print.
If the error above 30 seconds islarge (which will be rare) it is becauseCl has exceptionally high currentleakage. If Cl is not completely de-fective, the accuracy above 30 secondscan still be improved. Since Cl is arelatively high -cost item, the followingprocedure should be followed ratherthan to discard the capacitor. Set S3to the shortest time at which the ac-curacy is to be improved (say 50 sec-onds) and S2 to 5 seconds. ReplaceR15 with a 500K pot and adjust for anaccurate 55 -second interval.
The potentiometer can be left inas part of the circuit or the poten-tiometer resistance can be measuredand a fixed resistor substituted. Repeatthe procedure for the next highesttime setting of S3 (60 seconds andR16 in this example) after the select-ed resistor is soldered into the circuit.Leave S2 set to 5 seconds.
With switch S1 on and the FO-CUS -TIME switch S6 set to FOCUS, theenlarger will be continuously on. Af-ter selecting negatives and focusingthe enlarger, set S6 to TIME. Thelamp will be switched off and thetiming interval set by S2 and S3 canthen be initiated by pressing STARTswitch S4.
r
by FRANK H. TOOKER
Gate junction leakage in the FET isvery low-perhaps I nA at roomtemperature. Thus, loading of the RCtiming components is negligible. Leak-age resistance of the timing capacitoris the only significant loss in this partof the circuit.
The dc potential at the sourcejunction of Q1 rises as the voltageacross Cl rises. This potential appearsacross voltage -divider resistors R7 andR8. The potential across R8 is fed tothe cathode gate of the silicon controlled switch via R9. When this po-tential rises to the triggering level ofthe SCS (silicon controlled switch)(about 0.5 volt), the SCS turns on,applying voltage to the Sonalert unit.
The power supply uses a center-tapped transformer, a pair of rectifierdiodes and a fairly large filter capaci-tor, C4 The dc output across C4 isfed through resistor R11 and regu-lated at 15 volts by Zener diode D2.On the timer PC board, this potentialis regulated to 10 volts by R10 andZener diode DI before being appliedto the sensitive RC timing circuit. Ri-gid control of the timing -potentialsupply is thereby maintained. The re-mainder of the timer is supplied at theregulated 15 -volt level.
When the Sonalert sounds, andthe instrument is switched off, sectionS1 -b of the ON-OFF switch opens thetransformer primary circuit, whilesection S1 -a discharges timing capaci-tor Cl by short-circuiting it throughR4 and R5. The negative pulse thatappears across R5 at the instant ofdischarge is fed to the cathode gate of
RC ELECTRONIC INTERVAL TIMERSusually have a maximum time lapseof about 10 minutes. Beyond this de-lay, timers have been costly to build,have required relatively hard -to -getparts or have been inexpensive but in-accurate.
The timer described here isdifferent. It's not cheap to construct,but it won't flatten your wallet either.All components are available fromcatalog sources. The unit has two tim-ing intervals, the longer is 1/2 hr. Ac-curacy is ±5% for the low range;___10% for the upper range.
You can use a timer such as thisto get your breakfast eggs just rightor for timing long-distance telephonecalls. In the winter it will tell youwhen you've been under the sun lamplong enough; in the summer it tellsyou when to move the lawn sprinklerto another location. If you're a photog-raphy enthusiast, it will let youknow-even in total darkness-whenyour negatives are developed, fixedand washed. You'll find a host ofother uses once you have the in-strument built.How it works
A schematic of the 1/2 -hour inter-val timer is in Fig. 1. PotentiometersR1, R2 and resistor R3, together withtantulum capacitor Cl, determine thetiming interval. Resistor R5 has muchtoo small a value to affect the inter-vals significantly.
The voltage slowly developedacross Cl is fed to the gate of field-effect transistor Q1 via R6. The FETis connected as a dc source follower.
0
1
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5001,F.1 25VT.-.
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R910K
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2N5457
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G
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R7I5K
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C1 -220-µf, 10y, low leakage tantulum capacitor
C2-0 01 -AF, 100V, Mylar capacitorC3-0.022-gF, 100V, Mylar capacitorC4 -500-µF, 25V, electrolytic capacitorAll resistors V2W, 10% unless notedR1-1-megohm linear potentiometer Ohmite
CUI052 or equal)R2-5-megohm linear potentiometer (Ohmite
CU5052 or equal)R3-Two 10-megohm resistors in parallel
(see text)R4, R5-100 ohmsR6-47,000 ohmsR7-15.000 ohmsR8 -3000 -ohm wirewound potentiometerR9-10,000 ohms
+4
DI10yIW
R647K
R10-470 ohmsR11-220 ohms, 1 wattD1 -10V, 1 -watt, Zener diodeD2 -15V, 1 -watt, Zener diodeD3, D4-rectifier diodes (1N3754 or similar)Q1 -2N5457 field-effect transistorSCSI 3N84 silicon controlled switchSI-dpdt slide switchS2-spst slide switch71-30V ct, 50-mA power transformerLM1-120V ac neon pilot lamp (Leecraft
36N2315 or similar)Mallory Sonalert type SC628MISC.-5 x 4 x 3 -inch aluminum cabinet,
power cord & plug, pointer knob, rubberfeet, 3 -lug terminal strip, small L -brack-ets, rubber grommet, hardware
A
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R4
R3*5 MEG
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Fig. 1-Timer circuit is relatively simple. It uses an FET transistor and asilicon controlled switch (SCS) to do the hard work.
RIO470II
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the SCS via C2, triggering the SCSoff. Without this feature, the Sonalertwould continue to sound, eventhough the timer is switched off, untilfilter capacitor C4 becomes nearly dis-charged.
Capacitor C3 reduces the sensi-tivity of the SCS to line voltage transients. The circuit, however, may attimes be triggered by a strong tran-sient pulse, when the potential at thecathode gate is raised near the firinglevel.
The instrument is equipped witha neon pilot lamp to indicate whenthe unit is on and timing. (It's dis-couraging to wait for a 1/2 -hour inter-val and discover you've forgotten toplug the power cord into a wall re-ceptacle!)
R
Llost TO R8 TO RI TO SONALERT
Parts layout for the timer board. Foraccuracy use a glass -epoxy board
81
Parts layout on the power supply board.Phenolic board material is ok here.
RIO
R9
TO R3 S2 TO +15V
R6
88
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With S2 closed, any time intervalbetween 2 minutes and 16 minutesmay be obtained by setting poten-tiometer R2. When switch S2 isopened, a fixed 14 minutes is added to
0,
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Foil pattern for the power supply circuitboard. This is actual -size drawing.
R5
Foil pattern of the timer circuit board.Drawing s actual -size pattern.
121
SE R3 TIMER ASSEMBLY
POWER SUPPLYASSEMBLY
Another internal view shows parts loca-tons from a different vantage point.Follow this arrangement closely.
122
Inside the timer case you get a closelook at parts placement. Note the posi-tions of the two circuit boards.
the timed interval. The calibrated set-tings of R2 then cover a 16 -30 -min-ute range.
The setting of R8 determines thevoltage level across capacitor Cl at
which SCSI turns on. This adjust-ment is made to obtain the 16 -minuteinterval, when switch S2 is closed andpotentiometer R2 is set at maximumresistance in the circuit. Potentiome-ter R1 is adjusted to obtain the 2 -min-ute interval, when switch S2 is closedand potentiometer R2 is set at min-imum resistance in the circuit.
The setup exhibits some variationin the timed intervals as a result ofvariations in room temperature. Overthe range of 65-90°F, these variationsfell within the specified tolerances. Avariation of 3 min in lh hr representsa 10% tolerance.
Building a timerThe timer shown in the photos is
constructed in a 5x 4x 3 -inch utilitycabinet. Two PC boards are em-ployed: one for timing circuit com-ponents, the other for power supplycomponents. The timer -circuit PCboard is preferably of glass -epoxybase material. The power supplyboard may be phenolic.
Be particularly careful not tooverheat timer components when sol-dering this board. Even slightly ex-cessive heat applied to Cl or the FETcan materially affect circuit perform-ance. If the timer cannot be calibratedas described later, or if the calibratedpoints appear to shift without reasonor are otherwise erratic, and all solderjoints are clean and properly made,then component overheating shouldbe suspected.
The parts list calls for two 10-megohm resistors in parallel for R3.Using two parallel resistors like thisallows the value of one of them to beadjusted slightly higher or lower tobring the longer range of timed inter-vals within the 10% tolerance. Thisadjustment is desirable because, as thetiming extends beyond 16 minutes,the leakage resistance of Cl begins tohave a slight but increasing influenceon the timed intervals. As a result ofthis influence, R3 will appear to havea somewhat larger value when R2 is
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set at the 16 -minute calibration pointthan when it is set at the 2 -minutepoint. Thus, without compensation,all this effect could appear at the up-per end of the range of the longertimed intervals. By carefully adjustingthe value of R3 the effect can be di-vided between the high and low settings of R2, providing a more reason-able tolerance over the entire range.The setting of potentiometer R8 deter-mines the duration of the longertimed intervals, while the setting ofpotentiometer RI determines the du-ration of the shorter timed intervals.Before turning the timer on initially,set RI to put about two-thirds of itsresistance in the circuit, and set R8 tojust a little below midposition.
With RI and R8 set as described,set potentiometer R2 in its maximum -clockwise position (maximum resis-tance in the circuit). Make sureswitch S2 is closed, shorting R3. Thenmove S1 to On Using an accurate elec-tric clock with a sweep second hand,check the duration of the timed inter-val. It should be 16 minutes. If it isless, switch the timer off and adjustR8 to put a little less resistance in thecircuit. Then check the timed intervalagain. Try to set R8 so that the inter-val is accurate to within -1=10 sec.Check the interval several times, to
make sure of the best setting of R8,and to determine the repeat accuracy.
With R8 properly adjusted, turnpotentiometer R2 to its maximumcounterclockwise position (minimumresistance in the circuit), and checkthe timed interval. It should be 2 mm.If it is less than this, adjust potentiometer R1 to put a little more resis-tance in the circuit, and check the interval again. With R1 properlyadjusted, go back and recheck the 16-minute interval. Readjust R8 slightly,if necessary. Then recheck the 2 -min-ute interval, and readjust R1 slightly,if necessary. Go over these checksand settings several times.
Calibrating the settings of R2consists of locating the pointer knobpositions for the 4-, 6-, 8-, 10-, 12 -and 14 -minute intervals, and of mak-ing a tiny mark on the cabinet frontat each position, using a sharp scri-ber. This can take time, especiallywhen the exact settings for the longerintervals are involved. Having a goodbook to read during the interim eachtime will make the task seem lesstedious. Be sure to make a mark atthe extreme settings of R2 to locatethe 2 and 16 -minute intervals.
The optimum value for R3 maynow be determined, as described pre-viously. When this has been done, thetimer is ready for use.
DANCING CHRISTMAS LIGHTS
of red, yellow, green, for hours. Thisyear you can get every color of therainbow with a cycle time of-well, itstill hasn't been measured. All of thisin a tiny box, waiting to please bothfamily and friends.
By R. W. FOX'
REMEMBER THE COLOR WHEEL YOUbought last year, and the blinkinglights from the year before? They'reobsolete. Here's the color blender, thelatest in special lighting for thisChristmas season No more suddenflashes; no more monotonous repetition
A
RED
INTENSITY
GREEN
BLUE
RESULTANTCOLOR
GREEN VIOLET DIRK YELLOW BLUE-GREEN
YELLOW RED WHITE
The Color Blender consists ofthree identical circuits, one for blue,one for red, and one for green. Cornbinations of these colors will produceevery color in the rainbow. Figure 1shows graphically what is happeningand, by the way, the first time you turn
it on could very well be a happening!Due to the variation in componenttolerances, these circuits will not beidentical in performance. One maycome on faster than the rest; one maystay off longer. This will give a true randomness to the colors, which is not
Fig. 1-Output amplitude of three light channels as a function of time. Variation incomponent values cause each channel to randomly vary lamp pattern and intensity.
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PARTS USTCapacitors -50 volts unless noted R4-22 000 ohmsC1 -33µF, electrolytic R5-220,000 ohmsC2 -111F, 25 V, electrolytic R6, R7 R9-100 000 ohmsC3 -10µF, electrolytic R8-50,000 ohms, potentiometerC4-0.05 µF, paper Ql-D13T1Dl, D2, D3 -1N5059 SCRI-C106B1
Miscellaneous hardware-box, line cord,Resistors -1/2 -watt, 10%, carbon unless outlets (Amphenol 61-F1 or equiv )
noted circuit board, switch (Except for theR1-5 ohms, 5 watts box and line cord, three sets ofR2-1000 ohms parts will be needed to build aR3-82,000 ohms three -lamp controller)
1N5059
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possible with a color wheel or two,three or even fifty color wheels,since they operate similarly.
Figure 1 shows the output of thethree channels as a function of time.At the start the green light is onwhile the others are off; the red thencomes on and the color changes toyellow; then the red and green go offand the blue comes on. As this ishappening the color of the displaypases through a violet portion andthen to pure blue. In this picture thenext scene is all the lamps out ordark. Then the red comes on, followedby the green to give yellow, then theblue to give white light since all thechannels are at full intensity. At the
end the red lamps go out to leave ablue-green scene.
Inside, the Color Blender willhandle up to 25 miniature lights (6watt) per outlet on your Christmastree. If you intermingle three strings,one each of red, blue and green, withballs and tinsel to reflect the light,your tree will seem to come to life.Or, as another way, you could use upto 150 watts per outlet of flood lampswith color filters shining on a metallictree.
Outside, you could use strings oflights on a tree (be careful not toexceed the load limit on your circuit),or flood lights on the house or on a
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126
door covered with wrinkled foil Thewrinkling keeps the colors separatedbut allows them to blend in someareas to give a multiple color effect
After Christmas, there is no needto put the Color Blender away Justbuild a foil reflector with a largepicture frame and shine floodlightson it for a great addition to your gameroom.
How it worksThe G -E type D13T1 Program-
mable Unijunction Transistor, allowsus to build this unique circuit. TheDI3TI can be thought of as a com-plementary SCR. When the gatevoltage drops below the ar o.1e voltage,current flows from anode to cathode.This circuit uses this feature to phase -fire the C106B1 SCR (see Fig. 2).When the Color Blender is initiallyturned on, both Cl and C3 have nocharge. Capacitor C4 quickly chargesto a voltage greater than the D13T1gate voltage and triggers the D13T1,which in turn triggers the SCR, caus-ing the light to come on brightly Oneach succeeding cycle of operationcapacitors Cl and C3 have a higher
C2 SCR R2 C4 R3 R9 C3 D2
(UNDER BRACKET) (UNDER BOARD)
Fig. 3-Component layout for all three identical control channels. Parts are identi-fied on one channel only. Resistor R8 is adjusted to establish proper "on" sequence.
initial charge so that C4 cannotcharge to a voltage which wouldtrigger the D13T1 until much laterin the cycle. Since C3 charges at afaster rate through R7-R8 than CIand through R5, R6, R4, the lampdims slowly. When the lamp goesdark, Cl discharges faster than C3and the triggering angle of Q1 is ad-vanced and the light brightens again.
The purpose of RI in the circuitis solely for the protection of theC106B1 SCR. This resistor keeps thepeak current through the SCR withinits ratings. Although it wastes a con-siderable amount of power, about 4watts at full load, it is better thanreplacing the SCR whenever a lampburns out.
In addition to adding to thebeauty of your Christmas display, theColor Blender also increases lamp lifeat least 25 times. This increase in lifeis due to operating the lamp on halfwave. An added consequence of halfwave operation is lower bulb temper-ature, decreasing the chance of fire,hence giving greater safety.
With the board mounting, allthree circuits can be built at the same
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tab of the C106B1 are bent. CAU-TION: The tab should be held bylong -nosed pliers between the bodyand the bend while you are formingthe device.
To prepare your box, punchthree 11/4 inch diameter holes in thetop, one small hole in one end forthe line cord, one for the switch andthe box is ready. Insert into thebox the three outlets and fasten themin place with spring clips. The R1resistors can be attached on oneterminal and their free ends connectedto the switch Use heavy bus wirefrom .the other outlet terminals to theboard, and no other supports should beneeded to hold the board in place.
Once the circuit board is mountedin the box and the final connectionsare made, but before you put theColor Blender to use, you may haveto adjust the R8 resistors. If an as-sociated light comes on but damps outto a level between full on and off,R8 should be decreased. If the lampsnaps on, R8 should be increased.
By BRICE WARD
127
time, reducing the chance for mis-takes. Since one side of the line iscommon to many components, asingle bus running the entire lengthof the board may well be a good wayto start. To make the circuit compact,121 should be left off the board andwired directly to the outlet. This alsokeeps R1 far enough away from theSCR. Watch the connections of theSCR and Programmable UnijunctionTransistor. Reversing the lead ordercould very well destroy these units.The polarities should also be observedon the electrolytic capacitors and thethree diodes.
For safety's sake do not groundthe box to the line, for a reversal ofthe plug could be dangerous. If youhave a three wire system, though, byall means use that third wire as aground for the case.
Figure 3 shows the board as itwas constructed with all three circuitsso that it would fit in a 5 x 2 x 3 inchbox (Bud -CU -2106-A or equal). Youwill note in Fig. 3 that the leads and
capstan or what have you, and makesure a steady illumination falls on it(from an incandescent lamp or flash-light). Aim the probe, press a pushbutton switch and read the rpm direct-ly from the calibrated meter.
How it worksThe sensing circuit uses a cadmi-
um selenide cell designed to respondquickly to changes in light level. Othercells could probably be used, but the
HOW WOULD YOU LIKE TO READ THEspeed in rpm of any rotating shaftwithout making physical contact withit? Drill motors, lathes, drill presses,slot -car wheels, capstan drives, turn-tables and dc tape recorder drives arejust a few possibilities. Best of all, howwould you like to do this without fid-dling with dials or knobs?
This little hand-held tachometermeasures rpm this way: Place a con-trasting mark on the shaft, pulley,
Transistor Q1 and the photocell,then, form the sensing element circuit.Any device that can deliver a 1 -volt orbetter, positive -going pulse to the mul-tivibrator could be used to drive themeter circuit.
A pulse standardizing circuit uti-lizes a monostable multivibrator fol-lowed by a Zener diode. The mono -stable multivibrator, variously called a
Clairex unit (see parts list) will pro-vide excellent response out to the limitof the rpm range and probably be-yond this range.
Output from the photocell is cou-pled through capacitor Cl to Ql, usedas an amplifier with a voltage gain ofabout 50 (Fig. 1). The amplified volt-age pulse is coupled through C2 tothe multivibrator (Q2-Q3).
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Pulse counting is achieved with asimple half -wave rectifier. It appliesthe average power of the pulses to themeter, which smooths the pulses byacting as an integrator. A series ofwaveforms illustrate what occurs inthe circuit. All waveforms are positive -going above and negative below theapplied zero reference line.
one-shot or univibrator, has only onestable state, determined by the feed-back arrangement used. Here, thepulse width is determined by the vari-able time constant of C3-R7, and thepulse height is clamped to a 5.2 -voltlevel by Zener diode D3.
The voltage at point A in Fig. 1 isnegative -going with an amplitude ofabout 0.8 volt starting from a 4.2 -voltlevel This 4.2 -volt level will changewith the light level on the photocell orthe voltage applied to the circuit. (An18 -volt supply should give a 1.6 -voltnegative -going signal, permitting theprobe to be moved back from thepulsing light source.)
Since a fluorescent lamp tempo-rarily extinguishes on each half -cycleof 60 -Hz line voltage, pulse frequencyis 120 pulses per second, or (multiply-ing by 60) 7200 rpm. This gives agood source for examining circuit op-eration, and was used to calibrate thephototachometer.
This voltage pulse output of thecell is coupled by Cl to the base ofQ1 (B), overcoming the positive biason Qi and cutting it off. The result isdecreased current flow through R4and a consequent reduction in the volt-age drop across R4. Since R4 andQi's collector-emitter resistance rep-resent a voltage divider to ground(from the 9 -volt source), if the volt-age across R4 decreases, the voltageacross Ql's collector-emitter increasesand the voltage on the collector ofQ1 (C) increases with respect toground.
Multivibrator operation
In the multivibrator, Q2 is nor-mally cut off and Q3 is conducting.The collector-emitter current of Q3 isthen determined by the resistance ofR8 With a low Q3 collector voltage,less voltage is returned to the base ofQ2 (D) than is required to causeforward base-emitter junction bias.
When the signal on the collectorof Q1 swings positive, the voltage riseson the coupling capacitor C2. In try-ing to charge to this rising voltage, C2draws current through Q2's base-emit-ter junction, causing Q2 to conductand its collector voltage to drop (E).
Capacitor C3, which was chargedto about 7.6 volts, begins to dischargethrough R7, and drives the anode ofD1 (F) sharply negative, cutting thediode off. Since Q3's base-emitter current path (G) is through D1, Q3 is cutoff and the voltage at point H rises toa level controlled by D3. The 5.2 -voltpulse developed serves to drive themetering circuit, and supplies regener-ative bias to Q2.
As long as Dl's anode remainsnegative with respect to its cathode,Q3 will remain cut off, and the lengthof time that the anode remains nega-tive is determined by the time constantof C3-R7.
In short, the multivibrator has de-veloped a pulse of a width determinedby the C3-R7 time constant and aheight determined by the Zener diodeat its output. On completion of the C3discharge, the voltage on the anode ofD1 goes positive with respect to itscathode and begins to draw currentthrough Q3's base-emitter junction.Next, the voltage drops on Q3's collec-tor feed a negative -going voltage backto the base of Q2, causing Q2 to begincutting off. The positive -going voltagedeveloped on Q2's collector starts tocharge C3, reinforcing the switchingcycle by positive feedback.
Switching times for Q3 are ex-tremely fast (probably in the nanosec-
and range), and the entire standardpulse is generated in about 0.56 msec.
Figure 1-h shows pulses generat-ed at 7200 rpm, while Fig. 2 showsthem generated at 3600 rpm. The7200 -rpm pulses appear 8.5 millisec-onds apart. This means that roughly14 more pulses could be generatedbetween existing pulses, or a pulserate equivalent of 100,000 rpm.
At this rate, due to the extra en-ergy generated, the meter range wouldhave to be increased to 500 juA toread the total current.
As the system is set up, 10,000rpm is the upper limit, and R9 is ad-justed to give a full-scale reading onthe meter.
Fig. 2-Scope photo shows pulses at pointH with a 3600 rpm (60 Hz) input insteadof the 7200 rpm calibrating input (120Hz) shown in Fig. 1. Pulses appear every3 cm instead of 1.5 cm, halving thecurrent to the meter.
Adjusting R7 increases the pulsewidth to 0.7 msec .
ConstructionFor ease of construction, the unit
was built on a circuit board. Beforeinstalling R7, adjust it with an ohm-meter to about 3500 ohms or, if youprefer, use a 3600 -ohm resistor. Thephotocell (R1) can either be installedon the board or connected with a two -conductor cable for remote locationuse. Connect the negative battery leadto point A (Fig. 3) and the positivelead to point B. Points C, D and Ecan be used to raise the voltage on thephotocell.
To do this, remove R2 from itspresent location and install it frompoint C to point D. Install a secondbattery with the negative lead goingto the old R2 connection (extensionfrom point B) and the positive leadto point E. With this arrangement apair of 9 -volt batteries will apply 18volts to the photoresistor (increasingits sensitivity), but the circuit still re-ceives only 9 volts.
With 18 volts on it, the probe canbe moved farther from the device being measured. Depending on theamount of light striking the rotatingshaft, surrounding ambient light andother circumstances, a 9 -volt probewill function about 6" from the shaft.With the extra battery, this distance
could probably be increased to 12".The meter is installed with the
plus (±) terminal to point G and theminus (-) or unmarked terminal topoint H.
If you're a "scratch" builder orexperimenter, the transistors and othercircuit components are not critical.For example, any reasonably goodnpn silicon transistors should work inthe circuit. Reversal of all polarity -sensitive elements and the batteryshould allow operation with pnp typesof transistors.
The Zener diode is not criticaleither. If it's rated at less than thevoltage on the collector of Q3, andclamps the pulse amplitude, this canbe corrected by adjustment of R9.
The metering diode should be germa-nium, but this diode and Dl wouldprobably work with silicon devices.
The phototachometer can beused without a case, but since the me-ter movement supports the circuitboard it could be mounted in a case.Make a cutout for the meter and allowthe meter to support the circuit boardinside the case.
A larger meter movement couldbe used by drilling holes spaced a littlefarther apart on the printed circuitboard and again hanging the circuitboard on the meter.
132
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intermediate value, and then doublethis value as the probe is moved a littlecloser. It appears to be characteristicof fluorescent lamps that the lightpulse output is higher on one cyclethan the other. Therefore, the first me-ter jump represents a reading of everyother pulse from the lamp or 60 cyclesper second (3600 pulses per minute).
At the point where the metervalue doubles, adjust R9 for a readingof 36 on the meter, and the tachome-ter is calibrated. By proper adjustment
R7 -10,000 -ohm, Bourns E -Z Trim No. 3067.1-103 potentiometer
R8 -1500 -ohm, 1/2 -watt resistor
Q1 -2N2712 transistor (G -E)Q2, Q3 -2N697 transistorCR1, CR2-1N456 germanium diode
CR3-1N709 silicon zener diodeM1-50 liA, 1 9/16" square de meter (Lafay-
ette Radio 99 H 5049 or equiv)MISC-Circuit board, battery clip, 9-V battery
R7
Fig. 3-Component placement on the PC board. Text explains letters A-H.
multiplied by 200. (This is done quick-ly by adding two zero's and doublingthe result.) As mentioned earlier, afluorescent lamp will provide a 7200 -rpm pulse source. Since we want todetermine what the meter should readfor 7200 rpm, we reverse the proce-dure: divide 7200 by 2 (3600) andremove two zero's (36) to obtain asetting point for our calibration.
Hold the light cell about 12 inch-es from the fluorescent light and moveit in. The meter should jump to some
PARTS LIST
CI C2-0.1 sF, 75-V Mylar capacitorC3-0.22 sF, 100-V ElMenco 1DP-3-224 or
equal
R1--Clairex CL903C fast response photocell
R2-I megohm, 1,4 -watt resistor
R3-3.3-megohm, 'h -watt resistorR4, R5 -22,000 -ohm, 1/2 -watt resistor
R6 -47,000 -ohm, 1/2 -watt resistor
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133
If you measure very high-speeddevices, such as a model airplane en-gine, it might be necessary to reducethe overall pulse width by reducing ei-ther C3 or R7, and to reset R9 to indi-cate, say, 50,000 rpm full scale.
Other possibilities? Connect thephotocell to the circuit board using along cable. The very small probe canthen be mounted at a remote locationto monitor some shaft rpm not other-wise easily read.
The unit could be used as a fail-safe. The univibrator pulses could beapplied to a rectifying system used toenergize a relay. If monitoring shaftrotation, the relay would de -energizeand sound an alarm when rotationstopped
In short, the potential uses for thecircuitry are restricted only by yourimagination.
By ROBERT S. HAVENHILL
134
of R9, the phototachometer should becapable of reading pulses out to theequivalent of 100,000 rpm. The pho-tocell response time may become alimiting factor at or before this rate isreached.
Trying it outFirst, in reading something like
the propeller on a model airplane en-gine, remember that if the prop hastwo blades the indicated rpm will betwice the true rpm since the light isbeing chopped at twice the actualrpm. And if a disc has 30 segmentsthe indicated rpm will be 30 times theactual.
This applies for any rotating de-vice. Determine the number of full cy-cles for each revolution and divide therpm reading by this figure to obtaintrue shaft rpm.
SHOWING COLOR SLIDES is a good wayto recreate your vacation for friendsBy adding commentary on tape, you'rerelieved of repeating the same thing,and you can't forget what you said lasttime. But changing slides is still anuisance
The synchonizer changes slidesfor you automatically. All you need is astereo tape recorder. You put the voicecommentary on one track, and a syncsignal on the other. When friends dropin, you simply load the projector andthe recorder, turn them on, and theshow runs automatically. The recordedsync pulses advance slides as your com-mentary moves ahead. And there areno relays to chatter or stick.
The heart of this synchronizer is abidirectional thyristor, called a Triac.As you know, a silicon controlled recti-fier can conduct only on one-half of anapplied ac voltage. Why? Because anSCR is simply a diode with a gate. ATriac consists of essentially two diodesin reverse parallel, with a gate. It canconduct on both halves of ar ac wave-form and can be used as a full -wavecontrol and switch. A Triac can also begated by ac, which is quite useful.
Figure 1 shows the symbol for aTriac and one method of triggering it.When Si is closed the gate is con-nected to anode 2 through current-lim-
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Figure 2 illustrates a second meth-od of triggering: An ac signal is appliedto the gate -anode 1 circuit of theTriac through isolation transformer Tl.This second method is used in the tape/slide synchronizer on playback. Therecorded sync signal on the tape is fedto the primary of Tl. The signal thentriggers the Triac, which conducts andactivates the motor (or solenoid) inthe projector. The slide advances.
In the record mode, it's necessaryto simultaneously trigger the Triac (to
advance the slide) and produce an acsignal (to put sync on the tape). Thecircuit is shown in Fig. 3. Closing S1triggers the Triac and advances theslide. At the same time the 60 -Hz acto the motor passes through R1, causinga 1 -volt drop. This 1 -volt signal ispassed through T1 to the second tapechannel, while voice commentary is go-ing on the first channel. Fig. 4 showsoutline and terminal connections ofthe Triac.Operation-record
The synchronizer (Fig. 5) is con-nected to one recording input of thetape recorder through JI, and, to theslide changer through Pl. Si is set tothe record function. The tape recorder
Fig. 3-Drop across 111 is sync signal.
Fig. 2-Either ac or dc will trigger theTriac. Signal can also be applied betweengate and anode I to turn the device on.
Fig. 4-Typical Triac outline, terminalconnections, with mounting stud at top.
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Operation-playback
The cable connecting J1 to therecorder microphone input is now used
Fig. 6-Remote jack J3 isn't required . . . it is useful for remote operation of S2.Locking control is used for R3 to avoid accidental misadiustment of sync level.
137
is then adjusted to record voice from amicrophone on the other input channel.The tape is started and when you wantto change slides, S2 (or remote switchS3) is pressed This triggers the Triacinto conduction and starts up the slide-changer motor (or solenoid). In a mo-tor -operated changer, when S2 is released the cam switch in the slidechanger continues to feed ac to themotor or solenoid until the cycle iscomplete.
Meanwhile, as S2 is closed, thesync signal is passed through Tl, vol-ume control R3 and J1 to the inputof the recorder The level of the syncsignal can be set with R3; thus the vol-ume control of the recorder does notneed to be changed in going from rec-ord to playback.
S3 is an optional remote -controlswitch. It lets you put sync on tapewithout having to be near the unit.
to connect J2 to the external speakerjack of the recorder. (This jack mustbe on the same channel the sync signalswere recorded on.) SI on the tape/slidesynchronizer is set to the playbackfunction, and the recorder controls areset for stereo (or split -channel) play-back.
When you play back the tape,you'll hear commentary as usualthrough the tape speaker. But the sync -channel speaker should be muted, andthe sync signal goes into J2 and theprimary of T 1 . The signal is steppedup about 2.5 times by T1 and is thenapplied to the gate of the thyristor.
Note that closing either S2 or S3will override all automatic controlsand cause a slide change during eitherrecording or playback.
This synchronizer can be usedwith nearly any slide changer with re-mote pushbutton operation, and withany stereo or split -channel tape record-er with power amplifiers. It cannot beused with a tape deck since preampoutput isn't high enough to trigger theTriac. The SC4OB requires 1.5 to 3
R2 pi R J2
Construction
SC4OB
R3
138
LIKE THE IDEA OF TURNING SOME -thing on or off with your voice? Want tocontrol a transmitter or recorder so itoperates only when you speak? For avery small outlay of cash and time, you
can do it with this integrated -circuitsound -actuated relay.
The relay circuit itself uses a con-ventional transistor. but what providesthe audio amplification before that point
volts and 25 to 100 mA for triggering,and will control up to a 6 -amp load at120 volts.
Transformer T1 isolates the pow-er line from the recorder and gives afair impedance match between recorder output and the Triac gate. In therecord function the impedance matchto the recorder input is not good, butthis is not important as there is plentyof signal for recording.
The parts layout is not critical asall circuits are low impedance. Figs 6and 7 are top and underside views andshow the location of the few parts used.
The Triac is conveniently mount-ed on the small aluminum plate whichis insulated from and fastened to thecase with 1/2" insulated, threaded spac-ers. Transformer T1 is mounted on theopposite side of the cabinet (Fig. 7).P1, J1, J2 and J3 are mounted on thefront. As Fig. 6 shows. R3, Si and S2are mounted on the top of the case.
Resistor R I should be chosen toproduce a 1 -volt sync signal. The re-sistance value will depend on the volt-age value and the amount of currentdrawn by the slide -changer motor orsolenoid. If the motor draws about 0.3amp, which is normal for home -typechangers, RI should be 3 ohms Checkthe nameplate on your projector to find
TI
Fig. 7-Parts placement isn't criticalwithin the box as the circuit is chieflylow impedance. Mount Triac carefully.
out how much current it draws. Thisfigure may include the projector lampwattage, which you must subtract. Bet-ter still, use an ac voltmeter across RIand try different values until you comeup with a 1 -volt signal.
You will need a two -wire powercable with 'a receptacle to mate withP 1 on one end. The other end of thecable should match the slide -changerremote socket. For the sync connec-tions to the tape recorder, use phoneplug on one end, and whatever youneed on the other to match the recorderjacks.
IC SOUND -OPERATED RELAY
By LYMAN E. GREENLEE
How the circuit worksSound is picked up by the micro-
phone or small speaker and fed intoterminals 3 and 9 of the WC 183G (Fig.1). The audio voltage across the sec-ondary of the output transformer is rec-tified by a diode and charges a largelow -voltage capacitor in the base circuitof a pnp transistor. When the chargeacross this capacitor builds up to about0.3 volt, the transistor conducts andtrips the relay, which then remainsclosed until the voltage across the ca-pacitor has dropped to about 0 2 volt.
The delay must be long enough toallow for normal speech pauses. It canbe adjusted by choosing the proper val-
The sound relay can be built insidempst portable tape recorders. Compo-nents can be arranged to fit the availablespace. Layout is not critical, but inputleads from the microphone (or speak-er) used for sound pickup should bekept short to avoid feedback, hum andnoise. If these leads are more than 2 or3 inches long, use a two -conductorshielded mike cable. The shield goes toterminal 1 or ground and the two micro -
This complete "chassis" is just a piece of hardboard measuring less than 2 x 3 inches.
is a tiny integrated circuit, a Westing-house WC 183, encased in a flat plasticpackage that measures 0.14 by 0.25 by.055 inch. At 4.5 volts supply, it drawsless than 4 mA and gives a gain ofaround 90 dB. The IC, a differential am-plifier with four direct -coupled stages,comes in two kinds of packages-the"flat-pak" (WC 183G) and a 12 -leadTO -5 metal transistor -type case (WC183T). The "G" is the smaller, andthat's the one I used.
ues for R I and R2. R I can be a fixedresistor for predetermined fixed delay.or the arrangement shown in Fig. 2 canbe used for some control over the delay.If you use the transistor to drive the re-lay, capacitor CI can be much smallerthan if it were connected directly acrossthe relay coil -500 ILF is about right. IfCl is too large, there will be too muchdelay in pull -in and the first syllable orso will be lost.
I used separate batteries for theamplifier and relay because of the ten-dency of the circuit to cycle itself onand off when the relay is operated fromthe amplifier batteries.
Construction hints
we183
50/6V
50/6V
Fig. I-Complete circuitC1-500 p.F, 3 volts, electro-
lytic (Sprague TE1068or equivalent)
C2, C3 -10µF, 20 volts,electrolytic or tantalum(check for leakage ifconventional electroly-tic)
C4, C5-50 isF, 6 volts, elec-trolytic
D-general-purpose germa-nium or silicon diode(1N34, etc.) Select forlow forward -to -back re-sistance ratio.
IC-Westinghouse WC 183Glow-level audio ampli-fier integrated circuit.
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of the sound -actuated relay.
($7.00, Westinghousedistributors or CramerElectronics, 320 Need-ham St., Newton,Mass.)
Q-high-gain germaniumpnp audio transistor(RCA 2N2614 or 40263or equivalent)
R1 R2-See text and Fig. 2R3-pot, 5,000 ohms, with
switch (transistor -radiovolume control)
RY-miniature radio -controlrelay, spdt contacts,100 -ohm coil (JaicoGem or Deans Co.,$4.95 at Polk's Hobbies,314 5th Ave., New York,
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switchSPKR Small PM speaker
with 8- to 45 -ohm voicecoil
Tl-output transformer, 2,-000-2 500 -ohm primary,secondary to matchspeaker
T2-transceiver outputtransformer. Primaryimpedance 500 ohmsct; secondary (2) im-pedances 8 ohms, 3,-000 ohms. (Lafayettestock No. 99 C 6132)
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be used with a matching transformer.In that case, a secondary of 2,000 or2,500 ohms will be satisfactory High -impedance microphones can be matchedwith a transformer also. A good one totry is a transistor audio driver trans-former with about 10:1 ratio for dynamis mikes with 50,000 ohms impedance.Crystal mikes will need a 50:1 or even100:1 ratio. Impedance matching is nottoo critical for voice -operated relaytripping.
Note that the input transformer iscoupled through capacitors C4 and C5.Polarity is correct as shown in the dia-gram Both sides of the transformer dogo to the negative sides of the capaci-tors. These capacitors are necessary toavoid disturbing the internal biasing of
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phone leads to 3 and 9. The picturesshow the way parts were arranged forthe prototype, but there was plenty ofroom to spare on the 13/4 x 2-3/4 -inchcircuit board. There is no reason fornot making the whole device smaller.
A 45 -ohm intercom speaker worksvery well as a microphone. A speakerwith an 8 -ohm or 3.2 -ohm voice coil can
R - FOR VARIABLE T ME DELAY(OPTIONAL)
Fig. 2-An op-tional embel-lishment forvarying timedelay. Text ex-plains details.
cho Turnpike, Syosset, N. Y. 11791).The 3,000 -ohm winding goes to the relay circuit, and the 8 -ohm winding canfeed a pair of low -impedance head-phones for monitoring or testing.
The relay is made especially forradio control in model airplanes or boatsand is available from any supplier ofradio -control equipment. The contactswill carry 500 mA, which will do tostart and stop a miniature tape recorder.If heavier currents are to be interrupted,or if higher voltages than 9 or 12 are tobe used, an additional relay should beused in cascade to handle the extra load.
Working with the WC 1830Handle an IC with tweezers! The
most convenient way to mount one isprobably with a drop of cement.(Goodyear Pliobond cement is fine andavailable at local Goodyear tire storesand many hardware stores ) Put onedrop on the Bakelite board and anotherdrop on the back of the WC 183G. Al -
141
the device. If you check with your vtvm,you will find that the input terminals areslightly above ground. Westinghouse en-gineers suggest not using any externalcomponents that will alter this biasvoltage.
C2 and C3 are 10-p.F decouplingcapacitors. Solid tantalums rated at 20volts were used in the prototype, butany small low -voltage electrolyticsshould work equally well. Just be surethey are good before you use them.
Gain is controlled by a miniaturetransistor -radio volume control (R3).It can be bolted to the board that holdsthe other components. It will requireadjusting for maximum sensitivity indifferent locations because backgroundnoise level varies. It can be turned up asbattery voltage drops. Maximum gain iswith pins 4 and 8 shorted (minimumresistance)
Output transformer T2 is a specialmodulation transformer for a 27 -MHzCB transceiver (available from Lafay-ette Radio Electronics Corp., 111 Jeri-
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-if the background -signal voltage acrossCl falls below the relay dropout pointwithout it. This will depend on severalfactors and can he determined only byexperiment.
If the background noise level ishigh enough to trip the relay with R2out of the circuit, connect a variable re-sistor across CI and reduce the resist-ance gradually while observing the re-sult with the vtvm. Choose a fixed re-sistor closest to the resistance that willhold the voltage across Cl to below therelay dropout point of about 0.2 volt.R2 should be from 10 to 50K ohms. Ifit is less than 10K try a different tran-sistor and use a rolloff capacitor acrosspins 5 and 7 of the IC.
Any resistance across Cl acts as ableeder and reduces the sensitivitysomewhat. Adjust RI for the requiredholding period. If you need an adjusta-ble time delay, use a 50K pot and se-ries limiting resistor for RI (Fig. 2).If you do not need the variable de-lay, make a temporary hookup like Fig.2 to determine the value you want andthen substitute the closest value in afixed resistor. RI is not shown in thephotos
low both to dry for a few minutes untilthe cement is tacky, then carefully andfirmly press the IC against the board.
Carefully straighten and press eachlead against the circuit board so that allare held down by the cement. Leads areidentified by the black dot next to pinlead No. I. Use phono pickup arm wirefor connections. Use the smallest sol-dering pencil when soldering IC leads,get it hot and work fast.
Use nothing but the finest grade ofrosin -core solder. First, tin all leads tobe joined; then sweat them togetherwith a quick dab of the hot iron. Aclamp -on heat sink is handy for holdingwires in place while soldering. Practiceon some scrap pieces of wire until youcan make a good clean sweated jointfast. Just a quick touch of a hot iron isall you need. After all the connectionsare soldered to the WC 183G, coat itwith more cement to anchor everythingfirmly.
Connect the battery with the rightpolarity! IC's cost money. If you goof,you pay! Check all connections beforeyou install the batteries.
Testing and adjustingConnect your vtvm across Cl and
set it to the 1.5 -volt dc range. Turn SIoff. Turn S2 on and take a no -signalvoltage reading across Cl with RI andR2 not yet in the circuit. This readingshould be less than 0.1 volt. Now turnon SI and advance the gain control.You will reach a point where the relaytrips on background noise With thespeaker or microphone disconnected,you should be able to trip the relay bybringing your hand close to terminal 3or 9, without actually touching either ofthem. (Without a mike, this devicemakes a good "touch" relay.)
Observe the voltage levels at whichthe relay pulls in and drops out. Pull -inshould occur at 0.28 or 0.3 volt, anddropout at 0.20 to 0.22. If necessary,carefully adjust relay spring tension toset the pull -in point. Remember that in-creasing the spring tension will alsoshorten the time cycle You can omit R2
To avoid talking yourself hoarsewhile making adjustments, connect asmall speaker to your audio generatorand use it as a sound source. This willgive you a variable source of low-levelaudio. Keep the frequency in the rangeof normal speech, under 3,000 Hz Ifyou have trouble with high -frequencybackground noise, install a rolloff ca-pacitor (.05 p.F or less) across IC pins 5and 7. This capacitor is not shown onthe wiring diagram: it will not usuallybe required. and it reduces sensitivitysomewhat
When checking and selecting com-ponents, choose a transistor with lowleakage and a diode with a high for-ward -to -back resistance ratio. Cl mustalso have low leakage, and it is desirableto form it by connecting it overnightacross a 1.5 -volt battery. (Observe po-larity!)
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relay contacts to bypass them when thesound relay is not in use
143
By connecting a pair of head-phones across the 8 -ohm secondary ofT, you can monitor the signal and checkfor noise, motorboating, hiss, hum andlack of response from the amplifier.The headphone load will affect relay op-eration, so disconnect the phones whilesetting up the timing and using thesound relay. You can use the 8 -ohm out-put to drive a miniature speaker also.
To make a recording with thesound relay, set the tape recorder onRECORD and adjust its volume correctly.Everything must be set ready to go. Youwill probably have to cut one of thebattery leads to your recorder and run apair of wires to the sound -powered re-lay contacts The regular switch in therecorder will then be left on all the time.Add a shorting switch across the sound-
You may also want to install a
switch to stop the recorder when thetape is all used up. On recorders inwhich the on-off switch is not a part ofthe record-playback transfer, all youwill need to do is to hook the sound -relay contacts across the power switchIf you do that, include another switchin series to disable the sound relay incase the points should stick. This willalso enable you to stop the recorderwithout disconnecting the sound relay.
All recorders take a moment to getup to full speed. Experience will tell youhow to work with yours. You may needto say "uh" into the mike while therecorder comes up to speed.
in
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MUITIPTEX GENERATOR
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SUPPRESSEDSTEREO SUBCARRIER
RELATIVEAMPLITUDE
PLOTBO L -RSUBCAFR1ER SIDEJ3ANDS
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15 19 23 38MODULATION FREQUENCY- kHz
Fig. 1-FREQUENCY SPECTRUM in afrequency modulated stereo broadcast.
Theory of operationA basic understanding of FM stereo
transmission will aid the user in understanding the. generator operation and de-sign theory The FCC controls themethod of stereo transmission. The base -band spectrum is shown in Fig. 1. Themonaural information or average lettplus right (L±R) signal is contained inthe 0 to 15 kHz useful audio range. The
144
ON
PART 5
Test & Measuring Equipment
FM STEREO MULTIPLEX GENERATOR
The stereo generator described hereis designed for low cost and simplicity. Itcompares favorably to commercial unitsin stereo performance, but does nothave self-contained audio signals or ii.test signals. Only a 1 -kHz signal and ageneral-purpose scope are required foraligning this generator. An FM receivershould also be available. The generatorprovides over 30. dB stereo separationbetween 300 and 20,000 Hz. Total partscost is around $50, but many parts arecommon and can be found in your spareparts box.
This unit does have an rf output tocheck an entire receiver's performancewithout an external rf generator. Thehigh -frequency oscillator works at 106±2 MHz and is frequency modulatedby a variable -capacitance diode. A regu-lated power supply is incorporated andgives exceptional rf stability.
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C10-30 0F, 15 volts, tantalumC11, C13, C17-100 F, 6 volts, tantalumC21-4300 pF, 5% micaC25, C26, C27, C28-1000 pF, 2%. micaC34, C39-1000 pF, ceramicC36-15 20 volts, tantalum037, C38-33 pF, 5%, micaC40-100 pF, 5%, micaC42-500 ,,F, 30 voltsC22-820 pF, 5%, micaT1, T2 -Three 60 -turn windings on CF111-06
core (see text), Newark Electronics StockNo. 59F1510. $2.05
T3-24 volts, 0.3 amp transformer (BA.#13A903)
T4 -3 -turn primary, 1 turn secondary, #26enameled wire, 3/16" dia form, brass slug
Sl, S2, S3-spst toggle switchD1, D2 -1N276 diode, HughesD3, D4, 05 D6 -1N3064 diode, TID7 -1N536 rectifier, G.E.138-1N965B, 15 -volt Zener, I.R.D9 -1N5234, 6.2 -volt Zener, Mot.D10 -1N5140, Mot. or MV1624 (see text)Q1 -2N2222, Mot.Q2, Q3, Q4, Q5, Q6, Q7, Q8, Q9, Q10--
MP52923, Mot.Q11-MPS404, Mot.Q12-MPS918, Mot.Q13 -2N2218, T.I.IC-1-MC1531G op amp Mot. or MC1709C
(see text)J1, J2 -phone jacks.13, J4-teflon feedthru35, .16..17-bannana JacksF1-2/8 amp fuseCase -3" x 5" x 7" aluminum minibox
(Bud CU3008A)Miscellaneous: 12 pin printed circuit connec-
tor, 300 ohm twinlead, fuse holder, 1/16"single sided p.c. board 4'%x6 inches
This subcarrier was not included in thisgenerator since it has nothing to do withaligning stereo separation of a receiver.
The composite baseband signal shownin Fig. 1 now frequency modulates therf carrier between 88 and 108 MHz at amaximum deviation of ±75 kHz peak.Broadcast stations use pre -emphasis ofthe audio inputs which increases thelevel of audio frequencies above 2 kHz.This makes a de -emphasis necessary instereo FM receivers to restore correctfrequency response. This pre -emphasisat the transmitter occurs before multi-plexing. In other words, the subcarrierspreviously mentioned are not pre -em-phasized. This makes it unnecessary toinclude pre -emphasis in a stereo generator which is used for alignment pur-poses. If pre -emphasis is desired, it canbe applied to the audio input signals be-fore they are fed to this generator.
RI, R49, R52, R71-4700 ohmsR2, R22, R31, R43, R46 -potentiometer,
10,000 ohms, linear taperR3, R4, R12, R40-6800 ohmsR5, R37, R38-56,000 ohmsR6, R35, R36-5600 ohmsR7, R9, R17, R33, R34, R45, R47, R48, R50,
R54-10,000 ohmsR8, R18, R21, R23 R29, R70 R73, R75, R76-
1000 ohmsR10-15,000 ohmsRII, R13, R68-470 ohmsR14, R15-1200 ohmsR16-22,000 ohmsR19-680 ohmsR20, R28, R30-1500 ohmsR24, R25-47,000 ohmsR26-100 ohmsR27, R42, R60, R61 R62, R63-560 ohmsR32 R41-820 ohmsR39-2700 ohmsR44 -potentiometer, 50,000 ohms, linear
taperR51, R59 -potentiometer, 5000 ohms, linear
taperR53, R64, R65, R72-330 ohmsR55-3900 ohmsR56-100,000 ohmsR57-51 ohmsR58-390 ohmsR66-27,000 ohmsR67, R69-2200 ohmsR74-75 ohms, 5 wattsCl, C2-1500 pF, 2%, micaC3-3000 pF, 2%, micaC4, C19, C24, C35, C41-.01 ,,F, ceramicC5 -56µF, 15 volts, tantalumC6, C7, C20, C29, C31, C32-0 1 ceramicC8, C12, C15, C18, C23, C33-5 F, 6 volts,
tantalum
L -R information required for stereodemodulation is transmitted as an ampli-tude modulated 38 -kHz suppressed -sub -carrier signal. This will result in fre-quency components -±-15 kHz around 38kHz, or 23 to 53 kHz. The 38 -kHz sub -carrier must be reinserted in the correctphase at the receiver to obtain the complete stereo information. To do this, alow-level 19 -kHz pilot signal is trans-mitted which has a definite phase rela-tionship with the orginal 38 -kHz sub -carrier at the transmitter. This pilotphasing is one of the most importantproperties of the transmitted compositesignal and is discussed in greater detailin the section on alignment procedures.
A few FM stations also transmit a 67 -kHz storecast subcarrier. This signal is afrequency modulated subcarrier occupy-ing the spectrum from 59 to 75 kHz.
146
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Circuit descriptionTransistor Q1 in Fig. 2 operates as a
19 -kHz oscillator with the exact fre-
Fig. 3-PILOT SUBCARRIER signal gen-erated in 13 -kHz Twin -T oscillator Q1.
quency of oscillation determined by thetwin -T feedback network. The twin -T net-work makes a very stable oscillator onceR2 is adjusted for 19 kHz. The 11 -voltpeak -to -peak output of the oscillator (Fig.3) is fed to a phase -shift network throughbuffer amplifier Q2. The phase -shifter isalso isolated from the summer throughbuffer amplifier Q3. The phase -shifter pro-vides a means of correcting phase relation-ship between the 19 kHz pilot and the 38 -kHz subcarrier. The range of phase shiftavailable is about 120°. A maximumphase shift variation of 90° is necessary.
Transistor Q4 is the doubler amplifierwhich delivers two equal but oppositepolarity signals to diodes DI and D2which full -wave rectify the 19 -kHz sig-nal. Germanium diodes are used forhighest rectification efficiency. This dis-torted waveform (shown in Fig. 4) is
Fig. 4-DISTORTED SIGNAL resultswhen 19 -kHz subcarrier is rectified.
passed through an active bandpass filterconsisting of Q5, Q6, and Q11. This is atwin -T filter which has excellent selectiv-ity. The filter output is shown in Fig. 5.
The two audio inputs representing leftand right channels must be conditionedto give L+R and L-R signals forproper stero transmission. The left andright inputs are fed to the summer, Q10,
Fig. 5-THE STEREO SUBCARRIER iscleaned up by sharp 38 -kHz Twin -T filter.
which is a feedback amplifier that pro-vides greater than 20 dB isolation be-tween L and R inputs. The actual isolaLion is partially uetermined by thegenerator internal impedances used tofeed the L and R signal inputs. Sourceimpedances below 100 ohms should beused to give greater than 40 dB isola-tion. Transistor Q7 is operated as an in-verter to give -R at its output. This issummed with L by Q8 to give R-L atits output. R31 and R22 must be ad-justed to give proper stereo separation.
The R-L signal is used to modulatethe 38 -kHz subcarrier delivered by Q11.This is done -with a ring modulator withdiodes D3-D6. This type modulator isdoubly balanced with two center -tappedtransformers so the 38 -kHz componentis canceled at the output as desired. Theoutput of the modulator consists of theamplitude modulation components ofthe 38 -kHz suppressed subcarrier. Resis-tors R60-R63 are used to improve bal-ance or carrier suppression. The carrieris more man 40 dB below the Input,which is adequate.
Transformer Tl must respond past 50kHz without any resonances. Most com-mercial audio interstage transformers donot meet this requirement. Therefore, aspecial toroid transformer was designed.The construction details for this trans-former are in Fig. 6
Transformer T2 can be identical toTl. I used a commercial 2:1 centertapped transformer instead. At audio fre-quencies below 300 Hz the reactance ofT2's primary introduces an undesiredphase shift which results in decreasedstereo separation. This can be improvedonly at the expense of a larger trans-former winding or by reducing R65 andR64 which requires more transistor cur-rent. A higher current transistor couldbe used for Q9.
147
The total composite modulation con-sists of L+R audio, L-R informationon the 38 -kHz suppressed subcarrier,and the 19 -kHz pilot signal. These aresummed together by operational ampli-fier IC1. An op amp is used to givegreater than 60 dB isolation between thethree inputs. Summing resistors R47,R48, and R66 can be varied to adjustthe individual gains of each input. Theindividual gain is equal to R54 dividedby the series resistor. I used anMC1531G which is rather expensiveNewer and cheaper op amps are avail-able, such as the MC1709C, and can beused instead.
The summer output modulates vari-able -capacitance diode D10, which ispart of the resonant tank circuit of the100 -MHz oscillator. The diode, a Moto-rola 1N5190 Epicap, makes this a volt-age -controlled oscillator which results inan FM signal output. The center fre-quency of the oscillator can be variedabout ±2 MHz by adjusting the slug inT4. My unit oscillated between 102 and106 MHz. The exact frequency will beaffected by stray wiring capacitance andlayout. Be sure to prevent ground cur-rents from modulating the oscillator.This is the reason for the decouplingconsisting of R68, C35. and C36. Theoscillator is especially susceptible to 19 -kHz ground currents. Too much leakagewill cause the pilot to be transmittedeven though R46 is set at minimum. Theoscillator output can be suitably loadedwith a 300 -ohm twin lead. Diode D10can be any of a number of 10-pF unitsnow available One low cost unit is theMV1624 ($1.42). The difference in op-eration between it and the 1N5140($5.85) will be a slight difference inmodulation sensitivity but should benegligible.
A regulated dc power supply and anac rectifier circuit is used. The Zenerdiode reference provides a low outputimpedance. This is necessary to keepboth oscillators at their correct fre-quencies and prevent power line modu-lation of oscillator Q12.
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Construction and alignmentThe circuit layout is not critical ex
cept for the 100 -MHz oscillator, forwhich component leads should be kept
148
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to an absolute minimum. A piece of300 -ohm twin lead connects the os-cillator coil to the feedthru terminals inthe chassis. The circuit board is laid outso it can be unplugged from the chassis.It is held in place by one screw and athreaded standoff to the chassis.
The first step in aligning this gener-ator is to set the' frequency of the 19 -kHz oscillator. The easiest way to dothis is to monitor the signal at J1 with anelectronic counter while adjusting R2.Another way was devised which doesn'trequire a counter. A 19 -kHz signal isavailable in all multiplex demodulatorswhen the FM radio is tuned to a stationbroadcasting stereo. This signal can beused so compare with the generator pilotfrequency using the simple phase detec-tor shown in Fig. 7. R2 is adjusted untilthe phase -detector output contains alow -frequency beat note. This oscillatorcan be adjusted for a beat note ofaround 2 Hz which is plenty accurate-meaning the two frequencies are within2 Hz of each other. One of the phasedetector inputs should be greater than 4volts p -p to turn the diodes on. The
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Fig. 9-MODULATION WAVEFORMS. a-Left-only or right -only signal with 1 -kHzinput at zero pilot level. 13- and c.-Waveforms at J2 with right or left in-puts at 500 and 5000 HZ, respectively.
double exposure photo in Fig. 8. Nowthe pilot signal transmitted is in the
I.5K I.5K proper phase relationship for stereodemodulation.
The audio mixing circuits must nOwbe adjusted to give proper stereo separa-tion. First apply a 1-knz signal to theleft channel. The pilot level is turned tominimum. The L+R and L-R switchesmust be closed With the scope input atJ2, adjust R31 until one of the 1 -kHzenvelopes is minimum in amplitude asshown in Fig. 9-a. Figures 9-b and 9-c
chassis cover should be in place for thistest in case stray capacitance changes theoscillator frequency. This requires an ac-cess hole in the side or back of the unitfor adjusting R2.
Next, the 38 -kHz filter is aligned byadjusting R51 for maximum output atQ1 l's collector. Oscillation in this typeof active filter is possible. Therefore, the19 -kHz oscillator should be disabled byshorting R7 and checking to see that nosignal appears at Q11. The ac voltage atOil's collector should be near 10 voltsp -p and can be adjusted by selecting thevalue of R41.
The next step is to adjust the phaserelationship between the pilot signal and38 -kHz subcarrier To do this, you musteither sync the scope on one of the sig-nals or use a chopper input on the scopeif available. The chopped input methodallows direct viewing of both waveformssimultaneously. I used another method.The scope is externally synchronized bythe 38 -kHz signal at test point TP-1.Then the 38 -kHz waveform here isviewed on the scope and the scope set-tings adjusted so the sine -wave zerocrossing occurs at the center of the grid.Next the scope input is placed at J1 andR3, R4 are adjusted for a zero crossingof 19 -kHz at the same point as the 38 -kHz waveform. This is shown by the
The only calibration remaining is the19 -kHz pilot level required to give theproper oscillator deviation. I determinedthis with the help of an FM stereo re-ceiver. For this measurement the L+Rand L-R switches are in the OUT posi-tions. Somewhere in the receiver multi-plex you can check the 19 -kHz levelbeing received from the discriminatorwith a scope. Knowing this level, youcan receive the signal from this multi-plex generator and adjust the pilot levelto equal that from a broadcast station.The exact level is not extremely impor-tant but some multiplex demodulatorsare sensitive to pilot level as separationis affected. I found that 0.2 volt p -p 19kHz at J1 output is correct and shouldbe accurate enough for alignment. Atthis point the knob on the PILOT LEVELcontrol was adjusted to read "CALI-BRATE." I found it convenient to placean access hole in the chassis to permitexternal adjustment of T4's slug. This al-lows changing the oscillator frequencyto a spot where no local station is oper-ating.
COMPONENTS ON PLUG-IN BOARD arevisible when the rear cover is removed
Using the generatorThe composite modulation output is
used mainly for applying the stereo sig-nal to a multiplex demodulator forchecking it separately. For this, the 19 -kHz level is not calibrated and must beset to a predetermined level which thediscriminator will supply from broadcaststations. This is also true for the remain-ing composite signal level. The audio in-put levels to the generator must be keptbelow about 1.5 Vrms or saturation oc-curs.
Receiver separation versus fre-quency can be checked between 300 Hzand 20 kHz with this generator. Thisalso provides a convenient way to deter-mine if the receiver de -emphasis circuitis correct. The easiest way to measurereceiver separation is to apply an audiotone to only R or L of the generatorand measure the ratio of outputs onL and R at the receiver
The rf VCO in this unit is very con-venient. It lets you check the effect ofi.r. amplifier selectivity on overall sepa-ration. It also lets the technician adjust amultiplex unit incorporated in the radiowithout disconnecting it. A direct con-nection from the rf output is not re-quired as the receiver will pick up thesignal from a few feet away.
This stereo generator will result in aprofessional alignment job on any FMstereo receiver. Its relatively low costand versatility make it a valuable pieceof test equipment for any technician.
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can observe the input and output to-gether, instantly compare these signalsand note if distortion or phase shift ispresent. For working with electroniccounters and other digital circuits, adual -trace presentation is almost in-dispensable to find out the relationshipof one flip-flop output to another.
You might also use the dual tracefor checking a modulation signalagainst a modulation envelope or fordemodulation while testing a radiotransmitter. Uses are limited only bythe frequency response of the dual -trace switcher and oscilloscope. Insome cases the frequency limitationsmay be circumvented by converting ahigh -frequency rf signal to a lowerfrequency within the bandpass of theamplifiers.
This switcher may be constructedin a small box external to your scope.It is connected to the scope as shown inFig. 1. The two unknown signals are
TO THE INDUSTRIAL ELECTRONICSmaintenance man, the experimenterand the service technician the value ofthe oscilloscope is unquestioned. Butits role in sniffing out faulty ampli-fiers, measuring phase shift andanalyzing circuit operation can begreatly enhanced by adding a dual -trace switching unit.
Modern laboratory oscilloscopes,in the thousand dollar bracket, includedual vertical -input amplifiers with achoice of chopped -trace or alternate -trace sweeps, while some actually havedual -beam CRT's.
The cost of a laboratory versionis not practical for most of us, butyou can inexpensively add a choppedtrace capability to your present scopeat the vertical input.
With the dual trace, or dualsweep, you will be able to look at twowaveforms simultaneously. For in-stance, when testing amplifiers you
152
EXTSYNC
SYNC MAY BECONNECTED TOA, B, OR OTHERSOURCE.
Fig. 1-Inputs to switcher are combinedand fed to scope vertical input Ex-ternal sync is taken at input or source.
Fig. 2 -a --Trigger pulse in upper tracedrives a multivibrator whose output(bottom) can be viewed simultaneously
Fig. 2 -b --Top trace is a sine -wavemodulating signal, bottom trace is thedemodulated signal from rf transmitter.
Fig. 3-Heterodyne zero beat in FM sig-nal (vertical lines from negative peak).
SCOPE
DUAL TRACESWITCHER
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The superimposed sine -wave isthe modulating signal that gives reference points for tuning to the max-imum positive peak, maximum nega-tive peak and carrier of the signal. In
this way the peak -to -peak deviationcan be measured with a heterodyne
applied separately to the two inputsThe switcher output goes to the singlevertical input of the scope, and a syncline from one of the inputs is taken tothe scope's external sync input
Frequency response of the inputamplifiers is 300 kHz over the rangeof the gain controls. With the gaincontrols wide open so no attenuationof the signal takes place, the fre-quency response is up to 1 MHz. Thedual -trace switcher is easier to use ifthe scope has a triggered sweep tomake syncing of the trace independentof the input frequency.
Two examples of dual -traceswitcher use are in Fig. 2. Fig. 2-a isan oscillogram of a trigger pulse onthe upper trace, which drives a one-shot multivibrator giving an outputshown on the bottom trace. The sweepspeed is such that the chopping tran-sients appear as a broadened base.
A second example, Fig. 2-b, is anoscillogram of a sine -wave modulatingsignal on the top trace and a demodu-lated signal on the bottom trace of theoriginal modulation taken from an rftransmitter.
Fig. 3 shows a heterodyne zerobeat in an FM signal that appears as a
narrow band of horizontal lines run-
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Other partsTl-Triad F 94X transformer or equiv.S1-spdt switchMISC-3% x 7 x 31/2 -inch cabinet (LMB W -2C),
ac cord, knobs, terminal postsKit with PC board and semiconductors only is
available for $10.00 from Solid StateServices, 1720 Kimberly Dr., Sunnyvale,Calif. 94087
SemiconductorsDl, D2 -1N3064 diode or equiv.D3, D4 -1N4002 diode or equiv.Ql, Q2 -2N5163 transistor, Motorola or Fair-
child (selected for 1pss match)Q3, Q4 -2N3638 transistor, Motorola or Fair
childQ5, Q6 -2N5134 transistor, Motorola or Fair-
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emitter of Q3 is at a potential belowits base and is therefore off. At thesame time, Q6 is off and its collectoris "floating" so that no current flowsin load resistor R19. The emitter ofQ4 is, therefore, 0.6 volt higher thanthe base and this transistor is oper-ating in its normal mode. The outputfrom either Q4 or Q3 is developedacross R12, depending on which tran-sistor is on. The output voltage acrossR12 is fed to the input of the os-cilloscope in the setup.
Construction and useMost of the circuitry can be built
on the same -size printed circuit boardshown. The entire circuit may bemounted in an aluminum box. Fig. 5shows an inside view of the box. ThePC board is mounted vertically on theback panel. The output jack is on therear panel, where it is convenient toattach the vertical input of the scope.
After assembly has been com-pleted, an oscilloscope should show asquare wave of approximately 20 voltspeak -to -peak at 40 kHz on the collec-tors of Q5 and Q6.
Next, connect a generator (eithersine or square wave between 10 Hzand 100 kHz) to both inputs for afunction check. The scope sync shouldalways be obtained externally asshown in Fig. 1. A good sync sourceis at either input or the generator. Thescope sensitivity should be set at ornear maximum (0 5 volt p-p per inchor better).
All resistors 2/2W, 10%R1, R2-47,000 ohmsR3 -25,000 -ohm linear potentiometerR4, R5 -100,000 -ohm linear potentiometerR6, R7-4700 ohmsR8, R9, R12 R13, R15, R18-2200 ohmsR10, R11-820 ohmsR14, R19-2700 ohmsR16, R17-10,000 ohmsCapacitorsCl, C2-0.47 pF, 600 voltsC3, C4-10 µF, 15 voltsC5 C6-.002 µFC7-.05 }IFC8-250 µF, 30 volts
converter, where the converter os-cillator is on a low frequency that canbe easily measured. The oscillator de-viation times the harmonic number isthe measured signal's deviation. Theamplitude of the rf varies in the os-cillogram, not because of modulation,but because of the bandpass of thelow -frequency amplifier used after theconverter.
A schematic diagram of the dualtrace switcher is shown in Fig. 4. Twoinputs are attenuated by potentiome-ters R4 and R5 to a level within therange of the switcher's amplifier cir-cuit. The signal is amplified by the in-put FET's, Q1 and Q2, and applied tothe bases of Q3 and Q4, which act asswitches. These are alternately gatedon and off by the gating waveformsfrom Q5 and Q6. (Note that the diagram does not indicate the drain andsource terminals on Q1 and Q2. Thisis because the devices are bidirectionaland may be placed either way in thecircuit.
Transistors Q5 and Q6 are afree -running multivibrator whose fre-quency is determined by R16, R17,C5 and C6. Diodes Dl and D2 allowthe voltage at the collectors of themultivibrator to rise very rapidly andthereby improve the switching speed.
As an example of how switchingis accomplished, consider Q5 on; itscollector is therefore at ground. The
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Same size PC board and component placement are below.
C6
C7 R12
Note that the line between the two inner -most inputconnectors must go to plus 20 volts as do points "K"on PC board. Points "e" on board are for ground con-nections.
=
155
0 0
0 K
Operation of the position control controls R4 and R5 may be varied toshould move the two traces on the produce a convenient display size onscreen to any desired position. Gain the scope
SCOPE CAMERA
MOST EXPERIMENTERS HAVE AN OSCIL-loscope, yet nothing is quite so rare asthe experimenter with an oscilloscopecamera to keep a permanent recordof the results of his experiments. Onereason for this is the cost-a com-mercial camera designed for scopephotography will make a $400-$1200dent in the wallet, and can only takepictures of the scope screen. Muchas we appreciate the value of thispiece of equipment, few of us canjustify the expense.
This section describes the con-struction of a simple, inexpensivescope camera which, like Superman,is often disguised as a mild-manneredcamera for all the ordinary uses. You
by DALE E. COY
can buy a Polaroid Swinger for about$15. or the Big Swinger for around$20. Conversion to a scope camerawill cost another $5
Three things are required to con-vert these cameras for scope photo-graphy; modifying the shutter, adding asupplementary lens;, and a method formounting the camera to the scope.
Shutter modificationsThe shutter mechanism for these
cameras has one fixed speed of about1/200 second, which is much too fastfor scope photography. An added"bulb" position is needed With thistype of action, the shutter will openwhen the shutter button is pressed,
Fig. 5-Chassis view with cover removed from the case shows PC board mounted onthe back panel. You may want to mount the dual-trace parts inside your scope.
156
Fig. 2-Inside view of modified shut-ter. The tiny arrow points to bend madein shutter to provide a "bulb" position.
Figure 2 shows the inside of theshutter, as modified. The only changeneeded is to bend the end of theshutter leaf upward Make the small-est bend you can with needle -nosepliers. Then file the bent part downso that about Ms inch extends upward.
Modify the plastic shutter hous-ing next. Locate the spot on top ofthe housing as shown in Fig. 4, anddrill and tap for the stop screw to beused. This screw must be at least3/4 inch long (thread length), and nolarger than No. 4. If you can find aNo. 2 or 3 screw this long, it is even
Fig. I-Rear of the modified shutter.(A) is battery contact, (B) marks twoscrews holding shutter together. Lensopening is in the center of the plate.
157
Fig. 3-Modified shutter housing.Added screw (arrow) stops the shutterleaf. The screw should he flat and musthe at least 3/4" long (thread length).
and will stay open as long as the buttonis held down. Since most scope photosrequire exposure times between 1 and10 seconds, this is an easy shutter ac-tion to use. As an added bonus, theshutter mechanism does not have tobe cocked between exposures, so mul-tiple exposures may be made on eachpiece of film to place more than onetrace on the picture.
To get to the shutter, first openthe back of the camera and removethe battery compartment. Three smallPhillips -head screws hold the lens -shutter housing to the rest of thecamera Two of them are chrome, andthe other is black. Removing all ofthem lets the front of the camera dropoff. Place the front housing on a cleancloth so the lens will not be scratched.Remove the plastic rear cover plateand the flashbulb ejector from theback of the shutter housing. Youshould now see something like Fig. 1.
Although Fig. 1 shows the com-pletely modified mechanism with theshutter open, it also illustrates thenext operation. Remove the twoscrews at B. Grasp the tab at C withneedle -nose pliers and pull gently up-ward until the plate snaps free. Thenslide the plate out from under thebattery contact at A, and turn it over.
10
ONLY
290
159
better Carefully drill and tap the holeso the screw will lie flat, as shown inFig. 3. Since the top of the housingis angled, the hole will also have tobe angled. Now insert the screw, andmake sure it does not bind against thesliding blades as the red "brightness"knob is turned.
Put the pieces back together,pressing in at C (Fig. 1) to snapthe plate back on. Install the screwsPress the shutter release a few timesto check for binding inside. If thereis some scraping or binding, take the
shutter apart again. Binding may becorrected by one of three methods:(1) filing the threads off the stopscrew where it is inside the housing,(2) filing some more off the bentpart of the shutter leaf, or (3) usingthin shims or washers between themetal plate and plastic housing underthe two screws at B in Fig. 1.
A few trial assemblies will correctany binding. When the shutter is as-sembled and working properly, holdthe shutter release down and adjustthe stop screw so that the shutter isfully open. For this operation, thered knob should be fully clockwise(away from the "darken" end).
This completes the hard part ofthe modification. You may want tomark the stop screw, or install a nuton the screw so that it can alwaysbe turned in to the same depth Byremoving the screw and replacing itwith a shorter screw (1/4 inch or less),the shutter is returned to its usual
''SMALL SWINGER"FILM HEIGHT 54mm
250
10
DISTANCE
DRILL HERE --'----..:)R FLASH
Fig. 4-Drill and tap for the stopscrew between the D and I in DISTANCE.
LimWI--
Cf) LijWI -I(-) -IZ6-150
AO
5 _130=Ci 120I "BIG SWINGER"1- 110 FILM HEIGHT 73mmuw, 4 100m0 90
380
70
60
2- 5050 150 200
VALUE OF "S" IN MILLIMETERS
3 5 6 7 8VALUE OF "s IN INCHES
Fig. 5-Graph showing value of "S" (the focal length of the supplementarylens required and the focusing distance of the lens combination) for the 100 -mmSwinger lenses. The actual focusing distance will usually be about 10% lessthan Indicated on the graph.
POLAR 0I. 14 :". C ... I. . ..
operation and the camera wil worknormally.
Polaroid Corp. jealously guardsthe lens specifications for thesecameras. A rough measurement shows,however, that the Swingers have f/16lenses with a focal length of about100 millimeters. Most lens measure-ments are traditionally made in milli-meters (mm). There are about 25.4mm to the inch, so the focal length of100 mm means that the distance fromlens to film is just shy of 4 inches Asupplementary lens is needed to letus focus the camera from a distanceof something less than a foot, and tofill the picture with the image of thescope's CRT. Optics is a complicatedsubject, but since "rough" figures areall we need, we can use a "rough"formula:
place to obtain lenses is probably Ed-mund Scientific Co. You can obtaintheir free catalog by writing to 300Edscorp Building, Barrington, N. J.08007. The catalog lists a wide varietyof planoconvex and positive meniscuslenses with prices around $1, and a lot
160
(Lens Focal Length) X
S(Object Height)
Film Height
In this formula, the object height isthe height of the part of the CRTscreen we want to photograph. Thefilm height is the top -to -bottom mea-surement of the film image (about54 mm for the small Swinger and73 mm for the Big Swinger). Thevalue "S" is both the focal length ofthe supplementary lens we need, andthe distance from the front of thecamera to the CRT.
The object height chosen isusually not the diameter of the CRTbecause the pattern to be photo-graphed usually does not go all theway to the top of the tube. Instead, aslightly smaller height is chosen.
To simplify the problem, Fig. 5is a graph from which the correctsupplementary lens may be chosen.The other necessary information isthe diameter of lens needed. For theSwingers, the supplementary lensshould be at least 20 mm in diameter,with the most convenient diameter formounting about 28 to 30 mm The best
Fig. 6-Front of the modified camera.A plastic cable clamp holds supplementary lens in place. The added stopscrew is on the top just to the right ofcenter.
of other interesting gadgets. Their min-imum order is $2, so when you orderyour lens, also order a 21/4 x 31/4 -inchpiece of ground glass (No. 2143 for50) for use in focusing.
Mounting the lens on the frontof the camera is probably the easiesttask of all. It may be simply tapedto the camera lens tube or clampedon with a plastic cable clamp, asshow in Fig. 6.
Before mounting the camera, wemust find the exact distance at whichthe lens is in focus. With the supple-mentary lens in place, install a pieceof ground glass (or a piece of clearplastic sandpapered on one side) inthe back of the camera, where thefilm is located. Open the shutter, and
ON . IMP AO IPal SOO s. 1
Fig. 7 Test of the camera with a 238 -mm supplementary lens. Camera fo-cuses at slightly less than 9 inches andcovers a field approximately 4 x 5 in.
The actual distance from thefront of the camera to the CRT shouldbe slightly less than the distance mea-sured, because the scope image isactually formed on the inside of theCRT. For mast oscilloscopes, theimage formed is actually about IA inchbehind the front panel.
The scope mount is made fromaluminum chassis and boxes, as shown
move the camera until the image ofsome object (for instance, a ruler) isin focus on the ground glass. Thencarefully measure the distance fromthe front of the camera to the object.This is the distance that the front ofthe camera must be mounted awayfrom the oscilloscope. An alternatemethod of measurement is to take apicture of a test chart as shown inFig. 7.
i
Li
in the photo. Since scopes varyin size and style, the final design maynot match the picture shown, whichis for a 5 -inch Hewlett-Packard scope.The mount is held in place by theoscilloscope bezel, located inside thebox. The mounting screws pass throughthe bezel and through the mount, andare fastened to the front panel. Thebox is painted flat black inside to pre-vent reflections. Viewing holes withsliding covers are provided in eachside, so that the screen may be seenThe covers are closed before the pic-ture is taken. The piece of felt betweenthe two boxes shown also covers a slotwhich may be used for viewing. Thecamera is fastened to the second boxby two angle brackets bent from alumi-num. The lens projects through a holecut in the box.
When the mounting is finished,check the focus by using a piece ofground glass inside the camera as out-lined above. If the image of the scopetrace is in focus, the camera is com-plete. Small adjustments in focus maybe made by adding felt pads betweenthe camera and the mounting, if thedistance was made too short.Using the camera
Using the oscilloscope camera issimple. Just set up the picture on thescreen, close the viewing ports andhold the shutter open for a few sec-onds. Generally, exposure times willrun between 1 and 10 seconds. Many
161
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Fig. 9 Multiple exposure made bychanging scope's input and trace posi-tion. This is an easy way to displaywaveforms when comparisons are re-quired in the lab.things affect this exposure time, in-cluding the design of the oscilloscope,the frequency of the signal displayedand settings of the scope controls.The advantage of using Polaroid equip-ment is the ability to change exposuretimes and to see the results in just afew seconds. Because of the widevariation in setups, more specific ex-posure data cannot be given here.Shooting one roll of film with yourequipment will make you an expert.
ONE OF THE MOST VALUABLE COMMOD-ities in a shop or laboratory is benchwork space. I often find some little de-vice that I want to build but never dobecause of the space it will rob frommy workbench. For some time I'veneeded an oscilloscope calibrator likethis one but I didn't decide to build ituntil I realized that the proper placefor it was inside the scope.
Space requirements are small: Afew square inches of front -panel spaceand less than 12 cubic inches of elec-
tronics for voltage and frequency cali-bration and a variable -delay triggerpulse with a full spectrum of scopeuses.
The stick -anywhere calibratorsystem consists of three units. Fig. 1shows the Box, the power supplies andthe five panel controls. With the newminiature controls and switches, aminimum of scope front -panel space isneeded. If space is not available on thescope's front panel a separate panelcan be built.
The scope camera may be usedto make a permanent record of yourexperiments, to record transients andto photograph events which happentoo fast or too slow for the eye todetect on the oscilloscope screen. Itsversatility is limited only by yourimagination. Samples of the work doneby this camera are shown in Fig. 8 and9. Quality compares favorably withequipment costing 20 to 30 times asmuch. For most pictures, the "dark-ness" control (red knob) on thecamera should be set fully clockwisefor the brightest picture. For extremesharpness, you can rotate this controlcounterclockwise a bit, although thiswill cause an increase in the exposuretime needed.
This inexpensive scope camera issimple to build and performs well. Ifyou are reluctant to do the cameramodification yourself, most camerarepair shops will do it for a fewdollars. By removing the supplemen-tary lens and the stop screw, thecamera is returned to service takingsnapshots. This kind of bargain is
hard to pass up.
By JAMES ROBERT SQUIRES
3 -WAY SCOPE CALIBRATOR
R27 R28 R32 S3
C8
R29
R23 R24 R25 R26
R22
R30
Actually, the Box (Figs. 2 and 3)provides a calibration pulse variable inboth frequency (300 to 3000 Hz) andvoltage (16 Vdc to 25 Vdc), and adelayed trigger pulse. The delayed -trigger -pulse generator can be syncedto either the internally generated cali-brator pulse or to a negative -goingexternal sync pulse of 30 volts or less.The external sync pulse may be a sinesquare or triangular wave. Fig. 4 givespulse relationships.
The calibrator pulse is used to ex-amine scope sweeps as a general checkof frequency (horizontal time per divi-sion) and voltage sensitivity (verticalvolts per division). This calibrationpulse may also be passed through a pos-itive diode clamp and applied to theZ-axis of the scope (the cathode of the
POWER
SUPPLIES
ADJUSTMENTS -2
30 VOLTS
+6 VOLTS
4--0---i"I
CALIBRATOR BOX
ADJUSTMENTS- 2 FREQUENCY VOLTAGE
JI
P
Fig. 1-Calibrator block diagram. Sec-tions can be built into scope to save space.
E( -30V)D
PI
S4
SZ
163
K (GND)
J(+6V)
Controls may be mounted on panel scope or in a separate unit as shown here.
C7 C6 C5 C4
SI
C
TOSCOPEPOWER
INTER-FRONT PANEL
CONNECT CONTROLS
CABLE ADJUSTMENTS -5
-.. FINE PULSEWIDTH
CALIBRATEVOLTAGE
COURSE PULSEWIDTH
INT EXTSYNC
MILLIVOLTS -VOLTS
Op osr COCC W,
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gives data for using the Zener regula-tor at various voltages tapped from thescope supply. Assuming that you finda 60 -volt tap in your scope power sup-ply, you would use a 30 -volt Zener inseries with an 1800 -ohm resistor and a
MILVOLTS
C8.001
Fig. 3-Pulse-width and voltage -calibrator controls are mounted on front panel.
Cl-.05 µF capacitorC2- 002 ixF capacitorC3-27 pF capacitorC4-250 pF capacitorC5-.005 µF capacitorC6-.01 µF capacitorC7-.02 µF capacitorC8-.001 µF capacitorRI, R9, R19 -1600 -ohm, 5% 1/2 -watt resistorR2, R4, R6, R11 -560 -ohm, 1/2 -watt resistorR3 -120,000 -ohm, 1/2 -watt resistorR5 -100,000 -ohm miniature potentiometerR7, R17, R21 1000 -ohm, 1/2 -watt resistorR8 -6800 -ohm, 1/2 -watt resistorR10, R20 -4700 -ohm, 'h -watt resistorR12 -2500 -ohm miniature potentiometerR13 -5100 -ohm, %-watt resistorR14-62 000 -ohm, 1/2 -watt resistorR15 -100 -ohm, 1 -watt resistorR16 -16,000 -ohm, 5%, 1/2 -watt resistor
CRT) to provide variable blanking fortiming and sweep marking purposes(see Fig. 5).
Either a simple Zener regulatoror a more involved series and shuntregulator is the power supply. Fig. 6
4. °o.-rib
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5
ci 0IOFF
0.-2---1 02 COARSE
PULSE
WIDTH
1 1S2X 4 4 X1 0---H---0
X
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VARIABLE
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INT E X T
SYNC PULSE
H
FINE
PUR32
WIDTH looK
VOLTS 53
C4 250pF
7R22 20V2.IK
C5
Parts List
6R23 10V
I.025K
5R245V CALIBRATE
6100 4VOLTAGE
R25 2V SInon
R26 IVloon
R27600
R28400
3
2.5V
.2V
R29100K
R301000.
OUTPUT
TERMINALS ON PI
R18 -12,000 -ohm, 1/2 -watt resistorR22 -2000 -ohm, 5% 1 -watt resistor, see textR23 -1000 -ohm, 5%, 1 -watt resistor, see textR24 -610 -ohm, 5%, 1 -watt resistor, see textR25 -200 -ohm, 5%, 1 watt resistor, see textR26 -100 -ohm, 5%, 1 -watt resistor, see textR -27 -56 -ohm, 5%, 1 -watt resistor, see textR28 -39 -ohm, 5%, 1 -watt resistor, see textR29 -100,000 -ohm, 5%, 1/2 -watt resistorR30 -100 -ohm, 5%, 1/2 -watt resistorS1 -2 -pole, 7 -position miniature non -shorting
rotary switch (see lead photo)S2 -2 -pole, 2 -section, 5 -position miniature
non -shorting rotary switchS3, S4-miniature spdt toggle switchJ1-miniature 9 -contact socket connectorJ2-Insulated tip jackJ3, J4, J5-Miniature coax receptaclesQ1, Q2, Q4, Q5, Q6 -2N1303 transistorQ3, Q7 -2N428 transistorP1-Miniature 9 -contact plug connector to
match J1MISC-Transistor sockets, terminal strips,
hook-up wire, knobs
165
CALIBRATOROUTPUT VARIABLE
-30 V.
(SEE TEXT)
VARIABLE
MINIMUM DELAY 5µSEC.MAXIMUM DELAY 550µSEC.(SEE TEXT)
VARIABLE(SEE TEXT)
Fig. 4-Calibrator output waveforms. The one at a is a 1000-Hz calibration pulse;150 psec oN and 850 psec OFF. Its amplitude and pulse duration are variable. At bShe 30 -volt trigger pulse may be delayed over an adjustable range of 5 to 550 psec.
'241iiiiiiiiii
,
07
JI
0 V.
TIME HZERO
R5
J2
R12
OV.
01 Q4 Q6
R7
R3
R
R4
20V
a
R9
RIO
RI6
b
02 Q3 05
R2
Interior of calibrator box. Transistors are in sockets mounted on top edge.
Mounting board is I 7/s" x 3 5/16", with 7/4" x 3/4" corner section removed.
TIMEZERO
1 TO 5p.F 600WVDC
0,
I
INPUT TO'''.FROM CRTVARIABLE OFDELAY \ SCOPE
GENERATORIN
RS
BVL
RL cl5mA at6V
167
47K TO100K1/2 WATT
Fig. 5-The pulse output of the calibra-tor may be used for scope Z-axis blankingafter being passed through a diode clamp.
6 -volt Zener in series with a 3300 -ohmresistor.
Figure 7 illustrates a complete-30 -volt and +6 -volt combined sup-ply that could be installed within thescope. For pulse work this sort ofsupply gives much better high -fre-quency regulation and transient re-sponse than does a Zener regulator.
Panel controls are mounted wher-ever possible on the front panel of thescope. The photos show these controlsin open breadboard construction. Fol-low this arrangement if you build thecalibrator as a separate unit
A 5" x 71/2" piece of aluminumsheet is used for the chassis. You maywant to look under the dust cover ofyour own scope to see just how muchspace is available to house the cali-brator box. Location within the scopeis not critical so long as the circuit cardand other components are shielded bythe metal chassis Should you find thatyou have the room, you could purchasea larger chassis.
The circuit card is cut from Vec-tor prepunched board. As the com-ponents are inserted into the board,their leads extend through the card tothe other side and are used in wiringthe circuits.
Transistor sockets are mountedon the Box chassis with mountingrings. Orient the sockets with theemitter pin nearest the bottom or openside. This facilitates testing when theunit is completed Use No. 24 multi -strand wire to connect the transistor
sockets to the circuit card to minimizewire breakage during assembly.Calibrator box wiring
First connector J1 is wired, usingnine 8" lengths of stranded wirecolor -coded as shown in Fig. 2. Solder5" lengths of wire to the transistorsockets and fold out of the way for thepresent. Mount and wire the two po-tentiometers and test joint J2 as com-pletely as possible.
-30 V. SUPPLY
VS
40V60 V80V100 V
+6V SUPPLY
40V60 V80 V100 V
ZENER CONNECTEDAS SHOWN Rc 15 mA at 30V
Rs
600n,1 8 K
3.0K4.2K
ZENER LEADSREVERSED2 OK33K4 2K6.2 K
RL
1z MIN.2m A
VS FROM SCOPEPOWERSUPPLY
Fig. 6-How to obtain Zener-regulated-30 and +6 volts from scope's powersupply. The Zener's cathode always con-nects to the positive side of the source.
Next the components are insertedinto the circuit card and wired accord-ing to the photograph.
When the card has been wiredand is ready to be mated to the Box,it should be placed component sidedown above the transistor sockets. Allwiring between J1. the circuit card, thesockets and the potentiometers shouldbe made from this position. Wiring thecard in this manner permits the cardto be swung out of the way for servic-ing When all wiring is completed,tuck any loose wires under the cardand assemble with two 6-32, ,72."
screws.
5
168
When choosing a place to mountthe calibrator, remember that the fre-quency and voltage controls must beeasy to reach and adjust. The inexpen-sive transistors used in this unit mustbe kept away from heat. Heat arrives
R2 3911 2W
D2
40 CT100mA
CI1000µF/50V
T2 D4
26.8V CT IA
Parts ListT1-Power transformer, 40-V ct. 100 mA
(Triad F90.X or equiv)T2 Power transformer, 26.8 V ct, 1 amp
(Triad F40 -X or equiv)Dl, D2, D4, D5 -1N1692 diodeD3, D6 -6-7-V, 150-mW Zener diode Hoff-
man RS -6 or equiv)C1-1000-0, 50-V electrolytic capacitorC2-2000-0, 15-V electrolytic capacitorR1 -1000 -ohm, 1/2 -watt resistorR2, R6 -39 -ohm, 2 -watt resistorR3 -7500 -ohm, 1/2 -watt resistorR4 -1000 -ohm, 2 -watt potentiometerR5 -1600 -ohm, 1/2 -watt resistorR7 -430 -ohm, 5%, 1/2 -watt resistorR8 -500 -ohm, 2 -watt potentiometerQ1, Q3 -2N301 transistorQ2 -2N1303 transistor
02
R5D3 1.6K
R6 R739 430112
+ GND.
03
QI
C2 2000µF/15V
R4I K2WPOT
-GND
by conduction. convection or radia-tion, so mount your transistors ac-cordingly.
The emitter load of Q4 (Fig. 2)is made up of seven resistors (R22-R28, Fig. 3). The total value of theseresistors is 4135 ohms. To produce thevoltage steps of 20-, 10-, 5-, 2-, 1-, 0.5 -and 0.2 -volt, 1 -watt carbon resistorsare altered to give the exact resistancesneeded.
Using a three -cornered file, verycarefully file through the phenolicouter insulating body of the resistoruntil the shaft of carbon is exposed.With a good ohmmeter, measure theresistor value in ohms. For example,let us use R24, a 610 -ohm resistor. A5% 610 -ohm resistor was purchased;this means that the resistor might haveany value between 580 and 640 ohms.
DI,D2,D4,D5-1N1692D3,D6-6-7V,I50mW
ZENER DIODERS -6 (HOFFMAN)OR EQUAL
Fig. 7-A well -regulatedsupply improves high -fre-quency stability and transi-ent response. Magnetic fieldsfrom transformers T1 andT2 may make it necessary tobuild the supply on a chassismounted on the outside rearwall of the oscilloscope.
06
2N301-30V
ADJ
2N1303
RIIK
R8soon
2W POT
R37.5 K
+6VADJ
However, I have found that many 5%resistors are right on the money at 600ohms. Let us assume the resistor youget is 600 ohms. You have measured itsvalue on your ohmmeter and affirmedthis. You have, using your three -cornered file, cut through the phenolicbody of the 600 -ohm resistor.
But R24 must be 610 ohms. Fileinto the carbon of the resistance ele-ment a few strokes. Measure the re-sistance again. Notice that the resistorincreases its value. Continue filing withthe finesse of a brain surgeon until thehigher value is obtained. Work slowlyso as not to heat the carbon as youwork on it. As you work, continuouslycheck the zero of your ohmmeter.Alter all resistors requiring a resistanceincrease.
Careful work at this stage will re-sult in a series of resistors providingexact voltage division ratios. As eachresistor is filed to the needed value,apply a dab of Duco cement to the cutso that moisture will not cause futuredamage to the resistor
It is true that the power rating ofthe resistors has been reduced, but thatis why 1 -watt units were used. Only1/2 -watt units were needed, and theminute filing did not derate the resist-ors below that.
Use special care when installingthese units as their body has beenweakened. Once they have been as-sembled between decks of SI, you havea strong rigid assembly.
Remember that the filing technique only increases the resistor's valueand do not forget to seal the cut! Thismay sound like a piece of work butconservatively speaking it can savemore than $5 for 1% precision resistors to gain the same ratios. If youhave access to a precision or lab -typebridge, so much the better.
Operation
The voltage calibrator consists ofa variable -frequency multivibrator, Q1and Q2, driving Q3, a variable -voltage
switch. The output of this switch is thenfed to emitter follower Q4. Emitter re-sistance is ratio -divided to providevoltage ratios of 1, 1/2, 1/4, Mo, Mo,1/4o and Moo to ground. When thevoltage -control potentiometer ( R12 )is adjusted so that there are 20 voltsat the emitter of Q4, the voltages areas noted on the schematic of Fig. 3.These dividing ratios apply tor anyvoltage appearing at the emitter ofQ4. S3 provides additional divisioneither by 1 or 100. The ratios thenbecome 1/100, 1/200, 1/400, 1/1000,1/2000, 1/4000 and 1/20.000. Inother words, the calibrate pulse outputis in either volts or millivolts.
The variable -pulse generator istriggered either internally by the vari-able -frequency multivibrator or froman external negative -going square -wave or sine -wave trigger. Both courseand fine control of the variable -pulsegenerator is available.
Pulse width can be varied from afew microseconds to 550 psec. Pulsewidths for this Schmitt trigger dependto some extent on the input triggerwaveform. Therefore no data are giv-en for width values on switch S2. Out-put of the Schmitt trigger is bufferedby Q7, a fast switch. Fig. 4 illustratesthe time -pulse relationships availablewithin the Box.
Adjustments may be needed
Most scope calibrators operate at1000 Hz. The variable -frequency mul-tivibrator used in the Box covers from400 to 4000 Hz. Frequency adjustmentshould be made against a known fre-quency standard such as a 1000 -Hzcrystal. Approach the setting from thehigh -frequency end of the potentiom-eter. R5, the frequency control,varies the symmetry of the multivibrator and can be set to shut off thecircuit. When troublesh000ting thiscircuit, first be certain that the poten-tiometer arm is near the high -fre-quency end of its travel.
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DOT -BAR GENERATOR
IF YOU'D LIKE AN INEXPENSIVE,functional and compact Dot -Bar gen-erator to insure your color set alwayshas "hi-fi" color, this project is foryou.
Construction is simplified withthe actual -size printed -circuit drawingshown. The unit (Fig. 1) operates ona 9 -volt battery, fits in a small metalbox, and puts out horizontal and verti-cal lines, dots and a cross -hatch pat-tern.
A second similar circuit (Fig. 2)is a Cross -Hatch generator that can beinstalled in any set with a serviceswitch. It can be used when conver-gence circuits are adjusted
of the variation in tolerances of thecomponents, one transistor will con-duct before the other.
An increase in collector currentfor transistor Q1 causes a decrease inits collector voltage and a correspond-ing reduction in regenerative feedbackthrough capacitor C2 to the base oftransistor Q2. As a result, the currentthrough Q2 decreases steadily as thecurrent through Q1 increases until Q2
is cut off. Capacitor C2 then dis-charges until forward bias is re-estab-lished across the base -emitter junctionof Q2 Current through Q2 then de-creases until Q1 is cut off. CapacitorCI is coupled from the collector ofQ2 to the base of Q 1 so the cycle isrepeated
A positive input trigger pulse15,750 -Hz, from the horizontal sweepcircuits of the television receiver is
used to sync the multivibrator. Thefrequency of the multivibrator is setfor a multiple of 15,750 Hz by ad-justing R22.
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When adjusting calibrator volt-age, use at least a 20,000 -ohms -per -volt meter to set the emitter of Q4 to20 volts. You may want to cut holesin the dust cover of your scope to reachconveniently the two controls, R5 (fre-quency) and R12 (voltage), of theBox.
The variable -pulse generator hasboth coarse and fine pulse -width or de-lay controls. For example, with properminimum settings it is possible to ob-tain a 5-psec pulse of 30 volts ampli-tude. As mentioned before the rangeand extent of Influence of these con-trols depend on the type of sync ortrigger signal used.
Uses of the Calibrator Box aremany; they include variable sweepdelay. Z-axis blanking, amplifier cali-bration using voltage ratios, simplesignal tracing in radios. In photograph-ing scope traces, you will find thatZ-axis blanking is a fine way to addanother dimension of information toyour sweep photographs.
Remember that neither the cali-brator nor the variable -pulse genera-tor is a power driver and should not beloaded excessively. As an extra bonus,the Box can often be tucked neatlyaway in the dark recesses of yourscope.
By BENNETT C. GOLDBERG
How does it work?The function of the Dot -Bar Gen-
erator is shown in the block diagramin Fig. 3. The circuit is straipaor-ward and explained as follows:
Transistors Q I and Q2 form afree -running multivibrator which pro-duces a square wave output Because
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PARTS LISTResistors 1/4 watt or more, 10%R1, R13, R17-22,000 ohmsR2, R4, R7, R10, R12, R18-1000 ohmsR3-27,000 ohmsR5, R9-39,000 ohmsR6, R8, R14, R15, R16-2200 ohmsR11-6200 ohmsR19-330 ohmsR20, R21 -500,000 -ohm 1/2 -watt linear poten-
tiometer, Mallory SU-50 or similarR22, R23 -25,000 -ohm I/2 -watt linear potenti-
ometer, Mallory SU-29 or similarCalmat -orsCl C2, C7, C8-100 pF, 100V, 10% micaC3-12 pF, 100V, 5% micaC4, C5-0.033 14F, 50V. 5% disc ceramicC6-0.0043 j4F- 100V, 5% micaC9-0.002 AF, 100V, 10% mica
C10-0.001 uF, 500V, 20% micaC11-0.01 AF, 500V, 20% disc ceramicC12-0.47 j4F, 50V disc ceramicSemiconductorsQ1 -Q10 (Fig. 1), QI-Q8 (Fig. 2)-2N2219Q11 (Fig. 1), Q9 (Fig. 2)-2N2369D1 -D2 --1N914 diodesSwitches51 -2 -pole. 5 -position, nonshorting rotary.
Centralab PA -1003 or equalS2-SpdtS3-SpstMISC-I1 transistor sockets, 9 V battery
(NEDA No 1600 or equivalent), tip jacksand plugs, circuit board, sloping -panelcase at least 51/4" wide, 3" deep and 23/4"h'gh, test leads and clips.
tors Q1 and Q2 Q5 and Q6 as multi -vibrators. Transistors Q3 and Q7 arepulse shapers, Q4 and Q8 is the ORamplifier and Q9 is the inverting andoutput amplifier. Since the switchesare eliminated in this circuit, the out-puts of Q3 (a 0.1-psec, 180 -kHz posi-tive pulse) and Q7 (a 50-posec, 900 -Hzpositive pulse) are applied to thebases of Q4 and Q8 combining thesetwo frequencies at the common col-lector R7. This produces a cross -hatchpattern and is inverted by Q9 to givethe proper output phase.
How to build itA printed circuit was developed
for Fig. 1, but it can also be builtusing breadboard material such asVector board All parts should be ofgood quality, but nothing is critical tothe circuit except C3 and C6. Sincethey determine the pulse widths, theyshould be as close to the specifiedvalue as possible.
The transistors are silicon npnsmall -signal high -frequency types hav-ing excellent rise and fall times. I didnot try any of the economy planarpassivated transistors such as the2N3855 or the 2N3856, but do notsee why they won't work in this cir-cuit as they are very similar to the2N2219 and the 2N2369 transistors.
After completion, apply powerand check the current It should be
The multivibrator operates at180 KHz, which produces approximately 12 lines. There is enough vari-ation in the adjustment of R22 to pro-vide more or fewer lines as desired.
Transistors Q5 and Q6 operateas a 900 -Hz multivibrator. Its fre-quency is adjusted by R23. A 60 -Hzpositive trigger pulse from the verti-cal sweep circuits of the receiver isused to sync this multivibrator.
Transistor Q3 is a pulse shaper.Capacitor C3 and resistor R5 are con-nected as a differentiator network.Transistor Q3 is biased on by R5. Thenegative -going differentiated edge ofthe square wave of multivibrator Q1and Q2 turns transistor Q3 off. Thisproduces a positive, 0.1-eusec-wide,180 -kHz pulse.
Transistor Q7 is also used as apulse shaper, but a 50 -µsec -wide, 900 -Hz rate positive pulse is obtained.
Transistors Q4 and Q8 are in-verting amplifiers. Transistors Q9 andQ10 have a common -collector loadand perform an "oR" function. Tran-sistor Q11 is a low -impedance invert-ing output driver.
Switch Si selects the requiredmode of operation to form the desired output signal to the bases oftransistors Q9 and Q10. Switch S2 se-lects the correct output phase to thereceiver.
Circuit operation of the cross-hatch generator (Fig. 2) uses transis-
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Connect the ground lead of thegenerator to the receiver chassis. Con-nect the output of the generator to thegrid of the last video amplifier using atest socket adapter.
Now set the channel selector to alocal station. This will provide syn-chronization for the sweep circuits ofthe receiver and generator. Next setgenerator circuit to cross -hatch pat-tern and adjust R20, R22 and R21,R23 until the pattern stays in sync.
There should be 12 horizontaland 15 vertical lines when the gen-erator is in sync. Some minor adjust-ment of the vertical hold on the colorreceiver may be required.
Adjust brightness and contrast'controls for the sharpest patterns.
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near 50 mA for the dot -bar circuitand about 40 mA for the cross hatchcircuit.Using the generator
Both generators require externalhorizontal and vertical synchroniza-tion. To provide it, clip the horizontallead, J2, of the generator to the in-sulation on the red lead of the deflec-tion yoke (clip only, do not solder).Clip the vertical lead, J1, of the gen-erator to the insulation on the yellowlead of the deflection yoke If youhave any problem with vertical sync,connect clip J1 to any of the leads ofthe vertical output transformers orterminal "C" on the convergenceboard of color receivers like the RCACTC-15, 16 series
4
Fig. 3-Block diagram shows operationof Dot -Bar generator. After shaping, bothmultivibrator outputs are inverted andamplified by Q4 and Q8. Transistors Q9-Q10 operate as an on amplifier with func-tion switch SI. S2 selects output phase.
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lines and a black background on thereceiver picture tube. Program pic-tures will also be in the background.
THE POPULARITY OF SOLID-STATE ELECironic circuits has produced a rapidgrowth in the use of low -voltage ca-pacitors (e.g., 3, 10, 15 and 25 voltsdc) whose capacitances range from0.1 pF to 5000 tLF and higher. Thereis a need to know with reasonable ac-curacy the actual, rather than the rated,capacitance of electrolytics which aremanufactured with very wide toler-ances. Here is a simple method thatyou can use to measure capacitance.
All you need is a vtvm (or otherhigh -impedance voltmeter), a dc volt-age source, one or more referencecapacitors of known value and thesimple circuit shown in Fig. 1.
176
By MELVIN CHAN
In use, reference capacitor Cl ischarged to a voltage which does notexceed the voltage rating of Cl or C2(the unknown capacitor), whicheveris lower. The charging voltage is thendisconnected, and C2 is placed inparallel with Cl, causing the originalcharge on CI to be shared by C2 Atthis time, the meter will show a de-crease in voltage, which is a functionof the relative capacitance of the ref-erence and unknown capacitors. (Ob-viously, the voltages on both capaci-tors are equal at this lower level.)
The initial voltage (El) on refer-ence capacitor Cl, and the lower volt-age (E2) resulting when C2 is in pa-
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Reproduction of PC board used by author Board is shown actual size as it is used.
Now you can set S2 to position B fordots and A for other modes. This ap-plies the correct phase signal to thevideo amplifier grid, producing white
LOW -VOLTAGE ELECTROLYTIC TESTER
rallel with Cl, are substituted in theformula
R2
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R3J3 J6
177
or longer should be reformed beforemeasuring or return to service. Thisprocess consists of applying (with cor-rect polarity) the rated working volt-age to the capacitor until a voltmeteracross the capacitor stabilizes at thatvoltage. The time required for thevoltage to stabilize is a function of ca-pacitance, leakage current and the
J2
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Parts UstR1 -1000 -ohm potentiometer, linear taper
(see text)
R2 -220 -ohm, 1/2 -watt, 10% resistor
R3 -10 -ohm, 1/2 -watt, 10% resistor
S1 -2 -pole, 3-positi'n nonshorting rotaryswitch (Mallory 3323J or similar)
J1, J2, J3-Pin jacks or 5 -way bind'ng posts,red
length of time the capacitor has beenout of service. Reforming time mayrange from only a few seconds to aminute or more
1. Connect the charging -voltagesource and the vtvm as shown in Fig. 1.Turn RI (SET VOLTS) to reduce themeter reading to zero.
2. Set switch SI at FORM, connectCl and C2. Adjust R1 until the meterreading reaches the voltage rating ofCl or C2, whichever is lower.
3. When the meter stabilizes atthe rated voltage, throw Si to CHARGE.
Readjust RI to hold the voltage (El)on capacitor Cl at the level estab-lisnea in Step 2. (Meanwhile, C2 dis-charges through R3.)
Measuring capacitance
Fig. I-Unknown electrolytic capacitors can be reformed, discharged and measuredin a 1-2-3 procedure at the flip of SI. Voltage levels can be set by control RI.
J4, J5, J6-Pin jacks or 5 way binding posts,black.
C1 -100-µF 12 -volt electrolytic capacitor(Sprague 7E1119.3 or similar); Optional:15µF, 12 -volt electrolytic capacitor(Sprague TE 1129 or similar) 450 -AF,3 -volt electrolytic capacitor (Mallory TPG-4503T, Cornell-Dublier type NLW Electro-mite or similar)
:hassis-aluminum box, 3%" x 2'/a" x 1%"
4. After a few seconds throw S1to TEST. Read the new voltage (E2)on the meter.
5. Substitute the capacitance ofCl and voltages El and E2 in theequation above and calculate the ca-pacitance of C2.
For example, if Cl is 100 !IF, Elis 10 volts anu E2 is 7.25 volts, theequation becomes:
C2 = 100(10 - 7.25)
7.252.75=1007.25
= 38 I.LF (approx.)Obviously, the more precisely the
capacitance of Cl is known and thelower its leakage current, the more ex-act will be the value of C2 as calcu-
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lated from the formula. If you avoidmathematics whenever possible, TableI is for you. It gives the multiplier ofCl capacitance for charge voltages of3 and 10, at useful voltage -differencelevels. Thus, if El is 3 volts and E2 is2, El - E2 = 1 volt. Find 1.0 in theEl - E2 column and read 0.5 on thesame line in the 3 -volts column. If Clis 450 tiF, C2 is 0.5 x 450 or 225 F.
The resistance of R1 should beselected to limit its bleeder current toabout 15 mA. For example, with a 15 -volt dc supply R1 should be 1000 ohms(15/0.015). Resistor R2 limits thecharging current; R3 fully dischargesC2 after it has been reformed and be-fore it is paralleled across chargedcapacitor Cl.
Using a 100-µF, 10 -volt capacitorfor Cl permits measuring capacitancesranging from 1µF to 1200 µF. Theuse of 15-p.F, 12 -volt and 450-µF,3 -volt capacitors for Cl will extendthe range from 0.15 µF through 4950,i4F. If two or more standard capacitorsare to be used for Cl, they may beplugged into tip jacks (J2 and J5).
The parts in the basic test unitcost between $6 and $10. Be sure touse the 10% tolerance tantalum capac-itor as the reference unit for best ac-curacy.
Charge Voltage
0:200.200.31
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CI multiplier (see text)
The charging voltage may be sup-plied by dry cells connected in series,or by any 10 -25 -volt dc supply. It isnot necessary to charge Cl to morethan 10 volts when measuring un-known capacitors.
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HIGH ON THE LIST OF "HOT" ELEC-tronic items are those amazing little de-vices known as integrated circuits. Nowthat their prices have dropped so dra-matically, it's quite simple to incorpo-rate a couple of them in a multipurposetest instrument. Here's a practical anduseful project for audio testing.
True to their acclaim, integratedcircuits allow construction of the instru-ment with minimum fuss and few auxil-iary components. At the same time,they provide versatility, small size and,most importantly, low cost.
When used alone, the instrumentputs out a high -quality square wave(rise time under 450 nsec) at fiveswitchable frequencies ranging from 20Hz to 12 kHz. When the unit is usedwith an external sine -wave generator,variable -width pulses are produced. Thesquare -wave and pulse outputs are bothexcellent signals for testing and trouble-shooting audio components
Most important, however, is theinstrument's ability to gate or switch anexternal signal to produce tone bursts.The external signal to be gated can beany waveform-sine or square waves,white noise, even music or speech. Costruns under $20, and construction shouldtake no more than two or three eve-nings.
The tone burst is excellent for test-ing transient response in speakers, am-plifiers, tape recorders, crossover net-works and other audio components. The
gated or switched output provides aclose simulation of the rapid attack -and -decay patterns of musical material.The tone -burst waveform is also usefulin evaluating overload characteristics ofpower amplifiers.
Fig. 1 shows the schematic of thegenerator. Fig. 2 is a simplified blockdiagram showing its essential features.The unit consists of four subunits. IC1is connected as a variable -width pulsegenerator (Schmitt trigger). IC2 is afree -running multivibrator. Q1 and Q2are the gate driver-amplifier and thegate, respectively. Q3 and Q4 form acompound emitter follower to providea low -impedance output. A direct -cou-pled npn-pnp pair is used here so theoutput is at ground potential.
Sine waves introduced at input jackJ1 (Figs. 1 and 2) are converted tofast -rise pulses_ at the input frequencyby IC1. Pulse width is controlled by R3.Pulses, applied via C2-R5 to IC2, serveto lock in the square wave produced byfree -running multivibrator IC2 (thegating waveform) with the incomingsine wave (the gated waveform). Thetone burst will start at the point atwhich the sine wave crosses the zeroaxis. The input sine wave also is routedto gate Q2 by R9-R15, to be gated bythe output of IC2.
The multivibrator (IC2) producesa clean square wave without the round -cornered characteristic of the usualmultivibrator output. In addition, a sig-nal from 1C2 is presented at J3 to syncthe scope.
In the box specified, there is just enough room for batteries to fit behind panel jacks.
The gate (Q2) is a circuit fre-quently used in analog computer sys-tems. Extremely simple, it is relativelyfree from switching transients. It is alsohighly effective. Signal through theclosed gate is down more than 65 dB!Note, however, that, with R15 switchedinto the circuit by S2, "leak" throughthe gate is increased so that the signal isdown only 20 dB from the main toneburst. This feature is useful when youare interested in noting the behavior of
the device under test after the maintone burst is switched off.
In addition to its function as gatedriver, Q1 amplifies signals obtainedfrom the integrated circuits. Wave-forms up to 15 volts peak -to -peak areavailable at the output jack when theunit is operated as a square -wave orpulse generator.
Switch S3 selects the desired func-tion (OFF, PULSE, SQUARE WAVE, NON-
SI, C4 -C13
S2
R3
BATT 2
BATT
BATT 3
S3
Ic i.2-pL914 ( 2 )
2N1303
182
R R8F
RIGMEG
03
S3 POSITIONSI - OFF2 - PULSE3 - SQ. WAVE4 NONGATED5 GATED
19
K
J6
2N3393 OR 2N2926DRIVER /AMPL
3V2.7 K
0
15V
R122.7K
0 R13Io
RII 012 S3 -e 3.3K
4.7 KR1410K
1--0
C6 s -0
R153K
02
1N34
4
OUTPUT
5
S1
CAPACITORS -C4,9-.01C 5,10-.02C6,11- .1C7,12- IµF /6VC8,13- 5 is.F /6V
Fig. 1-For flexibility, input and output have phono jacks and binding posts in parallel.
GATED, GATED). Each battery is switchedinto operation only when it is necessaryfor the selected function. The signal ap-plied to R19 also is switched to pass thedesired output signal through the GAINcontrol. For the gate -open or nongatedfunction, R10 is connected between 15volts and the base of Q1. The driver isthus turned on, disabling the gate.Construction
The entire unit is built into a 5 x 4 x3 -inch aluminum box. Most of the cir-
cuitry is mounted on a perforatedprinted -circuit board. Capacitors C4through C13 are mounted directly onswitch S1. Fig. 3 shows the circuit pat-tern, and the photos show componentplacement. Letters A through 0 of Fig.1 refer to the terminations shown on thecircuit board. These points are con-nected to various positions on theswitches, input output jacks and pots.
The circuit pattern can be readilycopied using standard printed -circuit
R3 WIDTINPUT 250K
J7B
R2J1 CI RI
I2V5 + 10K
NC
NC
R44711
IC IPULSE GEN
INDEX (FLAT)
C247pF R5
2.7K
R7
°I 8
RI810K
RIO
I
C4 C9I-0 I-0
CIOI-0
C I -b
C7
CB1-0 1,
S2LEAK
J3SYNC
I
o
2 S3 -d
0GAIN
02,J3 4
2 FREQ
R927K
2NI302GATE
D
G
C3
560pF
3V
4 5.
2NI302EMITTER FOLLOWE7
-15V
7K
20
+S3 -a
; FUNCTION
I 2 3,z, 0
-, S3 -b
4 5 I0
2
+S3 -c
4 5 ;--,-.
3 40
5;_ j
V2 BATT 33 V
D
3 C14
560 pF
M
RI747K
N
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.7K
K
.4._..._
L: °
SPECIFICATIONSInput:
External signal generator-impedance approximately 7,-000 ohms at 1 kHz. Requires2 to 3 volts rms to drive pulsegenerator.
Outputa. Output impedance approx.
5,000 ohms (affected byposition of level control).Maximum output 15 voltspeak to peak (5.2 voltsrms). Square -wave andpulse outputs positive -go-ing from zero.
b. External scope -sync output.Performance:
a. Gating and square -wavefrequencies 20, 100, 250.5,000, 12 090 Hz nominal.
b. Pulse frequency-deter-mined by external signalgenerator-usable to atleast 500 kHz
c Frequency response ingated and nongated modes-flat from dc to approx.50 kHz.
PULSEWIDTH
183
RIN
Q2GATE
SQ.WAVEFREQ
Q3,4EMITTER -FOLLOWER
etching techniques. Careful hand ap-plication of ordinary nail polish makesan ideal and inexpensive way to applyan etch -resistant coating to the copper -clad board. After etching in warm fer-ric -chloride solution, the nail polish iseasily removed-with nailpolish remov-er, naturally!
The circuit board is fastened to thetop of the box with 4-40 hardware andsmall spacers, and the batteries are in-stalled in the bottom of the box. If youwire the lettered points on the boardto the appropriate points, the unit canbe wired quickly. Addition of rubberfeet, knobs, etc., will then complete theassembly.
After the wiring is complete, in-stall the batteries and connect outputjacks J4 and 75 to the vertical input of
GENERATOR
your scope. Connect sync jack J3 to thescope's external -sync input and turn onthe instrument. With function switch S3in position 3, you should get a cleansquare wave Si varies the frequency ofthe output. Now inject a 1 -kHz 8 -voltsine wave at 71 and J2. Place the func-tion switch in position 4 (nongate).You should get an unaltered sine waveat J4. In position 5, the sine wave willbe switched on and off at the frequencychosen by Si, producing tone bursts.
Adjustment of R3 will allow thetone burst to begin at the zero -axiscrossing of the sine wave (this is notnecessary, but it results in a more attractive display). Similarly, the gatecan be made to close at the zero -axiscrossing by trimming the input fre-quency slightly. Throw S2 and note that
PARTS LIST
R1, R14, R18 -10KR2, R13-4,700 ohmsR3-pot, 250KR4-47 ohmsR5, R8, R12-2,700 ohmsR6, R7-6,800 ohmsR9 -27K, 5%R10, R17 -47KR11-3,300 ohmsR15-3,000 ohms, 5%R16-1 meg (see text)R19-pot, 10KAll resistors 1/4 watt 10%, ex-cept R3, R9, R15.C1-5 ,aF, 15 volts, electrolyticC2-47 pF, discC3, C14-560 pF discC4, C9-.01 AF, discC5. C10-.02 !IF, discC6, C11-O.1 AF, discC7, C12-1 AF, 6' volts, elec-
trolyticC8. C13-5 /IF, 6 volts, elec-
trolytic11)--1N34IC1, IC2-µL914 Fairchild in-
tegrated circuit Availablethrough Fairchild distrib-utors
Q1 -2N3393, 2N2926 (GE)Q2, Q3 -2N1302 (RCA, TI)Q4 -2N1303 (RCA, TI)S1 -2 -pole 5 -position rotary
switchS2-spst slide or toggle
switchS3 -5 -pole 5 -position rotary
switchBATT 1, 2 -15 -volt battery
(Burgess U10 or equiva-lent)
BATT 3-two 1.5V Z -batteriesin series
Box- 5 x 4 x 3 -inch Miniboxor equivalent (Bud CU -2105A)
J1 through J5 -4 -way bindingposts
J6, J7-Miniature phono orcoax connectors
Battery holders -2 for Bur-gess U10 (Keystone 166or equivalent) 1 for 2 Z -batteries (Keystone 140or equivalent)
Perforated copper -clad board(Lafayette 19C3601 orequivalent)
Screws, nuts, spacers, wire,solder, etc.
LEVEL OUTPUT
Fig. 2-Generator circuit operation depends on the position of function switch S3.
R14 R9 R13
R4 ICI C2 C
R16 R 5 R12 Q115
14
RR
R8 +3V
Front of the perforated board contains most components with some room to spare.
L
R7
R5 0 Q2 1C2
3
1
Fig. 3-Back of the board above, shoving copper connecting paths to all elements.
184
RI
mui 1mq !Mil up!!
dim
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NIP IMP VIM
1.141
VII
111 1,1111
the "leak" during the gate -closed periodis increased to -20dB
Operation of the pulse generatorcan be checked by setting switch S3 toposition 2 and varying R3. Note thatR3 can be adjusted to give a symmetri-cal square wave; thus the unit will pro-duce square waves at the frequency of
Fig. 4-Samples of generator waveforms:
High -frequency square wave (12 kHz). Pulse output at approximately 10 kHz.
1,000 -Hz sine wave, gated at 100 Hz. 50 kHz sine wave gated at about 5 kHz.
As above, but with LEAK increased. As above, but with LEAK increased.
the external sine -wave generator in ad-dition to the five internally producedfrequencies.
With the unit in the "gated" mode,but with input jack J1 shorted to J2,check the gate offset. (This offset effec-tively places the tone burst on a slightpedestal ) It should be less than 0 I volt.
problem by making R16 variable so theoffset could be adjusted to zero occa-sionally as the batteries age. Such an ad-justment was omitted, however, to re-duce circuit complexity. In any event,the offset will not exceed about 0.1 to0.2 volt through the useful life of thebatteries.
This offset is caused by two effectsThe first and less significant one is theleakage current through Q2 when it is
cut off. Second, the positive and nega-tive 15 -volt supplies are not necessarilyequal in magnitude. In fact, as the batteries age, the offset will vary, sincebattery voltages do not change at thesame rate. One could overcome this
integrated -circuit operational ampli-fiers IC and IC2.
Integrated circuit IC2 is connectedas a voltage comparator with hys-teresis feedback. Because of the highgain of the amplifier, if the non -in-verting ( + ) terminal is slightly morepositive than the inverting ( -) ter-minal, the output will be the positivesaturation level of +14 volts. Con-versely, if the non -inverting input ismore negative, the output will be thenegative saturation level, -14 volts.
by NEIL HECKT
The heart of the generator is threeMotorola MC1709CG integrated -cir-cuit operational amplifiers. Each con-tains the equivalent of 13 transistors,2 diodes, and 15 resistors in a stan-dard TO -5 package.Theory of operation
The unit (Fig. 1) can be dividedinto three major parts. The oscillatorsine -wave converter, and output am-plifiers. The oscillator is a form of re-laxation circuit called a hysteresis os-cillator. It is composed of two
PARTS LISTSAll resistors 1/z watt, 10% unless notedR1-750 ohmsR2-potentiometer, 2500 ohms, linear taperR3-270 ohmsR4-22,000 ohmsR5-27,000 ohmsR6, R34-1500 ohmsR7-10,000 ohmsR8-5600 ohmsR9-15,000 ohms
R22.5K 27K
VERNIER
R32704
+15V
R 20220n
R211204
R22684.R2339.0R 24
KR28
07i5pF
IK
-15V
1C3
RII100K
013
R4
22K
012
p4P4
1
R15K
RI63
RI768.4
RII3120/1
R19220.0.
D9
40VCT
IA
R61.5K C6
10pF
18KR27
6
R10, R-35-potentiometer, 10,000 ohms, lineartaper
R11, R12, R31, R36, R37, R40, R41-100,000ohms
R13, R14, R38, R39, R42, R43-470 ohmsR15, R24-1000 ohmsR16, R23-39 ohmsR17, R22-68 ohmsR18, R21 120 ohmsR19, R20-220 ohmsR25-47,000 ohmsR26-18,000 ohms
.15V
R8 2
5.6 X JI
016I N474 4
5
D5
D6
R3947041,
COMPLETE CIRCUIT OF WAVEFORM GENERATOR. Regulated power supplydelivers +15 and -15 volts. Three IC amplifiers simplify construction drastically.
187
R7
5
R1347011
RI75
100 pF
10K
.001
01 C3
1 C4
D 7 1 pF C5 R12 RI4100K 470(1.
010
03
04
014 510.n.
B
R3700K
C9 -15V10.F +15V
R40 R42100K 47011
4-15
015IN 474 4
05
J3
R47
BA3W R4I
1001(
R915K
De
-15
BOTTOMVIEWMC 1709C6
R29
47 K LEVELR26 R30
25 K
R 31100 K
R3268 K R34
15KR44 R46
BR 5111 3.941
210 +
1000.1JF
6
RIO10K
LEVEL. D2
-15V '51/
R36100K
R38470fL
J2
ICI +
IC2 D
CH +/000p F
R45
51.112W
R35 0310K
R33 LEVEL
82KC815 pF
5 -ID4
MI
1
It
111
1-41}V
E11111111111.1111111.1.
4'1411.6.
R27 10,000 ohmsR28-510 ohmsR29-27,000 ohmsR30-potentiometer, 25,000 ohms, linear tapesR32-68,000 ohmsR33-82,000 ohmsR44, R45-51 ohms, 2 ohms, 5 1R46, R47-3.9 ohms, 3 wattsC1-100 pFC6, C9-10 pFC2-.001 uFC3-0.01 uFC4- 01 uFC5-1.0 uFC7, C8-15 pFC10, C11 1000 uF 30 volts, electrolyticC12-.0056 uF
Assuming the output is +14 volts,positive feedback applied to the non -inverting input via R9, keeps the am-plifier latched in the positive satura-tion state An increasingly negativevoltage is applied to the input resistorR7. The ratio of R7 and R9 is such,that when the input reaches -10volts, it cancels the +14 volt feed-back. As the input goes slightly morenegative, the amplifier switches to thenegative saturation level. It remains inthat state until the input exceeds 0volts when a similar, but opposite re-action occurs causing the amplifier tolatch again at the positive level.
Integrated circuit ICI is connectedas an integrator. If a positive voltage(E,,,) is applied to the input resistor,R5, the output will try to swing nega-tive. The voltage across the timing ca-pacitor Ct, (C1-05) cannot changeinstantaneously, however, and thenegative output thru the capacitorcancels out the positive input in an at-tempt to maintain the inverting ter-minal at zero volts. The cancellingcurrent charges the capacitor at alinear rate toward -14 volts atE,,/R5C, volts per second. The op-posite effect is true of negative inputs.
If comparator IC2 is latched inthe +14 -volt state, and some portionof the +14 volts is applied to the in-tegrator input resistor, the integratoroutput will run down linearly toward-14 volts. Since the integrator outputis connected to the comparator input,
the comparator will switch to 14volts when the integrator outputequals -10 volts. This applies the op-posite polarity to the integrator, andits output runs up linearly toward+14 volts. At +10 volts the compara-tor switches back to +14 volts completing one cycle The cycle then re-peats and the circuit oscillates at aconstant rate.
The frequency of oscillation is afunction of the applied voltage to theintegrator (E,,,), the integrator inputresistor (R5), the feedback capacitor(C,), and the trigger levels of thecomparator (E, max) according toF E,./4 E. max x R5 x C,. E. maxis set at 10 volts, R5 is 25,000 ohms,therefore F = E,./C, where C, is inµf. Resistors RI, R2 and R3 are chosento provide from -± 0 to -±-11 volts,and C, is switched from 1µF to 100PF providing frequencies from 1 Hzto 110,000 Hz.
The output from the comparatoris a 28 -volt p -p square wave while theoutput of the integrator is a triangleof 20 volts p -p. The triangle wave out-put of the integrator is converted to asine wave by the non-linear re-sistor/diode network R15-R30 andD7-D14.
Assuming the input to R29 andoutput across R30 are both zero in-itially, all the diodes are reverse biasedby the bias voltage divider R15-R24.If the triangle wave is rising linearlytoward + 10 volts, its slope is reducedat the output by the voltage dividing
J1, J2, J3, J4 -4 -way lacksTI-transformer, 118 -volt primary, 40 -volt
center tapped secondary, 1 ampBR1-full-wave bridge rectifier, 100 piV, lA or
four 1N4004 or similar diodesDl to D14 -1N914Q1, Q3, Q5 -2N4123Q2, Q4, Q6 -2N4125ICI, IC2, IC3-MC1709CG IC op ampMiscellaneous, 6" x 4" x 3" utility box, knobs
stand-off terminals, hookup wireDl to D14 -1N914Ql, Q3, QS -2N4123Q2, Q4, Q6 -2N4125BR1-MDA 920A-3ICI, IC2, IC3-MC1709CG
7-- NO DIODE CONDUCTS
/7-- - D 10 CONDUC TS...- - D9 CONDUCTS
7-* - D8 CONDUCTS...- - D7 CONDUCTS
action of R29 and R30. When theoutput reaches the bias level of D10,R25 is switched in parallel with R30,further reducing the slope. As the out-put increases still more, diodes D9, D8and D7 conduct in turn (Fig. 2),each reducing the output slope byswitching in R26, R27 and R28 Theinput triangle is therefore "distorted"into a quarter sine wave. As thetriangle voltage decreases back towardzero, each diode is cutoff in reverseorder to produce the second quartersine wave. For negative half -cycles,
T
189
TIME -i
FIG 2-DIODES MAKE A SINEWAVE out of a triangular wave.
INSIDE THE GENERATOR you can see how parts are set up.
SINE OUTPUT
-10
RIANGLE INPUT There are three output drives, onefor each waveform. They consist ofcomplementry emitter followers.Diodes, such as D I and D2, corn-pensa or the base -to -emitter drop ofthe tr istors. For positive inputs, thenpn transistor supplies the current tothe load, and for negative inputs thepnp supplies the current The 470 -ohm resistors limit the current to pre-vent damage to the transistors, as wellas provide an output impedence ofabout 500 ohms.Construction techniques
A 3 x 4 x 6 -inch chassis, such asa Bud AC -430 with BPA-1505 bottomplate makes an economical, compactcabinet Mount power supply com-ponents on the inside top of thechassis box using insulated standoffterminals (Grayhill type 18-1 or sim-ilar). Mount all controls and bindingposts symmetrically on one 3 x 6 -inchface. After all components have beenmounted in the chassis and on the PC
, connect the two thru twistedD11-D14 are cut in in a sim-
ila manner. The points at which theslope changes are not sharp, but arerounded by the non-linear forwardresistance characteristics of the diodesas the current thru them increases.
The output of the converter isabout 10 volts p -p. Operational ampli-fier 1C3 connected to give a gain of2 so that 20 volts p -p are available.
, 6r +
)
A
)1'
4
A -F. - i I-....
111-11
PRINTED -CIRCUIT PATTERNis half actual size.
PARTS LAYOUT shows componentlocation on the circuit board.
.-al..--..V4),D)
./ \L__.j
\Ca6 e\; -.,--'. A A_./4-\ \wig , 7.5- \L---.7i -(----=--' '
4 ),
Qµ0
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I 1
T T T-1129.9
RA8,1 122 IA D3/41
T pi4 Di
R15 Z18 gil R 6 Klg
1 I I
T rTT it T
Ri3 RaoDi 1 1
41
T T D3 (i';
R38
1'1 Da R.11 41 J-
9 9
1227 TR,,T e2z- T RI T g3T
pairs of wires as per the wiring list.Resistor RI is connected from the
top of R10 to the top of R2, andresistor R3 is connected from thebottom of R2 to the ground post.
The completed PC board is at-tached to the center of the bottomplate with 1/4 -inch spacers.
The completed unit should bechecked for proper operation Notice
there are no adjustments to be made.If all components are good, and thewiring is correct, the unit will func-tion immediately.
191
A checkout is completed, foldthe PC and up into the chassis in amanner similar to closing a book. Attach the bottom plate with four1/4 -inch No. 6 self -tapping screws.
I-
S
E
Electrolytic measurement,178
Electrolytic reforming, 177Epicap, 1N15190, 148
F
Feedback, amplifier, 19Feedback, audio crossover,
48Filter, 12 db/octave, 47FM stereo signal, 144Forward AGC, 28Free -running multivibrator,
170, 181Zener diode regulator, 105Zener regulator, 165
Iri IIII u :70
mu lime
B
Backbiased diode expande , 22Baffle, infinite, 44Bandwidth, tuner, 57Bass control, 53Bass -reflex speakers, 42Bidirectional thyristor, 134
C
Calibration, dwell, 90Calibration, multiplex gene-
rator, 148Calibration, phototachmeter,
133Calibration, tach, 91Calibration, timer, 118, 123Cam -operated "park" switch,
109Clairex photocell, 127Class B complementary out-
put, 64Color blenders, 123Comparator, voltage, 186Complementary output, Class
B, 64Crossover distortion, Class B
output, 64Crystal filter, stereo tuner,
30
D
Depth control, tremolo, 84Depth control, vibrato, 77Differential amplifier, 139Dummy load, amplifier, 54Dwell angle, 90Dwell meter, calibration, 90
ltz resonator, 44s audio crossover,
Humidity sensor, 103Humistor, 105Hysteresis oscillator, 186
IC, MC1304 and 1305, 7
IC, MC724P, 67IC, MC790P, 67IC, WC1830, 139IF alignment, stereo tuner,
37Infinite baffle, 44
K
Keyboard, 75
Light -tight housing, 25Light -variable volume expan
sion, 24Loudspeaker resonance, 43Low- filter, 50
MC13 305 IC, 7
MC724P IC,MC790P IC,Monostable multivibrator, 87,
128, 152Multiplex detector alignment,
8
r alignment, 38ator, variable-fre-
y, 169ical scale, 67
0Octaves, musical, 66One-shot multivibrator, 87,
128, 152
p
"Park" switch, 109Photographing scope wave-
forms, 161Pilot signal, FM stereo, 146Pitch control, 73Photocell, volume expander,
23
Reforming electrolytics, 177Regulator, Zener, 93Relaxation oscillator, 83, 110Relaxation oscillator, single
cycle, 114Resonance, speaker, 43RF alignment, stereo tuner,
37Ring modulator, 147RPM measurement, 12
Schmitt trigger, 181SCR, 110SCS, 119Separation, poor stereo, 57Shutter modifications, scope
camera, 156Sine wave generator, 188Silicon controlled rectifier,
110Silicon controlled switch, 119Sonalert, 119Sound to light converter, 79Speed measurement, RPM,
127Square -wave generator, 181,
188Stereo adapter alignment, 60Stereo indicator light, 31Stereo separation poor, 57Stereo signal, 144Switching headphones and
speakers, 53
T
Tachometer, calibration, 91Temperature sensor, 105Triac, 134Triangular -wave generator
188Tuning indicator, stereo
tuner, 31Twelve db/octave filter, 47Twin -T oscillator, 77
V
Variable-freqtiency multivi-brator, 169
Variable -width pulse generator, 181
Vibrato oscillator, 72Voice -operated switch, 138Voltage comparator, 186Volume control, 74
WC183G IC, 139
R
Rate control, tremolo, 84Rate control, vibrato, 77
A
AGC, forward, 28Alignment, multiplex detector,
8
Alignment, stereo adapter, 60Alignment, stereo tuner, 37Amplifier dummy load, 54Astable multivibrator, 105
1.
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