an electronic self scoring target for...checker indicates the charge state. bb ranger 35 the ranger...

ETIELECTRONICSTODAY INTERNATIONAL

THREE RAINDUCTAMETER

VHF INTE NCE

litTERFor High -Band Receiving Antennas

- Keep Radio -Pager Signals Out

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BB RANGERAn Electronic Self

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Vol 27 Issue:4 27th March 23 April 1998U.S.A. 54.95

Electronics Principles 5.0'A COMPLETE PC BASED ELECTRONIC

If you are looking for an easy and enjoyable way of studying orimproving your knowledge of electronics then this is the software for you.

Now includes the PIC16C84 & PIC16C71 hardware and instruction set,

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Graphics presentation hos been enhanced and speeded -up withnew menus and indexing which enables a quicker access andmore informative description of the extended range of five hundredand sixty electronics and mathematics topics.

The PIC 1 6C84 microcontroller hardware and instruction set has beenintroduced and brought to life through colourful interactive graphicswhere you can study the architecture of this device by changing thedata values to simulate all of the registers, direct/indirect addressing,program/data memory and input/output port configuration. Alongwith those analogue to digital functions of the F1C1 6C 71 If youwould like to learn more about the principles of these popularmicrocontrollers then it could not be mode easier.

Electronics Principles software is currentty used in hundreds of UK or .coverseas schools and colleges to support City & Guilds, GCSE Alevel. BTEC and university foundation courses. Also NVQ's andGNVQ's where students are required to have an understanding ofelectronics principles.

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Bectronlcs Principles 5.0 Is a significantode of our popular electronicsatonal software. Now containing

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olectronics formulae. It is extremely easy*o use. Just select a topic, which is always:yesented as o default diagram (no blankscreens!) and input your own valuesAlternatively, use those from any stanclaraelectronics text book to see the results asfrequency response curves. calculationslogic states. vottoges and currents etc.

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EPT Educational Software. Pump House, Lockram Lane, Witham, Essex. UK. CM8 2BJ.Tel/Fax: 01376 514008 sales@eptsoft demon co.uk http://www eptsoft demon co uk

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Switch, Delta, Visa and Mastercard orders accepted - please give card number and expirydate. Cheques & Postal Orders should be made payable to EPT Educational software.

ContentsVolume 27 No.4

0 Featuresc',_Projects

Next Issue 24th April 1998

RegularsNews

Understanding Robots 8Harprit Sandhi introduces the expanding family of "proyfanimabiegeneral purpose manipulators" and the mechanisms and controlloops that make them function.

Three -Range Inductance Meter 19To partner last month's Capacitance Meter. Robert Penfold'sInductance Meter has three ranges of full-scale value 1m1-1. 10mH.and 100mH.

Radio Interference Filter for VHF 24Strong legal transmissions. particularly the ever-expanding range ofradio -paging signals, can swamp VHF reception. E Chicken MBE'sfilter is designed to reduce swamping and clear reception.

LED Voltmeter for 12V battery check 29Twelve -volt lead -acid "leisurebatteries are designed to run downsignificantly before re -charging. Terry Balbirnie's four -LED batterychecker indicates the charge state.

BB Ranger 35The Ranger is an eprom-driven electronic score -board for a BBpellet -gun shooting range. A specially -built set of fall -down targetsenable an instant readout of the accumulated score.

GCSE Grounding: Sound Effects Module 46Terry Balbirnie continues his circuits for GCSE students. In thisissue: a six -sound effect module which can be adapted.

The Orphan DecibelThe Decibel is an unusual unit of measurement - it describes athing not by what it is. but by what it does. E.Chicken MBE FIEF:describes how to calculate power with decibels.

51

Spiced Circuits: Logic Simulation and Maths 56In the last of Owen Bishop's series on Spice -based circuitsimulation. he lookc at logic simulation, including B2Logic V3.0

Digital Tic Tac Toe 60An electronic game of N ;nts and Crosses which allows youto play against a PIC 16C64 microcontroller. By Rose andAndy Morell.

Line -Up Oscillator With GlitchWhen you are send : . II to a remote destination,it is necessary to know Mlicn stereo channel you are monitoring.Tony Sercomhe's glitch will tell you

63

Review: Peak DCA50 Component Analyser 73An advanced new meter for the identification of component types,pin -outs and functions.

4,5,7ETI PCB Service 67PCB foils 66, 68, 69Round the Corner 74

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06363 436344cawsmum

01369 436322

ETIELECTRONICSTODAY INTERNAT

Advanced IBM Disk Technology LoadsMore Than Before-Giant Magnetcreseuve (GMR)' disk dnve hearts, pioneered

by IBM Research scientists, are the essential componentbehind the worlds highest capacity desktop PC disk drives.The IBM Deskstar 16GP is a 16.8 -gigabyte drive capable ofholding eight times more information than today's averagedesktop hard drive. The 16GP can hold eight hours of full -motion video (MPEG-2 quality video), or, according to IBM,sufficient information to fit more than 16 pick-up trucks itprinted out as hardcopy (presumably on USA standard 11 x8.5 -in paper stock).

A pick truck is a traditional North American lightweightcpen-backed flatbed van with low sidings. Goods andpassengers are either stacked loose or lashed in place, usuallyaccompanied by a dog. Not generally robust enough for fullcommercial loading, pickups are widely used for localdeliveries, general agricultural transport and riding around.Loading capacity (depending on age and condition, and takinginto account the low canpressibility of paper) may beapproximately compared to one "transit" van. half a Luton boxvan, or a reasonable fraction of a corporation dustcart.

For large companies hke IBM. mass storage of this kindonce promised the 'paperless office", but the comparativelyslow spread of electronic documentation, and paperhardcopy's flexibility and freedom from compatibility problems,has tended to increase the amount of paper in circulation,rather than decrease it. In the future, mass storage byremovable hard disk may help to reduce the physical need forfiling space.

The GMR disk head is no bigger than the head of a pinand is the woddt most sensitive sensor for reading data on

hard disks. Accorcfing to IBM, the massive storage capabiltiesof the Deskstar 16GP allow television -lice sound and picture -quality multnieda programs on a suitable computer

GMR technology expects to be able to provide storage ofmore than 10 mullion bits per square inch on the disk platter by2001.

The Deskstar range has 10 disk capacities k choose from.For high performance as opposed to maximum capacity, TheDeskstar 14GXP family has a choice of thee differentc,apaortets from 10. gigabytes to 14.4 gigabytes running at7200 revs per minute. The 5400 RPM 16GP drive tan* hasseven capacities from 3.2 go 16.8 gB.

For more information see IBM Storage Systems Divisiontweb page: www.ibrn.com/storage, Dept. Star 30, or contactIBM UK Ltd.. South Bank 76 Upper Ground, London SE19PZ, UK. Tel 0171 202 3744, fax 1071 202 3792.

BUL.ELECTRICAL

EMPORIUM

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,1)1:1 NikArOkS0,1.1%

New Bulls On The Loose!Bull Electrical's latest catalogue Is n(

It features remote control systems.telecsopes and microscopes. batteries andbattery chargers, surveillance bugs andcamera accessones, video cameras. solarpanels and a complete wormery with 1000worms to convert kitchen waste intoorganic compost.

Bull have also started a new catalogueconcentrating on the scientific hobby ofhydroponic gardening. The catalogueincludes nutrients. heaters, pumps, lighting.accessones and how-to books. An idealway to utilise the rich organic nutrients fromyour wormery!

For more information and copies of theircatalogue, contact Bull Electrical, 250Portland Road, Bove, Sussex BN3 50T,UK. Tel. 01273 203500. fax 01273 323077.

-.1

\I Ott

ELECTRONICS TODAY INTERNATIONAL4

ETIELECTRONICSTODAY INTERNATIONA

All Micro Show at Bingley Hall in April

The SAMS'96 Spring Al Micro Show will take place on Saturday18th Apnl at Bingkay Hat. Staffordshire Showgroind. Weston

Road. Stafford. The show wit be open from 10arn to 4Prn, withplenty of free parking. Entrance is £300 for edits. 5Cp forchtten under 14, £2.00 for concessions: OAPs, RSGBmerrters. student cads. U640. Advance tickets are also C2.0Cplus a starved, self-addressed errvetpe.

The Showground is on the A518 Stafford-Uttoxeter Road.signposted from Junction 14 on the M6. There is a txis shuttlefrom Stafford railway station.

This is the 10th consecutive year of the Al Moro Show atBagley Hai Last yew saw 3.000 people and 100 trade standscovering al formats computing. nduring PC, Sfrialf, &sten.Amiga. Atari St and Atari 8 -bit computing supported by varioususer grows, along with stands seing accessories. software.books, components. shareware. media. hardware. ado, sateitea -id a big Bectronics Burg and Buy Stall. There arerefreshments, a cafeteria and a licensed bar from 11 am.

There wd be another Ai Micro Show in Bangley Hall onNovember 14th.

For further details. advance tickets etc. contact SharonAlward at Sharwarf Promotions, Knightsdae Business Centre.30 Knightsdale Road. Ipswich. Suffok P1 4JJ. Tel 01473741533, fax 01473 741361. Email: servicesittshaward.co.uk

Electronic Recycling Guidelines fromIndustry CouncilICER - the Industry Council for Electron c Equipment Recycing- has set up a Recycling Drectory for use in industries lookingfor serious suggestions about what to do with unwantedelectronic equipment

The cirectcxy fists some 30 LK companies whichin recycling different kinds of electronic and electrical

equoment mitered by category of equipment and therecyders who hands is, as well as an alphabetic frst ofrecyders OWN more information about the convenes.website giving access to the directory is also premed.

Anticipatirp that an EU drectne will raventualy reqUirerecycling is carried out by "approved recycles', ICER hasproduced a summary of 'best practice% defring thecriteria kr approved recyders". according to Gary Griffiths ofFrazier, who chars the 'GER recycing group. This includespoints such as:

-A recycler will need to: show the company hasicences or exemption certificates for ail sites n accordancewith waste regulations. and uses waste transfer notes andauthorised transport operations'. and class fy any wastefrom processes leg Contioled Waste, Special Waste) anddispose of tt property."

An ICER publir_ation, ICER Guidelines: Design for RecycingBectronics and Electrical Equipment s available, price £20 citePostage and packing (depending on destination) from PamGibson, CER 6 Bath Piece, Flyngton Street, London. EC2A

New Plasma Screen TechnologyReady to Hang on the WallPioneer's new television monitors are using new plasmatechnology that allows them to be slimmed down to Just 9cm (88mm. in fact) deep.

The immediate result of this ultra -slim profile is likely tobe more -Public Information Screens" in public places.The plasma screens are thin enough to be hung on thewall like a picture. making them useful in areas normallytno small or inaccessible for normal screens. For example.

the lifts at major airports could be fitted with plasma,careens giving the latest departure. arrival and check -in

information.Pioneer's innovative 40 -in colour plasma display

screens can be linked to video. PC CD-rorn. a normaltelevision tuner. even DVD. making them a versatile choicefor conference organisers and marketing organisations.

In due course the plasma screens are expected tocross over into homes for use with a TV tuner, to hang onthe wall or sit on slimline stands, far less bulky than

,rent TV tubes. The plasma screens' high brightness)0 candela per square meter - the highest level in theirld) and a 160 -degree vertical and horizontal viewing'Ile ensure that the screen is a screen that's seen - a

non -glare surface makes the screens suitable forlit environments (no more need to turn all the room

,'its down to cut off the glare and screen washout), whilelong lifespan (20.000 hours) and optional rain -proof

using making them durable enough for indoor orit door use.The base -model screens are available from April 1998

[,71 have built-in compatibility with TV tuners, amplifiers.Ps, external CPUs and DVD players, and are compatible

with PAL. NTSC and VGA video standards.For more information, contact Kate Moir or Julia

Savage, Storm Communications (Pioneer MultimediaPress Office) tel 01494 670444. Emailstormrnmrnacompuspr,y, r rur


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1:311 ki4.049This introduction to robotics by Harprit Sandhu explains what robots are and the

basic conditions needed to make them functional.

robot can be described as a programmabl'manipulator doing work that otherwise a hi:would have to do, in a manner at least some...'similar to the way a human would do it. Awashing-up machine, for instance, is not normally

: a robot. It washes dishes, but it does not place thedishes in a sink, use a mop, or stack them in the drying rackafterwards. The robot of popular fiction is a mechanical person;more often today. a robot resembles a part of a mechanicalperson. Most shop-floor robots are the emulation of one arm ofa human being.

Robot arms are used to weld. package. paint. position andassemble a host of everyday products. Parts from integratedcircuits to printed circuit boards and VCR tapes to automobilesare now wholly or partly made and assembled by robots.

There are three distinct component groups within anysophisticated robot(Ic) system. They parallel human functions:

1. Hardware: the bodyRobot arm and gripper: the arm and handVision system: eyes

2. The computer electronics: the brain

3.The software programming: the education

The mechanical part of the robot that does the physicalwork is designed to perform a specific family of tasks within aspecified envelope determined by the physical size of therobot. The majonty of the mobile robot population areAutomatically Guided Vehicles (AGVs). Their main use is thedelivery of raw matenal components. They provide anintelligent, mobile. programmable conveyor function.

A robot arm is a series of linkages connected up to work asa manipulator or arm. A motor controls each joint ormovement. The operation of each joint does not have to beindependent, but there are advantages to independentoperation. If operation is not independent, the computer canbe programmed so that for our purposes operation is virtuallyindependent. However. this makes the task more computer -intensive, because calculations may have to be made beforeevery move, and this computing time is no longer available forother computations. Even so, the mechanism is nowhere nearas complex or flexible as the human arm.

The modern robot arm is a six -axis device fixed to the shopfloor or to the machine it serves. It is controlled from a powerfulcomputer, usually nearby. The computer cabinet contains allthe logic components and amplifiers needed to run the robotShop floor robots can pick up pieces that weigh over 100

kilograms (220 pounds) and place them within about 1 mm(0.040 inches) of a desired location with ease. They can moveloads at about 1 meter (3 feet) per second and put them downas softly as you please. Gnppers or robot hands of many kindscan manipulate and control all manner of things.

Types of robotsRobots can be divided into families accordrig to the type ofwork they do. The major divide is between mobile andstationary robots. (See Table 1.)

Defining a robotIn some ways. a "robot" is whatever we agree to call a robot.We tend to use "robotic" to describe machines designed to actautomatically or autonomously. We might call an automatedvehicle a robotic jeep. Industrial robots are called robotic arms,and machine vision is referred to as roboticRobots in general:Are normally computer -controlled.Have components that move in some wayHave servo -motors incorporatedHave have a user interface to allow us to interact with them.The interface may consist only of a start button or akeyswitchCan be programmed to do tasksInteract with their environment with input and output signalsHandle or examine something external to themselves insome way

An AGV (Automatically Guided Vehicle) transporting paper rollsautomatically in a factory environment. Note the hand-heldpendant on the right. used for controlling the AGV manually.(Picture courtesy of Rocla (Finland)/Mentor (USA).)


It is worth noting that though playing chaos is very "human",we do not call chess -playing computers robots. If, however, wewere to add a rudimentary robot arm to manipulate the chesspieces, a mundane task compared to the ability to play chess.we would all agree that this was indeed a chats -playing robot.This shows that manipulation is an important element in ourperception of a robot. Yet we often call vision systems "roboticvision" even though no manipulations are involved.

ConclusionsWe conclude that a robot is a programmable. computer -controlled machine that manipulates products and/or tools todo work. It can make decisions and interact intelligently with itsenvironment. It can receive and send information to itsenvironment. It may or may not be mobile.

This is not the official definition of a robot stipulated by theRIA, but it sums up what the the official definition conveys:

"A re -programmable. multi -functional manipulator designed tomove materials, parts, tools, or specialised devices throughvarious programmed motions for the performance of a varietyof tasks.' Robot Institute of Amenca. 1979

A broader definition is: "An automatic device that performsfunctions normally ascribed to humans or a machine in theform of a human." Webster.

Robots as workersWe humans have an overwhelming interest in doing as little aswe can to get the job done. We design robots to improve onthe positive attnbutes and eliminate the negative attributes ofhumans. Almost the total effort is in this direction. The essentialdifference between the automatic machines of today andyesterday is that today's machines are programmable. Today,what they do depends on the information that we put intothem. Tomorrow, they will add intelligence, eventually massiveintelligence. They will be able to optimise their work and reactintelligently to all predictable and most unpredictabledisturbances. Gross unpredictable disturbances can behandled by shutting down, sounding an alarm, summoning ahuman supervisor, or rejecting the part. Minute changes fromcycle to cycle and part to part are harder to handle.They aresubtle and hard to define. detect and react to appropriately.

IntelligenceIn machines, intelligence is the implementation of sophisticated"If... Then...." strategies in which action depends on what hasalready happened. This looks intelligent, but it is really rigidlogic, perhaps with some randomness, range and flexibilityadded to the decision -making process. if. Then..." strategiescan be made extremely complex - this is what makes themuseful. Today, most robot language programming is done in theprogramming language C. C has around 28 instructions in it. I

say "around 28", because 28 instructions are defined althoughnot all are used. There is little limit to what can be built up withthese 28 kernels, and there are many sophisticated versions ofthe language. (Separate instruction sets address differenttasks). A language with only 28 instructions is relatively easy towrite (or port over) for a new computer design. All that has tobe done is to implement these 28 instructions.

Robot geometryTwo basic factors govern the motion and abilities of the robot:the geometry of the design and the sophistication of the

software. Robots come in many shapes and sizes. and thesoftware is optimised to the needs of the specific robot. In"three -space". our everyday three-dimensional environment,there are various ways to design a robot that will reach allpoints in its work environment. As a rule you need one motorfor each degree of freedom that you want to specify. So. atleast three motors are needed to reach a location in three -space (X. Y and Z coordinates). Another three motors areneeded to orient the hand in the three possible orientations(roll, pitch and yaw). This is the basic universal six -axis robot.

There are some mechanical design aspects of the humanarm that we should observe:

The distance from shoulder to elbow is greater than thedistance from elbow to wrist; which is greater than thedistance from wrist to knuckles; which is greater than thedistance from the first finger joint to the second finger joint Thedistance from the second finger joint to the fingertip is thesmallest distance on the mechanism.

The resolution of the system gets finer as we get closer tothe gripper and the work. There are also increasing numbers ofnerves in the human arm as we move from the shoulder to thefingertip. After the wrist, flexibility is provided by splitting thepalm, by opposing the thumb to the fingers, by removing onedigit from the thumb and by duplicating the fingers. Providingfingers of different lengths is a further refinement.

A 5 -axis, 6 -motor robot arm that can lift about lkg and run atabout 1 foot per second. This 32 -inch high Rhino XR-4 robot wasdesigned to mimic larger industnal robots in a teachingenvironment


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A Motorman SK6 robot arc -welding. The welding rod is fed inautomatically, and co-ordinated by the robot controller. All aspects ofwelding (arc detection, puddle size. depth of weld, wedling current, etc.)can be detected and controlled by the robot controller. (Picture courtesyof Motoman USA.)

This should give us a framework in which to think of thehuman arm as a robotic mechanism. This arm has done mostof what we need for milliong of vparc What does it tell us

about robot arms?

Industrial robotsThe moves we want our robot to make are dictated by thework we want it to do. The design will be an expression of theminimum capabilities needed to get that work done. The reachof each axis. the number of axes, the capabilities of thecomputer, the memory needed and the number of sensors (tcmention just some of the major design parameters), will all beminimised to produce the most economical robot possible(figure 1).

Robot geometries are defined by the shapes that therobot's design mimics. The most common robot configurationsare:

Articulated: Articulated robot armsact like the human arm. This is aflexible design that allows themost humanoid arm movements

Cylindrical: A cylindricalconfiguration dictates a robotwhose axes are designed to bespecified as cylindrical co-ordinates. This usually means anaxis that moves up and down likea cylinder, and another axis thatmoves in and out like a radiusvector.

Spherical: In a spherical coordinate robot the two angles anda radial sperification specify thepoint in space.

XYZ or Orthogonal: An orthogonalrobot is an XYZ machine. It hasthree sides that are arranged at

right angles to one another.

Gantry robots: Gantry robots have a robot gnppersuspended from a gantry that covers a large space

GrippersThe design of robotic grippers is a discipline in itself.Sophisticated gnppers have microprocessors dedicatedto their control. Almost all gnppers fall into the followingthree categories:

Vacuum Operated Gnppers: these allow the robot to pickup parts that have a flat surface. Small, delicate parts orlarge flat sheets often need vacuum grippers.

Pneumatic Operated Grippers: these allow simple open -close control. The fingers can have many shapes andconfigurations, including two-, three- and four -finger

units.

Electncally Operated Gnppers: these are the mostsophisticated type. They allow the greatest flexibility of useand need the most sophisticated software. They also need

the most maintenance.

SoftwareThe most important part of the robot is the software. Softwaredetermines what can be done with existing hardware and howit is done. Of course, if the appropriate hardware for a taskdoes not exist. the task cannot be carried out. and if thesoftware does not address the hardware in an appropriate way.even appropnate hardware is useless.

Sensors and transmittersSensors that the robot employs can be located either on therobot itself, on the gnpper, or in the robot's immediateenvironment. Sensors tell the robot controller about the robotitself, or its immediate environment. or about what is currentlyin the gripper. The robot can only take in information for which

Figure 1: axis designations in articulated and side -arm robots


LIGHT RECEIVERS

SIGNAL A zSIGNAL e( (

OV

LIGHT TRANSMITTER

(-7)

O

Figure 2: an optical encoder construction. Signals A and Bwill be 90 degrees out of phase. Here, signal B is On andsignal A is Off.

it has the appropriate sensors, and only transmit informationfor which it has appropriate transmitters. Only those things thataffect the robot and its operations are of interest to the robotoperating system. All other phenomena are ignored.

Sensors used with robots fall into the following categories.These are listed in "distance to robot" categories. A robot canonly receive signals from its environment through its sensorsystems. The robot emits signals to send information to itsenvironment. These can be electrical signals by wire. thephysical closing and opening of relays, specific light, soundand radio signals or some other form of signal.

Robotics and soundSound is easy to generate, detect and record. Using sound todetect distance involves emitting a specific frequency in shortbursts and wafting for the sound to bounce back from

surrounding objects. Emission anddetection are relatively time-consuming,so this can be done only 20 to 50 timesa second.

Sound moves across a space as aseries of pressure waves. If we sample asound 40,000 times a second (twice thehighest frequency we can hew), we canre -transmit that sound, and the humanear will find the reproduction completelyfaithful. However, less than 20.000 Hz issufficient for robotics.

Signal noise (such as a badconnection, or distortion) can be aproblem, but the biggest problem isnormal acoustic background noise - thekind of noise that makes conversation ina noisy factory impossible. Noiseproblems can be minimised by usinghigh frequencies and modulating themso that the signal can be more easilydistinguished.

Having our 20.000 bytes per second,we must analyse rt. The analysis iscomplex and time-consuming. ft ispretty certain that the sound will not bedeciphered neatly one byte at a time.

Some form of front end filtenng is used to sort out the correctsound so that we receive the information efficiently. Once weknow what is being said, we must decipher what it means. Inrobotics, this is made easier by limiting the machine'svocabulary. Voice recognition is already available for quite smallsystems (one of ETI's contnbutors uses rt on a PC), andtelephone companies are using it to some extent.

SLOT A

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SIGNAL A

SIGNAL B

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10 11 01 10 11

Figure 3: incremental optical encoder signals shown 90 degrees out of phase

16SIGNAL D

SIGNAL ASIGNAL B

SIGNAL C

Figure 4: an absolute encoder disk. Four signals are shownon this disk, which would give 16 divisions on a circle.

Running motorsThe two main pieces of equipment needed to control a motorwith a computer are optical encoders and power amplifiers.Encoders tell us what a motor is doing in terms of its rate ofmotion and overall revolutions, and amplifiers allow us tocontrol the motor by controlling the energy supplied. Bycombining these with the speed of a computer, we can controla motor very precisely.

EncodersRobot motors are usually linked to optical encoders using anoptical signal system to provide motion information. Thesignals sent to the computer are usually of the type that switch


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on and off with respect to time. The information is in the timing,and there are often millions of zeros and ones to read and sortout.

The position of a motor shaft must be measured preciselyto be controlled accurately. These measurements can be takenwith measuring devices that divide the position of the motorinto thousands of counts per revolution. Optical encoders candivide a circle into over 1000 divisions. Combining this withappropriate gearing to the robot arm achieves the necessaryaccuracy.

Optical encoders come as incremental and absoluteencoders.

Incronsontal oncodorsIncremental encoders (figures 2 and 3) provide two signals 9Cdegrees out of phase. Each signal is assumed to switch onand off for equal periods of time during each cycle. as shownbelow. The convention states that one complete on/off signalcycle represents a 360 -degree rotation.

When a motor moves in one direction, one signal leads:when rt moves in the other direction, the other signal leads.This tells the computer which way the motor is turning, andallows it to determine the progress and position of the motoreven after many revolutions. Speed is determined by takingtwo encoder counts a known time apart, and dividing thepositional difference hv the elapsed time.

Absolute encodersAbsolute the position of the axis they areconnected t .Av'lhtl' 41 single revolution; therefore, they areusually fixed not to the motor but to a robot arm pint. Whereasincremental encoders must have a known starting point tocount from, absolute encoders can provide the arm position atall times without reference to a known start -point (see figures4 and 5). Each ring on the encoder is divided into alternatedark and light segments to give a binary signal. Each nng hastwice as many segments as the previous ring, providing finerand finer positional information moving away from the centre.The nngs must be read simultaneously. Eight nngs provide aresolution of 1.41 degrees, or 1 part in 256 in a circle. Sixteenrings provide a resolution of one part in 65,536 (256 x 256), or0.0055 degrees.

MotorsNow that we have an understanding of how positional

POWER

696441

DIRECTION

Figure 7: a basic "H" Bridge amplifier. In this A, B, C and Dare large transistors that act as switches. Either A and 0 orB and C can be turned on at one time. Power wiring isshown as heavy lines.

sawn Al 7

SIGNAL a -

SIONAL C

saws 0 -1

POSITIc o t 2 3 4 5 6 7 6 9 /0 1' /2 13 4 15

Figure 5: signals from an absolute encoder.

L

ONSLOW

OFF

ONMEDIUM

OFF

ONFAST

OFF

ONE CYCLE

Figure 6: PWM signals. Energy fed to a DC moto

information can be obtained with the desired accuracy, weneed to understand how a motor is positioned to exactly theposition we are interested in.With regard to the motor, there are two things you need toknow over a period of time: how fast is it n, rig and how longhas it been running. With this information we can determinewhat the motor has done and what it is currently doing. Add tothis the very accurate clock in the computer, and we can dothe following:

Start a motor when we want; accelerate It at a desired rate:reach any velocity reasonable for the application; maintain thevelocity under varying loads; slow the motor down as wewant: slow clown the motor at a desired rate; stop the motor atthe exact point that we want; base the stopping point on timeor on revolutions turned; base the control on internal orexternal events: change the control algorithm at will and asoften as we want.

The speed of a DC motor can be controlled by changing theamount of energy supplied to it. Changing the load on a DCmotor also changes its speed. The motor is stabilised bymonitonng it and modifying the energy supplied to maintain thedesired speed. In a robot with an ever-changing load on themotor, these energy corrections must be made hundreds oftimes a second.

There are two basic ways of controlling motor speedelectrically: to control the voltage to the motor by an analoguemethod, or to turn a fixed voltage rapidly on and off. In a digitalsystem it is easier to turn the signal on and off to vary theamount of energy sent. This technique is called Pulse Width


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Figure 8: motor turning in one direction

Figure 9: motor turning in the opposite direction

Modulation (PWM). The length of the energy pulse is modifiedin each cycle (see figure 6). If the signal is on all the time. thepulse width is long, and the motor runs at a high speed. If it ison half the time, the pulse width is 50 percent and the motorruns at about half speed. If it is on none of the time. the pulsewidth is zero and the motor remains off.

The motor must be switched very rapidly for smoothoperation. About 200 times a second (Hz) is enough for mostmotors, because the mass of the motor and its attachedcomponents averages out the speed. However, these arefrequencies that human beings find very annoying whensomething loose in the motor windings starts to vibrate. An on -off frequency above 20,000 Hz is usually chosen. 40,000 Hz

the hen-ng rangy of most domestic animals as

The PWM amplifierThe ti2sic PWM motor amplifier is an H bridge (see figure 7)In this bridge, if transistors A and D are turned on, currentflows in one direction in the motor windings, and the motorruns in one direction. If A and D are turned off and transistorsB and C are turned on, the current flows in the other directionand the motor runs the other way (figures 8 and 9). Iftransistors A and C or B and D come on together, there is adirect short circuit from power to ground, destroying theamplifier. It is crucial that one set of transistors is shutcompletely before the other set comes on. Because the

switching of the transistors is very rapid but not instantaneous.overlap is a nsk. Logic circuits in the amplifier should ensurethat this does not happen.

A servo -motor must respond to commands within certainparameters to be useful in a robotic application. A motor wouldnot be performing its job if any of the following happened:

It does not turn on immediately when power is applied; it doesnot stop immediately when power is removed: it does notmove as fast as rt needs to: it moves faster than told to; itsoperation is not repeatable.

Motor control schemes should ensure that none of this occurs.Otherwise, the system should detect it and warn the operator.Usually, when the error between what the motor is supposedto do and what it does exceeds a certain number of encodercounts, the condition is flagged as a performance error.Nevertheless, there are always delays between an event, thetaking of readings. their interpretation, and initiating correctiveaction.

Spood controlThe control of motor speed by the percentage of time themotor is switched on (figure 10) is done with software. As themotor turns, the encoder counts go to a counter that thecomputer reads whenever it needs to. The computer uses thatinformation, and the time from its internal clock, to change thepulse width to the amplifier to maintain the speed and positionof the motor.

What follows here is an English -language -like program tomaintain a motor speed with the integrative tunctron only;

Label 1

Read the position counterCompare it with the lastreadingCalculate the motor speedIf it is faster then desired, lower

the currentIf it is slower than desired,

increase the currentTake care of other functionsGo to Label 1

The computer code will look less like English. but will funthe same function.

The control loop consists of a series of expressionsrepresenting the proportional, integrative and derivativecomponents of the energy needed by the motor. This is a PID

2s%J

5°%

'5%

100%

Figure 10: the PWM duty cycle. One cycle is shown.


equation, and we often refer to thisform of control as PID control. It isimportant to know that having threecomponents allows better controlof a motor.

The motors used to control-otors

(see figures 11 and 12) 'Servo"m La'. a slave)

,,t_,resses the 'deo the motortakes orders horn a changingsignal, here called an error signal. Ineffect, the motor responds to anerror signal by trying to make thesignal disappear. Whatever themotor is connected to affects themagnitude of the error signal, andthe motor is made to work or movein the direction that will reduce theerror signal. An external (to themotor -load) device is also able tomanipulate the error signal. Sincethe motor is designed to follow thesignal. we can make the motor douseful work by programming theerror signal.

ACTUALPOSITION

INFORMATION

POSTON ENCODER

POWERa

DESIRED4. POSITION

COMPARATORINFORMA T 'ON

ERRORSIGNAL

AMPLIFIER

Figure 11: motor feedback scheme schematic

We can now tell the motorexactly when to start, how fast to speed up. how fast to run,how long to run, how to slow down and exactly when andwhere to stop. We can tell the motor to repeat the cycle asoften as we like, we can cycle it just once. and we can modifythe cycle at will while the cycle is in progress. This allows us tomake the operation of the motor dependent on any conditionwithin the controlling computer or on any signal that can beread by the computer.

In Current technology. electric motors are put into motion bycreating opposing and attracting magnetic fields. The magneticfields are manipulated sequentially to rotate a shaft attached tothe rotating armature. Since at least one of the magnetic fieldsis created by passing an electric current through a winding,and we can control electricity, we can control the motor bycontrolling the electricity through its windings.

The following motor designs are suitable for robots:

Rotating Servo -motors: these are usually DC motors with aposition encoder attached to the motor shaft, usually anincremental optical encoder, to provide a feedback signalgoring the position and speed of the motor.

Linear servo -motors: these use the same technology asrotating motors, except that their armatures and fields arearranged in a straight line rather than a circle. Imagine cutting aregular motor along a radial plane, opening it up and laying itflat on the table. (Don't try this at home!) The field, thearmature and the commutator are in straight lines, and themotor armature moves across the field in a straight line.(Appropriate physical constraints and guides are added.)

Stepper motors: these move through a small segment of acircle each time the current to the motor is changed. There areseveral independent windings in the motor, and usually asequence of four electrical changes. The motor moves a stepdunng each change. There are also half steps and micro steps,created electronically. Stepper motors usually move between

0.9 and 15 degrees per step, and they do not normally needfeedback encoders, as we can count the motor's steps as wesend them. However, if the motor is overloaded, it can slip, andthe count will be compromised. Stepper motors usualy havelow speed and low torque characteristics. Their main use is inoffice machinery and other low power applications.

For the motor movement to be predictable, it is made to followa trapezoidal profile (figure 13): the motor accelerates at a fixedrate until it reaches its operational velocity. It then deceleratesat a fixed rated until it stops. The computer calculates theenergy needed by the motor to do its work in the timerequired. Motors operating in synchronisation with anothermotor must accelerate and decelerate for the same length oftime as the lead motor to keep their encoder counts in synch.

For very short moves, there is no constant speed operation.The motor accelerates, decelerates and stops, never gettingup to the running speed. This move profile is called a triangular(three-point) move.

It is very hard to run a motor at a very slow speed. Imaginetrying to run a motor at one revolution per day (gears notallowed). If we wanted to adjust the speed every second wewould need 24 x 60 x 60 = 86.400 encoder counts perrevolution to get one count per second, and we probably need1000 thousand times that for smooth motion. Let's assume anencoder that gives the minimal 86,400,000 counts: if we alsowanted to run this motor at 3000 revolutions per minute at itstop speed, we will need encoder counts for 3000/60 x86.400.000. or 4.320,000.000 counts per second. Though notimpossible, this causes problems. Minimum speed. maximumspeed and encoder counts are interrelated.

For a motor in a servo application, we want to define a topspeed that will use less than the maximum energy the motorcan accept. For example. if we want to run a device at 100rpm and we have geared the motor to the device with a 20:1ratio, the full speed of the motor will need to be 2000 rpm toachieve the speed we want. It is desirable that this full speed


be attained when the motor is being given as little as 25percent of the energy that it can readily accept (withoutoverheating or having other operational problems). The rest ofthe energy will be applied when there is a load on the device.This gives us the reserve power we need to meet the loaddemands of the application. A 25 percent duty cycle is not amagic number - the percentage would depend on the load thedevice was designed for. There must be plenty of reservepower to allow the system to be fully compliant: a compliantsystem is one that can do what we tell it without running out ofany of its performance parameters.

As the need for power increases, the size of the drivingtransistor increases. A large transistor is, very roughly, a lot ofsmall transistors in parallel. You can control a larger current byputting 8 or 10 small transistors in parallel with one another.This is tolerable in a hobby situation, but not in a commercialapplication, where special transistor arrays with safety featuresadded and designed specially for controlling motors are used(See figure 14.)

We need three basic inputs to control a motor amplifier: asignal to enable the motor amplifier; a signal to select direction

of motion(forward orbackward): anda signal todetermine thepercentage offull powerapplied (thepulse width).

A signal toenable themotor amplifieris important toensure that the

amplifier will remain turned off till the computer has come up toa stable condition and is ready to control the motor. This isspecially important with a robot motor, because the motormust not move until we are absolutely ready for it. or damagemay be done. This is achieved with fail-safe electronics andtying other signals to ground potential until they are ready to bereleased.

40 000 SECOND

4

FULL VOLTAGE

- 0 VOLTSPWM SIGNAL

Figure 14: power applied to motor

Model aircraft servo motorsRadio -controlled model aircraft use tiny servo motors.positioned by converting the length of pulse fed to them intothe position of the servo's output shaft. These servo motorsconsist of motor, a gearbox. a potentiometer and a smallintegrated circuit. The is converts the length of the incomingpulse to a desired output shaft position (represented by ashaft -mounted potentiometer). If the position of the pot doesnot match the desired shaft position, the motor is turned

:'is or backwards till the pot reaches the desired position.T ,,pant tr, the motor is then turned off. The situation is

so that correction is taking place all thelarf- (See figure 15.)

curacy depends on the accuracy of boththe pot and the pulse length. A resolution of about half adegree can be achieved with most model aircraft servos.

These servo motors run on three wires: the power line, thepulse signal line and a common ground. As model aircraftservos operate by converting a known pulse width to a motoroutput shaft position. they are relatively easy to use if a sourceof short pulses can be created. This is quite easy with the

BALL NUT

BALL SCREW

ENCODER

Figure 12: components of a typical servo motorinstallation. Gearboxes are usually _wooded to eliminatebacklash problems.

SPEED

CONSTANT SPEED PHASE

ACCELERATION

START

Figure 13: a trapezoidal motor move

DECELERATION

STOP

TIRE L___>

ARM

CONNECTOR

MOTOR

Mir

Figure 15: model RUC servo -motor

GEARBOX

ELECTRONICS

small single chip microcomputers now on the market. Theseinexpensive units are suitable for robotic experimentation.

(My book 'Introduction to Robotics" from Nexus givesdetailed plans and instructions on how to build and program awalking robot with such motors.)

Cover: The Mitsubishi RV-E2M Movemaster, a smallsophisticated table -top robot with a positioning "teach"pendant and controller. (Courtesy: Rixan. US Mitsubishidisnbutor.)


Three -RangeINDUCTANCE

METERA partner to last month's Capacitance Meter, Robert Penfold's Inductance Meter is

straightforward to calibrate and needs no scale recalibration.

This inductance meter is a companion project to thecapacitance meter published last month. Like thecapacitance meter. it is very easy to calibrate anduse. Only one inductor is needed to calibrate theunit, and no recalibration of the meter's scale plate

is needed. because the unit has linear scaling. The threeranges have full-scale values of 1mH, I OmH, and 100mH,which enables a wide range of inductors to be checked. Thisincludes high value components for use in audio filters, mediumvalue inductors as used in switch mode power supplies. andmany of the small chokes used in radio frequency circuits. The

cannot be to measure very low valuechokes of less than about 22uH in value. but these are littleused in practice. They are difficult to measure accuratelybecause the inductance in the leadout wires and test leads isquite significant in relation to the value of the componentsunder test. In fact, the inductance of a very small choke can beconsiderably less than the inductance of its leadout wires!

It is only tali to point out tnat tnere is a signt problem wnentrying to test inductors. which is that their inductance isinvanably frequency dependent. Most inductors are designedwith a specific frequency range in mind, and tend to havegenerally lower values outside that frequency range. In thiscase, the inductance is measured at a frequency of just over10kHz, which gives good results with medium to high valueinductors as they are usually designed for optimum results atlow to medium frequencies. Results with radio frequencychokes may seem to lack accuracy because the unit ismeasuring the inductance at a frequency that is below theirdesigned operating range. The meter is giving accurate results.but at an inappropriate frequency. Readings obtained whenmeasunng high frequency chokes must therefore be treatedwith a degree of caution. This is a problem that is common tomost inductance meters and bridges incidentally, and it is not ashortcoming specific to this particular unit.


System OperationAs can be seen from the block diagram in figure 1. thegeneral make-up of the unit is quite similar to that of thecompanion capacitance meter project. It operates in essentiallythe same manner, but things have to be swapped aroundslightly in the amplifier stage at the heart of the circuit. Thiscompensates for the fact that the impedance of an inductoruses with increased value, whereas the impedance of acapacitor reduces with increased value.

The oscillator stage of this circuit is simpler than theequivalent stage of the capacitance meter. As before, theaccuracy of the unit depends on the oscillator providing areasonably pure sinewave signal. In the capacitance meter. theoscillator operated at about 100Hz. and the sinewave signalwas generated using a triangular waveform generator plussome lowpass filtering. In this case a much higher frequency ofabout 10kHz or so has to be used, because the impedance ofthe test components are impracticably low at lowerfrequencies. There is potentially some advantage in using ahigher test frequency, but the operational amplifiers in thecircuit do not perform well at much more than about 10kHz,and this was found to be a good compromise figure. Thesignal is generated by a Wien oscillator, which has the usual

thermistor stabilisation to ensure kt!luuyt1simpler than the filtering method, using a Wien oscillator ismore expensive. However, at the higher test frequency of10kHz a Wien oscillator seems to give more consistent results.

An inverting amplifier receives the sinewave signal, and anegative feedback network governs the closed loop gain of this

-section network that has one of threeswitched resistors forming the input arm. These resistorsprovide the unit with its three measunng ranges. The inductorunder test provides the other arm of the negative feedbacknetwork. The voltage gain of the circuit is equal to theimpedance of the test inductor divided by the resistance of theinput resistor. At a fixed test frequency, the impedance of aninductor is proportional to its value. For example, it a 10mHinductor has an impedance of 10k, a 5mH inductor wouldhave an impedance of 5k, and a 1 mH inductor would have animpedance of 1k. Suppose the input resistance has a value of10k, this would equate to voltage gains of 1. 0.5, and 0.1 withtest values of 10mH, 5mH. and 1mH. Of more importance, ifthe output voltage were 1 volt with a 10mH choke. it wouldrespectively be 0.5 volts and 0.1 volts with 5mH and 1mHchokes. In other words, the AC output potential is proportionalto the value of the test component.

The output signal from theamplifier is fed to a precision half -wave rectifier and a moving coilmeter. These form a simple butaccurate AC voltmeter circuit thathas good linearity. The meterresponds to the output voltage fromthe amplifier, but as alreadyexplained this voltage is proportionalto the value of the test component.With the right voltmeter sensitivity.test frequency, and so on. the meterwill read directly in terms ofinductance. and the scaling will belinear. This avoids the need for any

METER

O PRECISIONRECTIFIER

INVERTINGAMPLIFIER

lJTEST

INDUCTOR

Figure 1: the block diagram of the inductance meter.

RANGERESISTORS

SINEVIAVEOSCILLATOR


R7

SI 1k0=-0%WOE

RA53

.LF35iNICY. 1.5411CNiC3 CA3140E

R11

91(1 SKY)-0020Crom

CS RI,ok

. 6

Figure 2: the full circuit diagram of the inductance meter.

CI100n

II ICJ

-2

R121C5N

1113

R14614

R151k

15C*

07.137

recahbration of the meters scale plate, and renders thecalibration process very easy indeed.

Circuit operationne tuil circuit diagram for the inductance meter is in figure 2.

The Wien oscillator uses IC1 in the conventional arrangementwith the positive feedback provided by R3, R5. Ca and C4.These set the operating frequency of the sinewave oscillator atabout 10.7kHz. Negative feedback is provided by R4 and TH1.with TH1 stabilising the output level of the oscillator. Precisecontrol of the negative feedback is essential if a low distortionoutput signal is to be obtained. Slightly too little feedbackresults in strong oscillation, and the output signal becomingheavily clipped. Fractionally too much feedback causes theoscillator to stall and the output signal to cease.

Thermistor TH I is the usual negative temperature coefficienttype, which means that its resistance falls as its temperatureincreases. Unusually, it is a self -heating thermistor that iscontained in an evacuated glass envelope so that it is, as faras passible, isolated from the ambient temperature. At switchon TH1 has a low temperature and therefore exhibits a highresistance. This gives relatively little negative feedback, highclosed loop voltage gain from IC1, and strong oscillation. Astrong current flow through TH1 reSults, causing itstemperature to rapidly increase. This produces a reduction inIC1 's voltage gain, and the oscillation decays to a low level.Now there is reduced current flow through TH1, its

14 3:6111111111111111111111111110

temperature decreases. and the circuit oscillates more strongly.After a certain amount of seesawing, the output level tends tosettle down and stabihse due to what is really a simple form ofnegative feedback action. The negative feedback stabilises theoutput level very efficiently, counteracting changes in outputloading, the supply voltage, and so on. An output level ofapproximately one -volt RMS is produced at the output of IC1.

IC2 is the operational amplifier that forms the basis of theinverting amplifier stage. R6 to R7 are the three range resistorsand S1 is the range switch. R6 to R8 respectively provide the1mH, 10mH, and 103mH ranges. Using an inductor in thenegative feedback of an operational amplifier tends to produceproblems with instability, and can easily lead to strong highfrequency oscillation. There are no major instability problemswith this circuit, but it is necessary to include R11 to damp thetest inductor slightly. Otherwise the circuit is apt to break intooscillation on the 100mH range. IC2 operates open loop withno test component in circuit, taking the meter beyond its full-scale reading. While this is unlikely to cause any damage to themeter, it is a possibility can not be completely ruled out.Therefore, S2 is used to place a short circuit across the testsockets until a test component has been connected acrossSK1 and SK2. S2 is then operated. removing the short circuitso that a reading can be taken.

The precision half wave rectifier is a very simple type thatuses IC3 as a non -inverting amplifier. The circuit is basically justa standard DC non -inverting circuit, but IC3 does not have anegative supply rail. Consequently, positive input half cyclesappear amplified at the output of C3 in the normal way, butnegative half cycles are not produced because the output ofIC3 can not go negative of the 0 volt supply rail. The outputsignal from IC3 is therefore a slightly amplified and half -waverectified version of the input signal. This signal is used to drivea simple voltmeter formed by R15, RV1, and MEI. RV1enables the sensitivity of the voltmeter to be vaned, and thisenables the unit to be calibrated.

With S3 set to the right hand position ME1 operates as asimple voltmeter which measures the battery voltage. The full-scale sensitivity of this circuit is nominally 15 volts. Thisprovides a simple battery check facility, and the unit will onlyproduce accurate results if the supply potential is more thanabout 10 volts. The current consumption from the 12 -volt


Figure 3: the component layout for the 50 -hole x 20-stnp stnpboard panel.

battery supply is about seven or eight milliamps, which givesan extremely long battery life. Note that a regulated supply isunnecessary because the thermistor stabilisation prevents theoutput level from IC1 changing significantly with vanations inthe supply voltage, and the dosed loop voltage gains of IC2and IC3 are also independent of the supply potential.

Constructors of this protect should note that the operationalamplifiers used in this device were chosen after carefulconsideration, and are not random choices. The device usedfor ICI must be capable of providing a good quality sinewavesignal at 10kHz into a fairty low load impedance. The amplifierused in the IC2 position must have high enough performanceto give good results, but should not be so lively that instabilitybecomes a rnaior problem. It must also be capable of stableoperation with less than unity voltage gain. The device used forIC3 must be a type that is suitable for use in a single supply railDC amplifier circuits. Using alternative devices for any of theoperational amplifiers in this circuit is likely to result in it tailingto work properly

ConstructionThis circuit is simple enouo' ' ''struction, and asuitable layout is provided figures 3 and 4 Theserespectively show the corn: side views of the

OOOOO .

. 41

N

p

..Figure 4: the underside of the component panel

board, which measure 50 holes by 20 copper strips.Construction of the board follows along the normal lines, with aboard of the correct size being cut out, the necessary breekSbeing made in the copper strips, and the two 3.2 millimetrediameter mounting holes being drilled. The components aridlink -wires are then fitted. The CA3140E used for IC3 is the onlystatic -sensitive component, but I would recommend usingholders for all three integrated circuits. Resistors R6, R7 andR8 should have a tolerance of 1 percent so that goodaccuracy is provided on all three ranges. Therrnistor TH1 has aglass encapsulation that makes it relatively fragile, and it shouldtherefore be handled with care. It is probably best to fit thiscomponent last of all. The RA53 thermistor is also available asthe R53, and either will work property in this circuit. It is quitean expensive component, and it seems to be worthwhileshopping around to find the best price.

The prototype is built into a small metal instrument case ofthe same size and type that was used for the capacitancemeter protect. The general layout of the unit is much the sameas for the capacitance meter protect, and the front panel layoutis virtually identical. The only difference is that pushbuttonswitch S2 has been added between the range switch and thepanel meter (see the accompanying photographs). Note thatS2 is a push -to -break switch, and not the more common


Figure 5: details of the hard -wiring (use with figure 3).

push -to -make variety. In general the exact layout used is notcritical, but try to keep the leads from the circuit board to thetest sockets (SK1 and SK2) as short as possible.

Details of the hard winng are provided in figure 5. whichshould be used in conjunction with figure 3 (for example. point'A' in figure 3 connects to point "A" in figure 5). This winngstraightforward and should not present any problems. St -

standard three-way four -pole rotary switch, but in this -

only one pole is used and 12 of its tags are left unconnectedThe battery pack consists of eight HP7 (AA) size cells fitted inplastic holder. There is just about enough space for thebatteries inside the case. but the holder will probably have tobe fixed to the rear panel or top of the case using double -sided adhesive pads. Do not simply leave the battery packrattling around inside the case, as it would probably damagethermistor TH1. Connections to the battery holder are via anordinary PP3 size battery clip.

Many inductors will connect direct to SK1 and SK2 withoutany difficulty, but some components have very short pins ratherthan leads. It can be quite difficult to make the connections tothese. and the best solution is to make up a pair of test leadsfitted with the smallest crocodile clips you can obtain.Alternatively small probe -clips can be used. The test leadsshould be as short as reasonably possible so that they add aminimal amount of inductance in series with the test

componentsIt is not difficult to remove the scale -plate of the meter and

add figures to suit the 1mH and 10mH ranges (the existing 0100 scale is coin: LA for the 100mH range). Normally the frontcover of a panel meter simply unclips. and removing two tinyscrews then frees the scale -plate. Rub -on transfers can thenbe used to add the new numbers. As rt is not difficult tomentally convert readings on the 1mH and 10mH ranges intomeasured values, and moving coil meters are easily damaged,it is probably not worth the nsk and effort involved in alteringthe scale -plate. If you do decide to modify the scale -plate.proceed very carefully. In particular, avoid touching the meter'sDo,nier and t,. n- In, .'110 the meter movement.

Calibration and use!ed against a precision

inductor roving a tolerance of one percent or better. In practicea 'ewer tolerance nnmponent will probably have to be used

because close tolerance inductors do notseem to be readily available. In fact most ofthe inductors currently on offer have atolerance rating of some 10 percent.Probably the best choice for calibrating theunit is a 10mH type 8RB inductor (as soldby Cirkit Distribution Ltd.). This componenthas a five percent tolerance rating, but fivepercent is the guaranteed maximum error.Actual ccrnnonentS tested were all well

..-lerance limits.To t' -e unit set S1 to the 10mH

range itne middle setting) and connect the10mH calibration component to SK1 andSK2. With S3 set to the 'inductance"position, press S2 and adjust RV1 forprecisely maximum reading on the panelmeter. The unit is then ready for use. Bearin mind the warning given earlier abouttesting high frequency inductors. Withthese components the measured value at

10kH2 is likely to be slightly different to the marked value.

ResistorsAll 0.25W 5 percent metalotherwiseR1,2R3,584,15R6R7

R8R9,10,13R11,12 100kR14R16

RV1

TH1

2k21k51k100R 1 percent1k 1 percent10k 1 percent10k

6k8150k10k min hor presetRA53 or R53 thermistor

CapacitorsCl 100u 16V radial electC2 47u 16V radial electC3.4C5C6

10n polyester10u 25V radial elect100n polyester

SemiconductorsIC1 LF351NIC2 LF411CN

IC3 CA3140E

MEISK1,2Metal Instrument case about 152 x 105 x 75mm,0.1 Inch stripboard with 50 holes by 20 copperflerlPg, battery connector (PP3 type), control knob,flTlit OIL holder, wits, solder. etc.

MiscellaneousS1 3 -way 4 -pole rotary (one pole

used)

S2 Push -to -break switchS3 SPDT mm toggle switchS4 SPST mm toggle switchB1 12 volts (8 x AA size cells in

holder)100uA moving coil panel meter1mm or 2mm sockets


RADIO INTERFERENCEFILTER for VHF

High -Band ReceivingAntenna

Reception can easily be swamped bylegitimate strong transmissions nearby,particularly by radio -paging. This filterhas reduced the problem dramaticallyfor designer E Chicken MBE FIEEG3BIK.

article describes a low-cost filter ofstraightforward construction, designed to removeor substantially reduce destructive interferencefrom strong local transmissions in the 120-150MHz range. such as interference to amateur

radio reception in the 2metre band, or to reception ofweather -satellite or air-traffic signals.

"Swampod"In some parts of the UK, legally authorised radiotransmissions on frequencies close to the edge of the144MHz amateur band can make reception of amateurtransmissions difficult due to close -proximity swampingeffects. For example, in my own locality, reception of 2 -metrelow-level signals front the Russian space -craft MIIR suffersbadly from a nearby Home -Office radio tower. This does notimply that such transmissions are in any way at fault. It issimply a technological fact of life that the rf input stages ofeven the best receivers can be swamped by strong localtransmissions, especially on adjacent frequency bands.Similar problems can also be encountered if you enjoylistening to air-traffic signals. And if you were to mention theword "radio -pager" in the presence of the 2000 or more UKweather -satellite receiving enthusiasts, you might be told in nouncertain terms to go wash your mouth out!

The reason for this powerful aversion to modern pagingsystems among 137 -MHz weather buffs is that since thatband was assigned to digital paging in the UK. weatherpicture signals from low -orbit satellites have been sufferingsevere interference throughout the UK. The very nature of thepager transmissions. coupled with the nationwide distributionof their radio -sites, has made this a very difficult problem todeal with, to the point where NASA may have to change afrequency on its proposed new weather satellite.

Many long-established weather -satellite receivers have had

The intenor of the Interference Fitter

to be redesigned to combat this new and destructiveinterference. In truth, my own receiving set-up does not suffertoo badly, but only because it is a modified ex-pmrcommercial receiver of very high technical specification with ahighly selective multi -stage tuned rf input circuit and verynarrow IF bandwidth. Although its narrow bandwidth causessome loss of detail on the received images, in practice thedefinition of cloud and land has up to now been adequate formy purposes.

Recently, however, following the announcement of extrafrequencies for further planned satellites, and bearing in mindthe cost of extra crystals for my ex-pmr set, I decided as anexpenment to modify a domestic VHF FM radio -tuner boughtas a bargain at the local car -boot sale. I re -tuned it to receive137 -MHz weather satellite transmissions.

It was a quality product with a tuned rf stage preceding thetuned mixer, and the usual upper limit of 108 MHz prior tomodification. Results were encouraging, with sensitivitysufficient to give full background quietening by the satellite fmsignals. but, alas badly disrupted by radio -paging


UMMtlit RI a+1jpPA.s,s111`00_flt a6

A good commercial filter beside the Interference Filter giving a sue companson An unusual feature of our design is the third trimmercapacitor, which can resonate the link -loop itself for optimum signal transfer at the required frequency.

transmissions even though my nearest tower is some 1.5kmaway! Now I knew how my fellow Wefax enthusiasts reallyfelt!

The problem of co -channel interference is not in itself new.It is and has always been of major proportions on commercialradio -communication sites. where receiving antennas have toshare the same tower as transmitting antennas on adjacentfrequencies. One effective and often -used cure to thatproblem is the installation of a sharply tuned band-pass filtersuch as that illustrated in figure 1. in line with the receivingantenna's coaxial feed -cable at the point where it connects tothe receiver. Based on that knowledge from my professionalexperience, a similar filter was constructed using readilyavailable low-cost components at a total outlay of about £6at 1998 prices. This totally cleaned up reception on my

mdiC3 )14

ANTENNA50,75 ohm 0 RECEIVER

Kv7Sehrs

°POUNCED DOUBLE -COPPERSCREENING BOARD

Figure 1 the circuit of the radio interference filter forvhf antennas

2metre amateur -band receiver.The fitter also removed the last vestige of radio -pager

interference to weather image reception on my modified ex-pmr receiver, and it even allows reception of acceptablyclean weather images from my bargain-pnced retuneddomestic fm receiver. As with all filters, there is some slightinsertion loss on the signals of interest. but not enough tocause concern, as the advantage far outweighs anydisadvantage.

Th circuitFigure 1 gives the circuit diagram of the filler, which is fullyreversible, with the antenna coax feed -line connecting to oneend and the receiver to the other via a short coaxial jumper -lead with suitable plugs. It consists of two separate butidentical parallel -resonant LC circuits each tuned to. forexample, 145MHz for use on the 2 -metre amateur band, or137.5MHz, which is about mid -band for weather -satellitesand is one of the most used frequencies. A metallic screenelectrically separates the two timed circuits L1 /C1 and L2/C2,and the required incoming signal is transferred from one tothe other by means of a closed loop. This consists of twosingle -turn link -coils L3 and LA connected in series. each ofwhich is inductively close -coupled to one of the resonantcircuits.

An unusual feature of this design, which makes it differentfrom a typical commercial version, is the addition of a thirdtnmmer capacitor C3 whereby the link -loop itself can beresonated for optimum signal transfer at the requiredfrequency. This also ensures maximum rejection of the radio -pager or other interfering signals.


ALUFAMM BOXWITH LID WI SI X ?wen DOUBLESIDED COPPER -CLAD BOARD

S0. 2SAPA SOLDERED TO TWO TAGS

0 0

C3 GROUNDED TOCOPPER SCRF F NOOAPP

o `OVAL I Orwn SUPPORT INSULATOR',CAE SCREW FIXED TO

SOLDER TAG UNDER TOP SCREW

Snow CLEARANCE MOLECCIANTIVIUNK law

BOTH NOES

Figure 2: the component layout of the radio interference filter for vhf antennas

The design of the filter assumes that the receiving antennafor the vhf high -band receiver uses coaxial cable ofcharacteristic impedance in the order of 50 to 75 ohms. Thisis impedance -matched into and out of the fitter by tappingonto the tuned -coil at a low -impedance position close to itsgrounded end. The actual position of the tap is not undulycritical.

ConstructionDrilis of sizes 1,2.3. 5. and 7mm are needed. The prototypefilter was contained in an aluminium box of size 76 x 51 x 25mm (3 x 2 x 1 inches) with a removable lid. Mounted centrallyon each end face of the box is a 50 -ohm BNC or uhf coaxialsocket chosen by you to Suit your own receiver.

A 50 x 25 -mm piece of double -sided copper -clad pcblaminate board serves both as the inter -screen for the tunedcircuits, and the mounting -plate for the coils and capacitors.The board is secured and grounded at its two bottomcorners by soldering each to a pair of solder -tags on the boxsides. These tags are longer than their 6mm fixing -bolts.which allows the board to be gripped between them beforesoldering to both copper faces. Countersunk bolts are usedto avoid mechanical conflict with the lid. Fixing of the lid isprovided by drilling 2 -mm holes through the lid and side -plates of the box to accommodate self -tapping screws. Twoscrews should be sufficient, one at each diagonal side.

Before soldering the screening board into the box, thecomponent fixing -holes must be drilled as described in figure3 using a 1 -mm drill which provides a neat fit for the 20swgwires, and then the four coils and three trimmer capacitorsfitted onto the board and soldered into place. Figure 2 givesguidance for mounting of the components, but positioning isnot critical, as the wire tails of the coils can easily be bent insitu to suit your particular layout.

Four coils of 12 -mm inside -diameter need to be woundwith 20swg tinned copper wire, after first having straightenedthe wire by pulling it through a soft cloth. Coils Ll and L2 areeach of three turns plus tails of about 25mm, with the turnsspaced to give a span of 12mm. Link -coils L3 and L4 are

each one single 12 -mmdiameter turn, again with 25 -mm tails.

Temporanly bend onewire ta,' -.IL t out of theway, Insert its otner tailthrough the appropriate holewith the coil horizontal, andsolder the wire to the copperon both sides of the board.Cut off any surplus tail 4re.From the opposite side ofthe board, do the same withthe other coil L2.

Solder the two common -tags of the green -coloured22pF trimmer capacitor C1to the grounded tail of coilL1, with the adjusting slotfacing upwards and thecentre -tag pointing inwards.With fine -nosed Miele,carefully bend the free tail ofcoil L1 and cut it to lengthsuch that it can then be

soldered to the centre -tag of trimmer C1. Do the same withthe trimmer C2 and coil L2 on the other side of the board.

This self-supporting coil capacitor method is mechanicallyadequate. Optionally but not really necessary, extramechanical support for each centre-tag/coil junction can beprovided by means of a 10 -mm insulated -spacer. single -holefixed to the bottom of the box by an M3 bolt. A solder -tagbolted to the spacer's free end would then serve as a firmanchor point for the solder joint.

One tail of each single -turn coil coupling -loop L3 and L4 isgrounded by passing it through a hole in the board. andsoldering it to both copper faces. Note that the 3 -mmclearance -hole, through which the other tail of each looppasses. is countersunk with a 5 -mm drill on both sides of theboard to trim back the copper -plane so as to prevent thewires from touching it. These two tails are soldered togetheron each side of the hole. but not to the board itself. Eachloop is then physically adjusted to be closely parallel to, butnot touching, its associated coil L1 or L2.

Now solder the yellow -coloured 65pF trimmer capacitorC1 into position on one side of the screen. The choice of side

The fitter boxed, showing the three tnmmers and an old-style 50pcoin for compansonl


2 tern HOLESCLOSE -SPACEDFOR LI AND L2

CUT CORNERS TO CLEARFOONG mum -

2 Inn, HOLESCLOSE -SPACEDFOR L3 AND L4

4 16

412

644-lo

125

00 0 0

4- -

12 5

so

20n-un 20swp WIREFOR GROUNDING C3

3mrn CLEARANCE5rrIn COUNTERSUNKBOTH SIDES

Figure 3: dnlling details for the copper -clad laminate 50 x 25 -mm screening board

is not important, but the trimmer should sit snugly against thecopper with its adjusting slot upwards, and the centre -tagfacing inward. The trimmer's two common -tags are solderedto the short piece of 20swg grounding wire which is solderedthrough the board at 6mm from the edge, as Shown in figure3. The centre -tag should be in contact with and soldered tothe union of the two link -loops, close to where they passthrough the clearance -hole in the board.

Fit the assembled screening -board into the box with eachcorner gnpped between a pair of solder -tags. then solder thetags to both copper faces. It will be necessary to snip eachcorner to clear the solder -tag fixing nut. If necessary, shorten

the two socket -pins to avoid unintended contact with thecoils, then solder a wire link from each pin to the turn nearestthe earth end of its own coil.

Finally, drill three 7 -mm diameter holes in the top face ofthe lid located so as to give clearance -access to theadjusting slot of each trimmer. An easy way to determine thedrilling points is to press a thin layer of Plasticine or wax onto the inner surface of the lid and to gently press it down onthe tnmmers. This leaves a clear impression of the trimmeradjusting -slots in the Plasticine. which can then be used as adrilling template. After drilling the trimmer adjustment -holes.set the moving plates of each trimmer capacitor to aboutmid -way, then fasten the lid into place.

Installing and tuning the filterThe filter must be connected between the antenna and thereceiver, as close to the receiver's antenna input socket aspossible. The antenna's coaxial lead plugs into either end ofthe filter. A short length of 50 -ohm coaxial cable fitted withappropriate 50 -ohm plug at each end is required to make !'connection between filter and receiver. The only adjustmentnecessary. is to tune each of the three trimmer capacitors formaximum received signal strength with the receiver set to saythe local repeater channel for 2 metres. or to 137.5MHz forweather -satellite use. For convenience, a signal -generatorshould be used as the signal source. It should preferably be atone -modulated vhf fm signal generator of 50 -ohm outputimpedance. but a dip -oscillator would suffice. Alternatively.the 2 -metre repeater or weather -satellite signal itself could beused.It is very important that an insulated trimming tool is used onthe tnmmers, not a screwdriver. When tuning is completed,label one socket as Antenna and the other as Receiver.

It should be noted that.because of the filter's veryhigh selectivity, it may not bepossible at first to hear theoff -air signal through the filterbefore it has been tuned. sousing a signal generator maybe the best option. However.

generator is notavailable. adjustment of thefitter on an incoming radiosignal in the band120.450MHz can besimplified as follows:

Remove the filter -lid. Set allthree trimmer vanes to mid -mesh. Connect the antennato thg receiver and listen for,

say. the 2 -metre repeater signal. or air-traffic signals on orabout 120MHz, or weather satellite signal on 137.5MHz. Thelatter would best be done during an overhead pass. when thesatellite can be heard for 15-20 minutes. While the requiredsignal is present, disconnect the antenna from the receiver,leaving the antenna cable unconnected. Connect the filter tothe receiver. then touch the free antenna plug onto theadjusting slot of the yellow 65pF trimmer, allowing the signalto be heard. Adjust the green 22pF trimmer nearest thereceiver socket for maximum signal. Now touch the antennaplug on to the adjusting slot of the other green 22pF trimmer,and adjust the yellow 65pF loop -trimmer for maximum signal.Connect the antenna plug to its own socket on the fitter, andadjust its nearest green 22pF trimmer for maximum signal.Refit the lid and give a final adjustment for maximum signal to

three tnmmers.

Cl

C2, C3

Ll L2

LI L4

SK1, SK2

Maplin(Adjust sockrequired signal.)12 -mm dia. 20swgwire single -turn links12 -mm dia. 20swg 3 turnsspaced to 12 -mm span50 -ohm square socket, bnc oruhf 50239 Maplin YWOOA orBW1350

2 x 50 -ohm twist -on plug, bnc or PL259 to suitreceiver, Maplin JK21X or HL9SD: 2 x M3 6mmbolt and nut, countersunk. Maplin BF36P; 4 x M320 -mm bolt and nut, pan -head, Maplin JY22Y; 6 xM3 solder tag, Maplin LR64U: 2 x 10 -mm insulatedspacer (optional), Maplin FS36P: 0.5-m 50 -ohm 5 -mm UR76 coax cable, fitted each end withappropriate 50 -ohm plug; 50mm x 25mm double -sided copper -clad laminate pcb board, MaplinFA5SK; 0.5m 20swg tinned copper wire, MaplinBL13P; aluminium box with lid, 6 x 51 x 25mm (3 x2 x I in). Maplin AB12: preset trim -tool, MaplinBR491.

-11111split catalogue numbers are given for guidanceAS* and any suitable alternatives can


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It

LEDVoltmeter

for 12V batteryIf 12 -volt leisure batteries are run down very far, their life -span decreases

dramatically. Terry Balbimie's four -LED charge-state indicator helps to monitorbattery health.

Twelve-volt 'leisure" batteries are the starpower source for equipment in a caravan Cgboat. They are used to operate the low-voltagt:-appliances such as lights, water pump,television, etc. This type of battery is also used

mateurs to operate transmitting equipment inremote iocations and by those taking environmentalmeasurements in the field. When used in conjunction withan inverter, they also serve to provide a 240V 50Hz supplyfor mains equipment. such as computers, where no actualmains sunp1v is available.

How to kill a batteryLeisure batteries are a member of the lead -acid family. The

most familiar one is the car battery. However, leisurebatteries are specially designed for cyclic use, whereas acar battery is used chiefly in float mode. Cyclic means that

the battery is run down significantly before re -charging. In

float mode only a small fraction of the total charge isdrawn at any time. The battery is then re -charged at a lowrate and over a prolonged period - even continuously - asin the back-up supply for a burglar alarm. Batteriesdesigned for float applications may be cycled occasionally.such as when the car fails to start in the winter and it runsflat. Car batteries, however, are not designed for cyclic useand wilt fail fairly quickly under such conditions - hence the

need to buy the correct product and to avoid using acheap car battery for any of the purposes mentionedearlier.

Even a leisure battery will fail much more quickly if it isregularly discharged to its rated low point. Typically, if abattery is only run down by one-third of its total charge. itmay be expected to cycle some 1000 times. If the depth ofdischarge is 50%, this figure falls to some 500 times. If it isrun down to its low point, the life may be expected to dropto some 100 to 200 cycles only. This is not alwaysappreciated by users.

Dep dischargeLead -acid batteries suffer from one major disadvantage:they must always be maintained in a satisfactory state of

charge. If they are allowed to run down below the ratedlow point, permanent damage is likely to occur. The battery

will then fail to accept a full charge. Under extremeconditions of 'deep' discharge such a battery will becompletely ruined, especially if it has been lett that way forsome time. It is interesting to note that a battery lett with aterminal voltage of less than about 12.3V (50 percentcharge) will slowly deteriorate anyway.

Although leisure batteries are described as having a 12Voutput, this is only an average figure. A fully-chargedbattery will develop an off-load voltage of some 13Virrespective of size or manufacturer. As the battery is rundown, the terminal voltage falls. At 12.3V, some 50 percentof the charge remains. At about 11.7V it may beconsidered flat, even though some charge still exists.Below about 11.5V the battery is in a dangerously lowstate and must be re -charged as soon as possible. The

state of a battery could therefore be found by using adigital voltmeter. However, some non -technically minded


people would find difficulty interpreting the readings, andmany would not wish to use their expensive instrument forthis purpose anyway.

Thermometer displayMost of the time, the user simply wishes to know theapproximate state of the battery, and this battery checkcircuit fulfils that need. On the front panel of the unit arefour LEDs (light -emitting diodes) arranged thermometer -

fashion. The colours used are green. yellow, orange andred to indicate "good", "serviceable'. "poor" and "flat"respectively. These will operate in turn whenpredetermined voltages are applied to the input. There is.of course, a further state where none of the LEDs are on.This indicates the point when the type of rapid damagereferred to above is likely to occur.

The circuit is powered from the battery beingmonitored, drawing about 1.5mA plus a further 15mA foreach illuminated LED. Thus, when all four LEDs areglowing (battery fully charged) the total requirement is alittle over 60mA. The device should not be left connectedpermanently to the battery because even this small drainwould eventually run it down. A large battery of 60Ahcapacity would take several weeks to do so but a smallone of. say, 1Ah capacity would take only a few hours.The intention is that the meter is used every now andagain to monitor the condition of the battery.

How It worksThe complete circuit for the LED battery checker is shownin figure 1. It will be seen that the design centres on IC2,which contains four identical operational amplifiers, IC2a.IC2b. IC2c and IC2d each used as a voltage comparator.The specified lc is particularly good for this application. Itis a robust bipolar device and works correctly from singlesupply rails as are used in this circuit. Also, the circuit willnot be damaged if it is connected to the battery withincorrect polarity. Each op -amp section has two inputs,the "+" (non -inverting) and the (inverting) one. Thetheory of operational amplifiers states that, if the non -inverting input voltage at any unit exceeds the invertingone. the appropriate output will be on.

The circuit is connected to the nominal 12V batterythrough fuse, FS1. A supply is then provided to the 5Vreference device, IC1. This operates in conjunction withload resistor, R3. to give a precise 5V between its ends.IC1 may be regarded as a superior type of zener diodeand is represented by the same symbol. This referencevoltage is connected to the inverting inputs of all tour op -

amps (pins 2, 6, 9 and 13). Meanwhile, each non -invertinginput (pins 3, 5, 10 and 12) receives a voltage dependenton the adjustment of presets RV1, RV2, RV3 and RV4respectively. These are connected as potential dividers andthey operate in conjunction with fixed resistors, R1 and R2which are common to all units. The purpose of theseresistors is to narrow the range of adjustment of thepresets so that they are more easily adjusted at the end.To make the operation clear, suppose RV1 is adjusted sothat exactly 5V exists at IC2d non -inverting input, pin 3,when the battery supply is 11.8V (i.e. a poorly -chargedbattery). Suppose also that the other presets are adjustedso that each non -inverting input develops 5V with supplyvoltages of 12.2V (IC2c). 12.5V (IC2b) and 12.8V (IC2a) -

that is, increasing states of charge.

Suppose the battery is in a very poor state of chargeand only 11.5V exists between its terminals. It will be seenthat all the non -inverting input voltages are less than 5V.None of the op -amp outputs will be on and therefore allLEDs will be off. Now suppose that the battery voltage lustexceeds 11.8V. The voltage at pin 3 rises above the 5Vreference at pin 2 so IC2d output, pin 1 is high. Thus, thered light -emitting diode. LED1, will operate via current -

limiting resistor. R4. In a similar way, as the battery voltagerises above 12.2V, 12.5V and 12.8V, the non- invertinginputs of IC2c, IC2b and IC2a will exceed the 5V referenceexisting at the inverting ones. Thus, orange. yellow andgreen light -emitting diodes LED2. LED3 and LED4 willoperate in turn via the appropriate current -limiting resistor.By adjusting the presets to the required working voltagesat the end of construction, the colours will show atpredetermined points and the device will behave as anarrow -scale voltmeter.

The op -amps are used without positive feedback. Thismeans that the switching action will not be sharp. In otherwords, the LEDs will come on and go off gradually over asmall range. For the present purpose, this does notmatter.

ConstructionConstruction is based on a single -sided printed circuitboard (PCB) and the component overlay is shown in


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figure 2. Begin by soldering the is socket and two linkwires in position. Follow with all the resistors (including thefour presets) and capacitor. Solder the LEDs in placetaking care over their orientation (the cathode end ismarked -k- in figures 1 and 2). Note that with LEDs 1 and4, the cathode end is connected to the lower track on thePCB whereas with LEDs 2 and 3 it is the upper one - if thepolarity is incorrect, the LED will not work. The tips of theLEDs should stand approximately 20mm above the circuitpanel and should all have exactly the same height so that

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they appear neat in thefinished device. Take IC1 and.holding it so that the flat faceis towards you and with theleads pointing upwards. cut offthe right-hand end one closeto the body. This lead isconnected to the substrateand has no effect if leftunused. Solder the tworemaining leads in position withthe flat face of the ic towardsthe lower edge of the PCB.

Solder test probes to thepoints labelled °+12V nom'(nominal) and 'gncl". The +12Vone should be red and the'gncr one. black. These couldbe easily made. However, inthe prototype unit. a pair ofinexpensive meter leads wasused with the 4mm plugs cutoff the other end. These give amore professional appearance.Connect the in -line fuseholderin the positive lead as shown inthe photograph. Adjust thepresets as follows: RV1 andRV2 fully clockwise; RV3 andRV4 fully anti -clockwise. Theseadjustments are as viewedfrom the edges of the circuitpanel (that is. not from IC2position)

Complete the construction of the circuit panel byinserting IC2 into its socket, taking care over theorientation. Insert the fuse into its holder. This should havea value not exceeding 100mA. Note that the use of a fuseis essential because. under accidental short-circuitconditions. a lead -acid battery can deliver an enormouscurrent. This could cause wires to become red hot andresult in burns or even fire. Also, in the event ofconnecting the probes to the battery with incorrectpolarity, a larger current than normal will flow. If left for

more than a second or so, the fusewill blow. The op -amp and voltagereference ic seem to survive,however.

Figure 2: the component layout of the LED voltmeter battery checker

; e Stint:If you have a continuously variablepower supply unit and a digitalvoltmeter available. you could adjustthe circuit with the aid of these. Ifyou do not have this equipment.adjustments may be made bydischarging the battery undercontrolled conditions. This takeslonger but will probably producebetter results because they arebased on real experience.

If you are using the power supplymethod. connect its output(observing the polarity) to the input


probes on the unit and also to the voltmeter. Set the outputof the power supply unit to 12.8V and adjust RV4 using atrimming tool or small screwdriver so that the green LED isjust completely off. Reduce the voltage to 12.5V and repeatwith RV3 until the yellow LED is lust off. Repeat at 12.2Vadjusting RV2 so that the orange LED is just off. Finally, dothe same at 11.8V adjusting RV1 so that the red is just off.Slowly increase the voltage again and note that each LEDcomes on in turn over a small range.

In the absence of the above equipment, charge the batteryfully and connect a suitable bulb to act as a load. Do notdraw too large a current in an effort to speed up theprocess. Aim to run the battery down over a period of atleast 20 hours. This may be done in more than oneoperation so it will not be necessary to get up in the middleof the night to adjust a preset!

Start by finding the Ah (amp -hour) capacity of the battery- this will be printed on it somewhere. A leisure battery willhave a typical capacity of 60Ah. This means, in theory. that itcould supply 1A for 60 hours. 2A for 30 hours, 3A for 20hours and so on. With a small load connected, it will appearto have a larger capacity. For example, it would supply oneamp for more than 60 hours. Conversely, the capacity will beless with a large load so it would not maintain a load of 60amps for a full hour.

With the amp -hour capacity known, divide it by 20. Thiswill give the approximate current that must be drawn to runthe battery down in 20 hours. For example, the 60Ah leisurebattery would need a load of 3A to do this but a smallbattery, of say 6Ahr capacity, will need only 0.3A or 300mA.Now choose a 12V bulb, or bulbs to roughly provide thisload. To find the current needed by a bulb. divide thewattage figure by 12 (the nominal voltage of the supply). Foran approximate result, you could divide by 10 instead. Forexample, a 10W car -sidelight bulb could be considered todraw 1A or a 21W flashing indicator lamp. 2A. If you appliedthe 21W bulb to the 60Ah battery, it would take some 30hours to discharge and this would be a suitable time. For thesmallest batteries, you would need a panel light bulb. Theseare available in a range of power ratings from car accessoryshops. A typical one is rated at 2.2W which works out atabout 0.2A. This would be a suitable load for the 6Ahbattery mentioned above. One practical point - sincefilament bulbs become very hot in use. care must be takenthat they do not cause burning. They must be placed on asuitable heat proof surface or (better) suspended in the air

Connect the bulb to the battery and note how manyhours it has been alight to the point where it begins to dimnoticeably or until a voltmeter shows 11.8V. This time willprobably not be the same as the theoretical value.Remember, this does not need to be done in one continuoussession. Re -charge the battery. Now decide on suitabletimings which will be used to decide "good", "serviceable""poor" and 'flat" states. Suppose it took 30 hours for thebulb to go slightly dim. it may be decided to regard timesgreater than 20 hours to be 'good" (green just off after thistime) down to 15 hours, "serviceable" (yellow just off), downto 10 hours, "poor" (orange just off) down to 5 hours 'flat -(red just off). Less than 5 hours (all LEDs off) will indicatedangerously discharged condition. In actual use. the unit willbe used with the battery off-load. It is therefore best if thebulb is disconnected before making an adjustment. Wait for

10 minutes or more so that the voltage stabilises beforeadjusting the corresponding preset.

The circuit panel may be built in any small plastic box whichcan accommodate it. Measure the positions of the LEDs onthe PCB and drill holes in the lid to correspond. Take care todrill them in a perfect straight line and make them a tight fit.The PCB will then require little extra support. Drill a hole inthe side of the box for the probe leads to pass through. Youwill need to remove them from the PCB, pass them throughthe hole and re -solder them in position. Apply some strainrelief by, for example, tying a knot in the wires. Leave someslack inside the case and check that the wires cannot bepulled free in service. A piece of foam plastic on the base c'the box will probably be sufficient to support the circuitpanel. Check with the lid in place. If necessary, makeadjustments to the presets over a period of use to obtain thedesired effect.

When using the LED voltmeter, make the test with thebattery off-load and preferably after it has rested for a whileThis will enable the terminal voltage to settle down to thecorrect value

If a routine test reveals that the yellow LED is off(indicating about 12.5V). the battery should be charged assoon as pocsible. This will prevent the slow deteriorationreferred to earlier. In service, this will not always be possiblebut the battery shOuld certainly be charged before the redLED has gone of

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ELEL:1 uLJAy i IRNATIONAL34

BB RangerThis is a combination of an eprom-driven control box, a custom-built fall -down target

and an LED scoreboard for BB and other lightweight pellet guns, by Bob Noyes

The BB Ranger target unit

his project was designed as an electronic score-board for a BB shooting range. Both of my sons havegone through the BB gun fad. starting with srnal,lightweight, very low power guns and waking up toquite accurate pieces, some of which look similar to

reel guns although their plastic appearance ensures they can't

ea* be confused with the real things.For target practice aid n order for the boys to pit their Wks

against those of thew friends I derided that some kind of electronscore board was nagured to gve an instant read-out of theaccumulated score. There was nothing on the market that would

do so this project. 138 Ranger, was born.For those of you not fernier with BB guns, they are, in many

ways, similar to air guns, usrig a powerful spring or puff of air, or

both. to propel a smell Ernm round plastc pellet - coign* rt was asmal bal bearing, hence "BB". Unlike real guns. there is noexplosive element in BB guns and licences aen't required forownership; they are not lethal weapons although. as a precaution.eye protection should always be worn by everyone around when

the guns are in use. The project is suitable for at guns as wel asBBs. but ar rifles are too powerful aid may damage the targets.

Although a bullseye type target would have been preferable. rt

was rot easy to construct we on which switches could be

moulted to detect the score. As a convenient compromise,nstead of a conventional target with the smallest mg berg thehighest score aid the lager ergs the lowest, we used separatendividual fold -back targets (figure 1). There are ell targets, withscores raging from 10 to 80 points. It goes without saying that the

smaller ones carry higher scores. In the prototype. the 80-pontsscore target measured 25mm high by 63.5mm wide: 70 points,

51mm by 63.5rnm, each target thereafter being 25mm taller thanthe one before but scoring 10 points less until the 10 -ports target.which was 203mm high. These eight targets were mounted ri astiaylrt line, but you can choose whether to mount than nascending order, or al mixed up with a gap of about 38mmbetween each one. As can be seen In figure 2. they are mounted

n such a way that, when hit. they fal backvards.Out of sight behind the body of the assembly, a magnet is

mounted n each of the targets. The type used was a door switch

ice those used n alarm systems - it is readhy available and easy to

mount, as a flange with countersunk holes is built into the magnet

and the reed switches.The magnet is mounted into the fall -back pat of the target. aid

the reed switch into the body of the target assembly, so that when

ELECTRONICS TODAY INTERNATIONALSS

Figure 1: a suggested layout for the fall -back targets, from front and side. Each playerhas up to eight shots, and the highest cumulative score wins. Standing up the targetsresets the score in the display.

the targets standing up the magnet is n range of its reed switch.The switch s closed in these circumstances but when the exposedpart of the target is hit It falls back and the magnet mounted on rt isout of range of the reed swtch. rnakng an open arcut The saneis repeated for the other seven targets. A 9 -way cable was used toconnect the eight reed switches, pus a common to al of than, tothe control box. This cable should be several metres long to keepthe control box wel away from the target area. A plug and socketwas used to connect the cable to the control box.

Board and paint.R.A kiVC 1U .-oostrukA eight fold -back targets if you don't

want to. So long as the ones used score n the range 10.20. 30.:O. 50. 60, 70 and 80 the arcuit will W. although wee inks mustoe used to connect the unused connections permanently from thecontrol box to OV. This just Ws the control that the unused targetsare still starring and don't play any pal in the scoring. If you want

more than apt targets. then a new program is required n thecontrol box. See below.

The whole target s made out of MDF board as this is realtyavailable and reasonably paced. The finished targets may bepanted or varnished to protect than from the ravages of time (andlads).

The exposed pats of the target were covered in tidy -backedfoam, the sort of stuff that pipes can be covered with. This absorbsthe impact of the pelet hitting the target and helps prevent it

rebounding to the firing position. The front of the body of the targetassembly slopes back again to deflect the pelet. This surfaceused to show the score for each target.

This Ranger project is rust one possible type of construction.Any senior type may be butt. and so long as a ht proctuaas anopen circuit and a miss. or target sill staridrig, produces a shortarcut (and up to eight are used) rt should be compatible with theelectronics n the control box.

LARGER TARGETSLOWER SCORE

FOAM TOCUSHIONFALL ATTOP OFEACH

TARGET

PI voT ALARM DOORREED SWITCHAM) MAGNET

Figure 2: how the fall -back target was constructed

FOAM ON FRONT TO ABSORB IMPACT AND STOPBALL BEARING FROM BOUNCING BACK

4M BALL BEARING PELLET

ANGLED TO DEFLECTBALL BEARING

REED SWITCH IN CONTACTWITH MAGNET WHEN HIT THETARGET FALLS BACK AND THEREED SWITCH GOES OPENCIRCUIT THE SCORE IS THENSHOWN ON THE DISPLAY

A very large cardboard coverwas made to go over the target

fitly n order to trap thepelets so they could be usedagar. The °Nona was wel overa metre long, 53cms high and53ans deep. which allowedenough room to reset the targets.that is. to stand than up agar. Italso retailed about 95 percent ofthe pellets used. The rade of thebox was covered with canvasfrom an old tent to stop thepelets embedding themselvesinto the soft cartboard. I usedwallpaper paste to stick thecanvas to the cardboard. To resetthe display. just stand up al thefallen taroetc and It wr read zero.

The 2716 epromAt the heart of the control PCB isan eprom or erasableprogrammable read only irwiury.The one used is the 2716. whichis the srnalest n the 27xx range


The rear of the targets

of eprems, and can be poked up very cheaply as surplus stock.Larger mermcnes such as 2732 and 2764 can be used so bog astheir control signals and extra acktess Ines are catered for.However, the PCB in the project is designed for the 2716 only.

Pee NM* a)For some reason. axons are often considered only as a pan of

a computer system containing software for the operating system,but it is just another component with a specification. In the castwe are using Rs memory. even when the power is removed, to holdevery combination of score from the eight targets. There is a totalof 256 combinations of score from al targets up. that is. no scoreor zero to all eight targets down and a score of 36 points. The eightinputs from the targets' reed switches go to the 8 bwest addresslines of the eprom. that is. the '10" goes to AO (note AO not Al).'20' goes to At . -30" to A2 and so on. The remainng addressfines, A8. A9 and A10 are corrected to zero. tf a larger 2732 isused. All must also be grounded. Now every combination ofscore sets its own address. i.e. with the "10" and "20 and "30"down but al the others up the address is as folows:

A7 A6 AS A4 A3 A2 Al AO0 0 0 0 0 0 0 0

At the address a score of "6" is stored n the rr Tory: trigs takes

the form of DO, DI, D2,133 which holds the tens or next most

significant decade:

DO D1 D2 D31 2 4 8

so data stored is:

DO D1 D2 D30 1 1 0

and D4, D5, D6 and D7 hold the ri List significant ctrarie:

D4 05 D6 071 2 4 8

As can be seen when adding up the score, the least significantdecade or was is always zero. so does rot have to be held nrrierTTcry. Orly the most significant and next most significant, orhundreds and tens. must be held in inerni_iy. So. n the case where

the "10, "20" and "30" ponts tayets are down and all the othersare up, the most significant digit holds zero and the next mostsignificant hods 6. With the permanent zero in the least significantdecade, a score of "60" wr show on the display. When you buythe 2716 eprom. it we either be blank, or. if it has been reclaimed

from a circuit. it we contari a useless program. In that case the oldprogram must be removed with a UV eraser. The eprom is usuallyexposed to UV fight for 10 minutes or so, which should blank the

eprom memory.The program for the project can then be loaded in via an axon

programmer. or a ready -programmed 2716 can be cbtaried readyme, for £16 including postage and pang (see the end of thisaide). The mg -am is fisted in figure 4.

The erne diagram of the BB Ranger appears in figure 5 Theintimation in CO - D7 is decoded by two 4511 BCC) to seven -segment decoders, IC2 and C3. These ics give outputs suitablefor switching on the separate elements n the seven -segment LEDcisplays. If only a smal readout is required. D4L1 and DIL2 supplythe current limiting resistors reqLinad for the two decades ofcommon cathode displays. The third least significant decadecisplay is wired so that al segments but the 'g' segment are on.so that it reads zero. This tares the torn of 6 x 270n resistors fromeach segment: a. b, c. d. e and f corrected to +5V, and thecommon connected to OV. Any smal seven -segment commoncathode displays will do. but they al have different en outs. The pin

AT SCOPE K

Al SCOPE 70

AS SCORE 10

AA SCORE 10

4.3 SCOPE 40

PIRA-:

SKTA-3

SKIMSATRE

SKTA4

A2 SCORE 30 SPRAT

t SCOPES) SKT4-1

AO SCORE ,0 BATAS

DO NS A. C2-7

Di PRA O. K.2.1

CE NSS C 11:2-1

ON SUPPlv

Figure 3: the 2716 eprom pin -outs

.5V SuP9Lv

AB NOT USED CONNECT TO DV

AS NOT USED CONNECT TO OV

NIP SOT WIC CONNECT TO .5v VIA Ri

CE OUTPUT ENABLE OAR CONNECT TO DV

ADO NOT USED CONNECT TO OV

CE GNP SELECT SIR COMECT TO OV

DT MS A 1C3-4

DI NS C C3-2

DS LASS C11

NSA CIT03 NASD C24


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NOTE: SOCKETS A. B. C t0 a 0.1' CONNECTORS

Figure 6: the component layout of the Control Board PCB

4 Score 65 Score 5

6 Score 47 Score 38 Score 29 Score 1

10 OV common

OutputsTo large display

B Function To1 OV

2 Keyway3 +5V4 NMS t

5 NMS g6 NMS a7 NMS b8 NMS c9 NMS d10 NMS e

OutputsTo large display

C Function To

Et 0

SOCKETS 0 AND E ARE 10. WAY 0 15r RIGHT ANGLE PCB HEADERS DA. 1.3AND 3 ARE 14 CRISPIN ONLV1441611101.1111ED FOR 1110R RIENSTORIL

Figure 7: the layout of the Display Board. The centre bar is not required in the lowest significant digit. ask stays shows 0.80 identical 0.2 -in LEDs are used

ELECTRONICS TODAY INTERNATIONAL3.

-1101-

I 171 411

DIRECTION OF LEDs IN WWI

FLAT ON LEDSNOWS VE SIDE

i2V

0 V

R INTERNAL

INPUT TO DRIVER FROM4511 EITHER DIRECTOR

SOCR VIA DR. RESISTORS

DARUNOTON

Figure 8: connecting up the LEDs for the Display Board. Thelowest significant digit has the bottom of 100R grounded, as thisalways reads 0

1 +5V

2 MS f D83 MS g D54 MS a DIO5 MS b D96 MS c D7

7 MS d D68 MS e D4

9 OV D210 Keyway

inversion takes place as a high on the base will produce a

low on the colectcr. In order to factrtate this, the large

displays use a common anode configuration. Greet caremust be taken when fitting the LEDs - al 80 of them Owefigures 7 and EQ. it only takes one LED round the wrongway to stop the whole segment from iuminating. Its a

good idea to buy al the LEDs at the same tine, as this wdensure that they are the same type; it wil make a clearerdisplay than using odd ones, as LEDs vary considerably in

colour and brightness. If a segment is out when it shouldbe on, the chances are that one of the LEDs is at fait. Theeasiest way to find out which LED is out is to short eachLED on the offending segment out in tun. VVhen the others

*urinate. the one being shorted out is the one at fait -either dead or the wrong way round.

To enhance the contrast of the display. the component

side of the display PCB should be sprayed matt blackbefore assembly; this prevents any reflection from 819 PCB.A red filter is required to cover the display, wtich againinproves the look of the fnal project as well as blanking outeverything but the illumnated LEDs.

TT., li.l'..- .

The power supply to the display must be capable of producing inexcess of 500mA. so a rechargeable battery is a , ". -

this project can only be safety used out of doors. it must not bemains powered for obvious reasons. Too many accidents arecaused by the misuse of electncal power in the garden, fromexcessive dampness to faulty extension leads so to eliminate theseproblems this projects completay battery powered. The batteryused in the prototypes was a 12 -volt 2.3 -amp hour. the sort of

battery used in alarm systems as a battery bacierp. It goes withoutWhen using a dartngton. or any transistor for that matter, an saying that it will need recharging, so a fused socket is provided for

MATERIAL lOrnon MEN

ALL SWITCHES FUSES AND CONTROL PCB MOUNTED ON REAR PANELAS WELL AS TARGET SOCKET EXTERNAL BATTERY IN AND CHARGER INPUTFIT TO SUIT. GREAT CARE MUST BE TAKEN WITH THE POLARITY OF THEEXTERNAL BATTERY INPUT AS WELL AS THE CHARGER INPUT

PANELS 2rnrn ALUMINIUM

' SOUARE WOODEN STRIPSTO ADD STRENGTH

Figure 9: the structure of the Ranger control box

240Enrn

LARGE COVERED AREATO CREATE BRACE.SPRAY PRUDE MATT BLACK

2Torn ALUMINIUM PANEL

DISPLAY PCB MOUNTED ON PILLARSFROM THE INTERNAL SUPPORTS

BATTERY HELD IN POSITIONBY WOODEN BLOCKS FIT TO SUIT.


CONTROL PCB

.0,12VOVI

A101

10

C

10

Is.%V. 4, sclIfPnv PCB

'V' III 0 99 40 II 06 9 6 9

IA 4 4 44 *NOT WIRED PIN FOR

PIN SEE PIN CUTTABLE

10 n10

ONDISPLAY ON

IOFF

2AINT

POWER SELECT y EXT

INS401 SEE TEXT

EXT 2A

12V

1

EXT I2V IN

Figure 10: the external connections for the BB Ranger system

this on the rear of the control box.The batteries will differ depending goon type style.

manufacturer and so on, so no precise chargng currents or triesear be given. Manufacturers' information must be followed.Although the internal battery will provide several hours ofcontinuous use, for prolonged use it is recommended that a carbattery is used as an external source of power. This must be fusedat 2 amps as dose to the battery as possible. usng an dine fuseholder. If rt is gong to be corrected by a plug and socket. the leadfrom the battery must have shrouded female contacts. This

eliminates the possibility of shorting out the pns.A selector switch should be used to make sure that this external

battery carrot be connected across the internal battery, as greatdamage may result. Only one battery source can beselected at any one time. Great care must be taken toensure the polarity is correct, as, although fuses are fitted, alot of damage can result before a fuse blows. A diode canbe fitted in series; this must be the 1N5401 type. Althoughit will add a 0.7 -volt drop, this should be compensated bythe fact that a 12 -volt car battery, when fully charged. iswell above 12 volts.

Me control box is also constructed out of MDF board(figure 9) design should include a lip over the displayto pr.,..;LIU a shaded area so that the display can be seen inbright daylight. The size of the box is not important so longas it is large enough to contain all the parts: the maincontrol PCB. the large display and the battery. Again, onlyrecommendations are given as the type of battery and otherdetails will differ from one to another. The power set-up for

IAo--

E XT CHARGER IN

ONLY CHARGE WITH POWERSWITCH IN OFF POSITION

The control box from the rear, showing connections

The control box from the front, showing the score display


The Control Board

ResistorsR1

R2

51L1

DILI

DIL2

CapacitorsclC2, C3C4

1k

10k

8 x 3k3 single -in -line resistor unit7 x 270R 14 -pin dual in line resistor unitOR 7 x 270R one-third watt resistors7 x 270R 14 -pin dual in line resistor unitOR 7 x 270R one-third watt resistors

100uF 16V radial2u2 16V radial47oF 16V radial

SemiconductorsIC1

1C2, IC3104

2716 programmed eprom - see text4511

7805

MiscellaneousSKTASKTB

10 -way 0.1 vertical PCB header10 -way 0.1 vertical PCB header

SKTC 10 -way 0.1 vertical PCB headerThe heatsink on the prototype was RS (Electromail) part number 263251. Other suitable heatsinks can be used If bolesare drilled - see text.Suitable casing (see text); link wire, solder. etc.

The Display Board

ResistorsDILI. 2. 3pins will fit.

Capacitorsci,czc3

14- or 16 -pin 100R dual in line resistor units of 7 or 8 100R resistors. Only 14 pins are required, but 16

100uF 16V

SemiconductorsIC1, IC2 ULN 2003ALEDs: 80 x 0.2 -in red LEDs: all the same type to make an even display

MiscellaneousSockets D, E: 10 -way 0.156 right-angled PCB headersMatt black paint for PCB and pellet -retaining cowling.Wire, solder, etc.

The Target10 -min MOF boad for construction; door -alarm type reed switch and magnet combinations for each fall -back target:9-w/wog/big to connect reed switches; small pivots for individual fall -back targets; sticky -backed foam strip (see text);

rd to make pellet -retaining cowling.

source. Fuses, fused socket, selector switch etc as appropriate for batteries

the Ranger is shown in figure 10.Since the original was built a couple of years ago, the

control box has found other uses. One, for instance, wasused in a Roll the Ball game at a fete where light -dependentcells were used under holes in the game. When a balllodged in the hole, it shut out the light to the LDR and ascore was registered on the display. Again, there were eightholes with scores 10 - 80: the object was to roll five ballsdown a short runway and get them to lodge in the variousholes, the winner to score 210 points and win a prize.

For a pre-programmed send a cneque or poster orderfor £16 to me. Bob Noyes. 13 Bowfell Close, Tilehurst,Reading RG3I 6QR.

Blank 2716 eproms from Farnell Electronic Components:Canal Road. Leeds. LS12 2TU. Tel 0113 263 6311 or othersources. Second user ICs are often available as salvagefrom Greenweld Electronic Components Tel. 01703 236363.


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Sound Effectsmodule

Terry Balbirnie continues a series of adaptable circuits for GCSE projectsmodule based on the HT2860, a three -oscillator sound effect chip

In this series we are describing some electronicmodules which could be useful to students ofGCSE Technology or similar courses at school orcollege. The circuits may be used in whole or in

part for practical projects. All circuits have the potential forfurther investigative work built in. which could be useful for themore adventurous student. But you can use the circuits asthey stand or with only "cosmetic" modifications.

All the designs are laid out on Veroboard. This has theadvantage of being similar in layout to the circuit diagram.Some students find PCB layouts difficult to relate to the circuitdiagram. Such details as exactly what the circuit is to be usedfor and how it will be housed are left to the constructor.

The circuit this month is a Sound Effects Module. This will

with a

provide a effects of four alarms plus a horn and an ambulancesound, and the effect is determined by the setting of a set ofminiature switches on the circuit panel. An external push-button or other type of switch is then used to activate thecircuit and the sound will be given as long as it is operated.Alternatively, the unit can be operated by some other circuit orsystem using a reed relay to provide isolation between theparts. In this way, no particular care need be taken to make thetwo sections 'match" - for example, if the two circuits usedifferent supply voltages. Any source of between 5 and 12Vand capable of giving 10 to 20 milliamps will be sufficient todrive the coil of the reed relay and hence operate the newcircuit.

The Sound Effects Module may be made into part of a toyor a game. It could be used to provide a different sound witheach aspect of the game. or for different functions of a toy. It

ELECTRONICS TODAY INTERNATIONAL4.

EXTERNAL RELAYTRIGGER COIL

RELAYCONTACT

EFFECTSWITCHES

SW1 SW6

Figure 1 the circuit of the Sound Effects Module

EXTERNAL., RELAYTRIGGER cot

RE LAYCONTACT

EFFECTSWITCHES

SW +. SING

.

Figure 2: the stripboard layout of the Sound Effects Module

Di1N4001

.4.5V

OV

DI,14001

ol

4SV

OV

may be that, once the sounds have been heard, a littleimagination could be used. They could be called laser gun" or"phaser missile" for example! By adding a suitable amplifier, themodule could be used as the basis for a burglar alarm systemor another high-powered warning device.

Circuit descriptionThe circuit for the Sound Effects Module is shown in figure 1.The circuit requires a supply voltage between 3V and 4.5VHere, it is provided by three "AA" size alkaline cells in asuitable holder, giving a nominal 4.5V. Diode DI providesprotection to the circuit should the battery be connected in thewrong polarity, since then it would not conduct and no currentwould flow. In fact, the voltage applied to the circuit is only3.8V or thereabouts due to the forward voltage drop of thisdiode.

The circuit centres on IC1, which is a soundeffects chip. The effects are pre-programmedinto it. so very few additional components areneeded to make a working system. The qualityof the sounds three built-inoscillators (referred to as OSC 1, 2 and 3) andthese require two external resistors and onecapacitor to make them work. R1 is connectedbetween pin 1 (OSC2) and pin 2 (OSC 1). Theother resistor, R2, is connected between pin 2and pin 5 (OSC3). The capacitor is connectedbetween pins 13 (the OV 'negative" supplyinput) and 14. Pin 4 could be used to operatean LED while the system is working, but theredid not seem to be any point in using it. Inaddition, the lc has several 'test" pins (3, 15and 16) and, again, these are not used. Thepositive of the supply (connected via diode DI)is applied to pin 6. The output signal isprovided between pin 17 and the OV line but it

can only give a small current. Some students might like to try apiezo transducer connected to these pins direct. However.most people will wish to use a small loudspeaker. This needs asimple amplifier to provide sufficient current to operate it. Thisis the purpose of transistor 01. Current from pin 17 enters itsbase via current -limiting resistor R3. The much larger collectorcurrent then flows through the speaker. Note that the speakermust be of the high impedance type as specified. Do not usean 8 -ohm speaker which is the most common type. Note that,even with the amplifier, the sound will not be very loud butshould be sufficient for many purposes.

The required effect is obtained by making one of the "effectpins" 7, 8, 9, 10, 11, or 12 high (that is, connecting rt to supplypositive). This is achieved by pre -selecting it using one of theminiature switches, SW1 to SW6 ("effect switches"). Theseswitches are in a plastic case exactly the same shape as an


Figure 3: the reverse of the stnpboard showing cut-outs

integrated circuit. The required effect pin will then be madehigh when the trigger switch is operated. This switch isshown as a push-button type although any other type ofswitch could be used appropriate to the application. Whenthe trigger switch is actuated, the loudspeaker will soundwith the desired effect. Some students will wish to provideindividual connections to the effect pins so that differentswitches can be used to give the various effects.

The alternative triggering method is via the "make"contacts of reed relay. RLA1. Its "make" (normally -open)contacts, connected in parallel with the "trigger" switchworks in the same way. Its coil could be energised by asignal from another circuit as described earlier.

ConstructionIf the circuit is going to be triggered by an external switch,there is no point in including RLA1. Use this only if thecircuit is to be operated by some other system.

The topside stripboard layout (component side view) isshown in figure 2. There are several inter -strip links and anumber of track breaks needed. Make the track breaks firstthen solder the link wires in place. If the circuit does notwork at the end. it is almost certainly due to a strip notbeing properly broken, a break or link wire being left out ora blob of solder or sliver of copper bridging adjacentcopper tracks - you have been warned!

Solder the lc sockets (one for IC1 and one for the effectswitches) and all other components in position. Theswitches could be soldered to the PCB direct but it isbetter to use a 12 -pin dil socket for them. This size ofsocket is not readily available. However, it is an easy matterto file a 14 -pin unit to size. Alternatively, a 14 -pin socketcould be simply soldered to the PCB with the extra twopins at the bottom left unused as shown in the photographThe reason for using an ic socket for the effect switches isbecause all strips to the left-hand side of them areconnected together using a piece of bare copper wire onthe copper strip side of the circuit panel. While solderingthis wire, considerable heat will be generated and thiscould damage the switches. Using an ic socket dispersesthe heat and avoids any likely damage. When solderingwire, make sure it does not touch anything else and cal,a short circuit. Take care to mount diode DI and thetransistor with the correct orientation. Solder the PP3-tynebattery connector to the '+4.5V and the "OV" points.

Solder short pieces of wire to the trigger ("trigg") positions.Set all but one of the effect switches off (the "on" directionis clearly shown on the switch body). If the unit is to betriggered from another circuit, solder wires to externaltrigger ('ext tngg") points.

Insert IC1 taking care over its orientation. This is aCMOS device and could be damaged by static charges. Toavoid problems, touch something earthed - such as awater tap - before handling the pins. Insert the effectSwitches if they have not already been soldered direct tothe circuit panel.

TsstinqConnect the speaker to the wires marked "[Si". Insert thethree "AA" size cells into the holder. Touch the two lrigg"wires together. A sound should be emitted by the speaker.Try using other trigger switches while keeping the rest off.

There is an interesting property of the circuit whichshows itself when more than one effect switch is on whenthe circuit is tnggered. If this is tried, it will be found thatthe effects for which the switches are responsible SOund insequence for a few seconds each. The order in which theysound depends on an in-built priority, and this may befound quite easily by trial and error.

More adventurous candidates may wish to try increasingthe volume emitted by the speaker by splitting the supplyinto 4.5V to the actual circuit and a higher voltage for thetransistor section. It would also be possible to add anintegrated circuit power amplifier or to use an externalamplifier.

The standby current requirement of this circuit is only8uA approximately so it may be connected to the batteryfor long periods the need for an on -off switch.

ResistorsR1

R2

CapacitorsCl

SemiconductorsIC t

Di

120k560k1k

1000pF

HT28601N4001

MiscellaneousSW1 6 Set of six SPST sub-miniature PCB -mounting switchesLS1 Miniature 60 - 70 ohmloudspeakerRLA1 Reed relay with SPSTContacts and 5V 500 ohm coil - if required (seelext)0.1 in matrix stripboard; holder for three "AA"edit and alkaline cells to fit; PP3-type battery01111111actor; 18 -pin MI socket; socket for PCB-1116allillang switches if required - see text.

IC1 and all other components are available fromMaplin. The reed relay is order code J1112N.


Microchip PIC and Motorola HC11 based development ToolsPIC Microcontroller Programmers Original This is our original programmer for 16C5X. 16C55X.16C6X. 16C7x. 16C8x. 16F8Xdevices Price : £40 for the kit, or £50 ready built. Serial - This programmer programs the newest PIC devices in a single 40 pin multi -widthZIF socket Will program 16C55X, 16C6X. 16C7X. 16C8x. 16F8X. 12C508, 12C509. PIC 14000. Also In -Circuit programming. Price : £40 forthe kit, or £50 ready built Introductory - Will program 8 pin and 18 pin devices 16C55X. 16C61. 16C62X, 16C71. 16C71X, 16C8X.16F8X. 120508. and 12C509 Price £22 for the kit (not available ready built). Note All our programmers operate on a PC. using astandard RS232 serial interlace (COM1. 2. 3. or 4). No hard to handle parallel cable swapping All programmers are supplied withinstructions. Windows programming software. MPASM. MPSIM and PICDE (Windows based PIC assembler )PIC or HC11 Windows Based Development: PiCDE SIM and HC11 DE allows assembly and simulation of your PIC or HC11 projects inone Windows program Incorporate multiple files. view help file information directly from the code, edit within project, build and track errorsdirectly in the source then simulate. Simulator allows 3 breakpoint types, follow code in the source window, set breakpoints directly in code.Run programs, or single step. or step over subroutines. Track vanable values and trace for display on the Trace Analyser Input stimuli includeclocks. direct values and asynchronous serial data. Profile your program examine frequently called routines which are timed and use theinformation to optimise out bottle necks PIC Version Simulates up to 50 times faster than MPSIM NEW - 32 bit version allows full use ofWindows '95 NT4 0 facilities Cost 130.00, or f25.00 for existing and new purchasers of any of our programmers. Please specifyWindows 3.1, or Windows '95 (32 bit) and either PIC or HCII versionPIC BASIC FED's PIC BASIC products - straightforward. capable. powerful, rapid development. Operating on a WindowsDevelopment Environment our modules need no assembler or UV eraser to program your PIC's. and operate from a senal link to your PCThe 16C74 module features - 8k EEPROM. up to 2000 lines of BASIC. 27 lines of programmable 1'0. 8 ILD inputs. Interrupt driven sepalRS232 interface. Penpheral I2C bus interface. LCD display dnver routines. up to 178 bytes for vanables and stack. extendible with optionalexternal RAM arid all the standard 16C74 features Ask about the 16C57 version.Compiler The FED PIC BASIC compiler for the 16C74. It produces hex code to program your 16C74 directly with no need for externalEEPROM Compatible with the EEPROM versions of PIC 16C74 BASIC modules develop on an EEPROM based module then compile andprogram your PIC crops directly16C57 Module Kit (8k EEPROM. 4MHz) £25.00. Pre -built £30.00 16C57 Module Kit (8k EEPROM. 10MHz) £31.00, Pre -built £37.0016C74 Module Kit (8k EEPROM. 4MHz) £35.00. Pre -built £42.00 16C74 Module Kit (8k EEPROM. 20MHz) £40.00, Pre -built £46.0016C84 chip programmed with BASIC - £25.00 Compiler - £60.00. or £50.00 when ordered with a module

PIC and HC11 devicesPIC 16C74/JW Erasable 20MHz £24.00 PIC16C558 £5.00PIC16C74-04P OTP 4MHz £8.00 PIC16C74-20P OTP 20MHz £11.00PIC 16C57 -04P OTP 4MHz £5.00 PIC16C57-10P OTP 10MHz £6.00PIC 16C84 -04P 4MHz £6.00 PIC16C84-1 OP 10MHzHi £800PIC16F84-04P 4MHz £6.00 PIC 12C 508-04P OTPPIC14000-04P OTP 4MHz £10.00 PIC 14000/JW Erasable £2300PIC12C508-04P OTP 4MHz £2.70 Motorola MC68HC811E2 Ring for details

Ask about other chips!

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The OrphanDecibel/

Power variations are better represented in a logarithmic rather than a linearmanner. This is accomplished by the use of the decibel - but what is a decibel?

By E.Chicken MBE FIEE

-D13000 cearty mean one tenth of a Lei, Put rave you wondereowhy one never hears of a abet', or for that matter a millibel,megabel or kiobel? Well. the truth is that they do not exist This isnot guile the truth. perhaps. because in theory the 'ben is still outthere somewhere but its story is rarely told - or perhaps that shouldbe "toted', as the bet lost one when it first appeared more than70 years ago. The lone bel was doomed to disappearancebecause of its nccnvenient dimension. somewhat ice (and hkewsedespite its eminent inventor) the farad.

Named after Alexander Graham Bel. nventor of the telephone.the bet replaced in 1924 the 'transmission unit' which had beenntroduced the year before nto the field of lie telephony by theAmerican Telephone and Telegraph Company (ATT). The bet wasMerited to equal 10 of ATT's writs'.

The 'transmission unit' itself had replaced an even earlierconcept based on the ratio between the decrease in signal powerproduced by a given telephone cable and that produced in onemile a of standard cable. In 1924. an international advisorycommittee on long distance telephony in Europe, n cooperationwith American representatives of the Bel telephone system. agreedto recurnmerrJ the adoption as standard of ether the bel or theneper, both of which were loosely referred to as units.

The bet was to be a unit based on logarithms to the base to.whie the neper (named after Napier) would be based on Napenanlogarithms to the base 2.71828. The neper was later used to someextent n Europe, but is not often met today. Both units werentended as a means of expressing power ratios. and gain or lossof power -related quantities such as voltage aid current. But inpractice. one bet was too large. so it was scaled down some 5years later to one tenth of a bet, becoming the deabel, abbreviatedto dB. (Arid n retrospect, 4 looks remarkably like the equvalent ofthe eater 'transmission urvt' that it sought to replace.). But sncethen. the decibel (dB) has become a very usekl and much usedmathematical tool, particularly n electrical engineering andacoustics. although I will oily consider the former here.

PowerStratty speaking. the original mathematical expression for betrelates oily to power raters. Two powers (say P1 and P2) are saidto to differ by N bets (where N is a number), when

they power ratio P1 / P2 = 10"

which when converted nto logarithm form becomes:

N bets =109,0(P1/ P2 )

arm because 1 bet = 10 cieorcis , this ixol xnes

N decibels = N dB = 10 logo (P. / P2 )

Its important to note that this expressos defines a number ofdecibels. not the decibel as a unit. so. strictly speaking, one canrefer to a decibel or to one decibel. but not to the (decibel. Nor isdecibel a metric unit of measurement or quantity, which s why itdoes not appear in the Systeme International ciliates or SISystem. In that metric system. the product or quotient of any twoquantities is the unit of the resuftant quantity for example. volt xampere = watt. each of which can be expressed in decimalmullaples or sub -multiples such as mega-, mil-. and so on.

Decibel however , ca -not be multiplied or divided by any SI Uri(to produce another SI Unit, nor can rt adopt other decimal multiples

silo-multales in place of deo-. It is an orphan n the field ofelectrical engineering measurement. because it is really no morethan a mathematical tool (as is the logarithm), whereby theproducts or quotients of large numbers can be more simplyhandled by the addition or subtraction of de quantities. Examplesof this are given later.Note that a ratio such as 2/1 produces a positive number ofdecibels. vvtereas a 1/2 ratio would produce a negative logarithmand a negative number of decibels. Positive decibels add tosirnuate multplicabon. whereas negative decibels subtract tosimulate division . for example:

(+10dB -3dB) = Ix 10/2) = x 5 = 7dB

But for convenience of use. rather than refemng to power r bus. it

is better to refer to power gas or loss. In which case, +derepresents a power gain. arid -dB a power loss or attenuation.

VoltageAs sad eerier. dB can also be applied to derivatives of power suchas voltage and current. but using a slightly modified expressionbased on the relabonshp power P = E/ R. as follows

N dB = 10 bglo (PI/P2)

now re-wrrtten as:

N clE1 = 10 logic (EIOR

(E2)1/T4

and canceling the R/Ta = 10 logic (E1)'/(E2)'= 10 logic (E,/E2)'


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SMART HIGH QUALITY ELECTRONIC KITSAll kits are complete with PCB and other components in a blister pack.

If you want more information about them, we have copies ofthe illustrated Smart catalogue available price £1.

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which. by using the mathematical expression logio 1x1'=2b9,o (x),

can be re -written as:

2 x 10 logic (E,/E2)

hence, n terms of voltage E:

N (IR = 20 no. (E -/E2)

Example 3: Voltage -gain of 36 dB is here converted to rts numencal

voltage gar),

36dB= 20c1B+ 10d6+6dB= 10 x 3 x 2= voltage gain of x60 (cf x63.1 true)

Example 4: Vottage-gain of 126dB is here converted to its

numerical voltage -gain.

Current 126dB = (20+20+20+20+20+20+6) dB

arty ri terms of current I, using the relationship power P = 12 R, = (10x10x10x10x10x10x2)

oeids : = voltage -gain 2)(10 (cf 1.995x106 true)

N dB = 20 log,2 (1,42) ft is clear from the above that:

Some examples follow. Note that al examples deliberately use In terms of power:

approximated numerical values:3 dB means a multiplication by 2, re twice the signal power

In power terms: 10dB means a multiplication by 10. re ten tries the original power

a power-gan of x2 ( ratio 2/1), gives: &milady, a negative dB figure moles division:

N= 10 log,,2(2)= 10x0.3.3dB-3dB means a division by 2. ie half the original power

a power-gan of x5 (ratio SO). gives: -1013 means a division by 10, e one tenth the ongnal power

N= 10 log,o(5)= 10x0.7 = 7 dB

a power -gain of x10 (ratio 10/1), glues:N = 10 Iog,c (10) = 10 x 1.0 = 10dB

And n terms of voltage:6dB means a multiplication by 2. re twice the original voltage20c1B means a multiplication by 10, re ten tines the cognal y-)' )(10

In voltage terms: &milady . a negative dB figure implies drown:

a voltage-gan of x1.4 (ratio 1.4/1), gives:

N= 20 logio (IA) = 20 x0.15= 3 dB

a voltage -gain of x2 (ratio 2/1), gives:

N= 20 log,o (2) = 20 x 0.3= 6 dB

a voltage -gm of x3 (ratio 3/1), gees:N= 20 log.0 (3) = 20 x 0.5= 10 dB

a voltage gain of x10 (ratio 10/1), gives:N= 20 log... (10) = 20 x 10= 20 dB

Applying the above dB values to obtain an acceptably nearapproximation of power or voltage gain:

Example 1: Power -gain of 33dB is here converted to rts numerical

power gam

-6dB means a drown by 2, re half the ongnal voltage-20dB means a division by 10, e one tenth the ongrel voltage

In amateur electronics an radio. decibel usage leans more topower and voltage and rarely to anent. Now, beer n mind that toproduce a two -fold increase in received rack:ravel voltage(considered by some to be = one S -point) naquires not justdo kung , but quadrupling of transmitted power (.+3dE1+3d13)! It isthen obvious that 1dB (= x1.26 power a x1.2 voltage) and even2dB (= x1.6 power or x1.3 voltage) represent not very significantgains/losses either n power a voltage terms, hence can be

ignored fa many practical purposes. asPectak if only approximatevalues are sought.In this case, it is possible to 'guesstimate' quite easily the power orvoltage gain/loss ecjuvalent to a given number of dB. by

commrtting to ri en cry the timing sox relationships:

Power3dB = multiply or divide by 27dB = multiply or divide by 510dB = multipty or divide by 10

33dB = 10dB + 10dB + 10dB + 3dB Voltage:

=10x 10 x 10 x2 6113 = multiply cx divide by 2

= power gan of x2030 (cf x1995 true) 10dB = multiply or divide by 320dB = multiply or divide by 10

Example 2: Power -gain of 67dB is here converted to rts nurnenca

power gain.67dB = (10+10+10+10+10+10+7) dB= (10x10x10x10x10x10 x5)= power gan of 5x16 (cf 5.01x106)

Example 5:Anterna gar of 16dB:=(power) 10d8 + 3dB + 3dB= x 10 x 2 x 2 = x40 gan (cf 39.8 true)


or= (voltage) 10d + 6c113

= x 3 x 2 = x6 gain (cf 6.3 true)

Example 6:Power gain of 47dB= 10dB+10dB +10dB+10dB+7dB= x10 xl0x10x10x5= 5x10 (Cf 5.01x10)

Example 7:

A coaxial cable is quoted as having attenuation of 6c13 per100rnetrets at 100MHz. This means a power loss of 6dE1/100m = (-3d13-3dB)/100m = (2'2)/100m = "4/10Ornetre. Signal power is

thereby reduced progressively by rahle losses, to one quarter of its

former value for each 100rn of the cable's lei gth. or to one halfrower per 50rn of cable and so on

Absolute power , voltage, or current - - - -s as applying ti

vortage ratkrs. trlat rs, to relative power or voltage. andnot to ahonlute values in watts or volts.

This difference is important n practice. For example. it a radioamateur were to say 1 have increased my power by 20 watts". thatwould be of no significance if his angina power was 1003W, butvery significant if he had increased from 1 watt to 21W. Sirnilarty. a3dB increase in power could mean a change from 2watts to4watts, or from 4kW to 8kW, so to be meaningful in absolute termsone would need to also state the origna reference power level.There is however a way around the. and that is to express the ratioin +/-dB relative to some arbitraty chosen datum or ' zero lever.Datum levels have been riternabonatypurposes. three of which are given below as being of direct interestto the radio amateur:

dBm = dB relative to one mihiNatt (mW) of electrical power.whereby OdB = 1 mWdBW = dB relative to one watt (W) of electrical power, whereby0dB = 1WdBd = dB power or voltage gain (typicaty of a been antenna)relative to a half -wave dipole antenna.

Note that d8d as applied to antenna gain is sal only relative. in thatrt does not provide an absolute power in watts or level in volts. Itimplies an increase n power or voltage over whatever is expectedfrom the major lobe of a dipole antenna in a simiar location. andthat applies equaty to either reception or transmission.

Example 833d8rn = power increase of >2003 above 1 mdwatt = (1/1000 VV)x 2000 = 2 watts-33dBm = power decrease of 2000 below 1 millwat = (1/1000W)-2ULJ0 =0.5 rnicrowatt33d8W = power increase of x2003 above 1 watt = (1VV) x 2030 =

20(X) watts-33dBW = power decrease of 2000 below 1 watt = (1)Arl" 2000 =0.5 milliwatl26dBW = power increase of x400 above 1 watt = (1VV) x 400 =400 watts16dBd = power gain of x40 relative to man lobe of dipole antenna16dBd = voltage gain of x6 relative to man lobe of Vole antenna

In electrical engneering, OdBm is taken as being 1rrdiwatt into 600ohms (hence Od13--0.775V rms pd). whereas in radio it is more

ikely to be lmW into 50 ohms (hence 0dB = 0.224V rms pd). Thsis important, for example when using a signal generator the output

attenuator of which is calibrated in dB , and you might wish toconvert that into microvolts pd to determine the sensitivity of areceiver.

Some other dB terms of radio interest are:

dEN = +/-dB relative to 1 voltdBuV = +/-dB relative to 1 miaovoltdB = antenna gain relative to the theoretical Isotropic" (equalresponse n all drectxxis) radiatordBc = dB relative to carrier level. for use in spectrirn analysis to

quantify noise, spunous signals and distortion products such astkiTTIfj

Other appk:aticns of dB n recto include the measurement ofreceiver sensitivity, ackacent charnel selectMly. squelch sensitivity,intamodulation rerection. mage refection and S -meter hnearity.

Power v voltage gainrre'lli yd r orr.polier can correctly be

quoted in dB, even if the input and output impedances arenot equal. If however the input and output impedances reallyare equal, then voltage dB will numencally equal power dBand vice -versa. that is. 3dB voltage -gain = 3dB power -gain,but only when Zin = Zout. That is because there can only bea direct relationship between voltage dB and power dB if theimpedances across which the voltages are measured areequal, to cancel out in the previously stated dB formulabased on P=E2/R. But in real life. amplifier voltage -gain isoften quoted in dB. even when the input and ouputimpedances so with due care.

Antenna gain is often quoted in dB. This is an example ofwhere Zin = Zoutt because the feed impedance is a constant.so that the dB figure applies equally to power -gain or voltage -gain.

Example 9:A 16dB gain antenna would produce:

a power -gain of 16dB = (10d8+3d13+3dB)= (x10 x2 x2) = x40

Or,

a voltage -gain of 16dB = (10d6+6dB)= (x3x2) = x6.

ConclusionIt is useful for all radio and electronics enthusiasts to have atleast a basic understanding of decibels. even if only toappreciate the meaning of those mystical figures that appearin antenna adverts and equipment reviews, or to translateinto watts the 26dBW given as permitted transmitter outputpower in the UK Amateur Radio Licence. Of course, the fullutility of decibels really comes into its own in complexcalculations such as those for radio link -paths for earth -moon -earth or meteor -scatter communications. But whetherit be for simple or complex calculations, a pocket scientificcalculator makes light work of conversion from numerical gainor loss into +/-dB and vice -versa, by using the log key whenconverting from gain to dB, and the inv. log key for dB togain - not forgetting of course to multiply or divide by 10 forpower and 20 for voltage.


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ELECTRONICS T. NTERNATIONAL65

SPICEDCIRCUITS

Circuit simulation with software, by Owen Bishop. This month, part 7 - Logicsimulation and a maths package.

n the previous part (Vol 27 Issue 1) we looked atsimulation of logic circuits by conventionalSPICE -based simulators. But even SPICEcannot be expected to be all things to alldesigners. It was written for designing integrated

circuits. Subsequently it has been very useful in numerousapplications relating to discrete analogue circuits, but it ha'.limitations with logical circuits. When we describe a logiccircuit, we do it in terms of gates, and assemblies of gates(such as flip-flops, counters and registers). Logical analysis isbased on truth tables.

When we simulate a logical circuit using SPICE, thenature of SPICE makes it operate at a more fundamentallevel, calculating the currents through the transistors andresistors that go to make up each gate. Analogue analysis isbased on Ohm's Law, Kirchhoff's Laws and transistor outputcharacteristics, plus a great deal more. This is slow if thecircuit contains more than a few dozen components. Logicgates usually consist of several transistors and othercomponents, so that they perform in a very robust andrepeatable way. Analysing them by analogue methodsinvolves many time-consuming calculations for which, inpractice, there is no need.

For instance, if the voltage at the input of a CMOS gate isgreater than half the supply voltage. it rates at a high input.With a 5V supply, it does not matter if the voltage is 3V, 4V,or 5V or something in between. So there is no point itmaking the simulator iterate around a resistor/transistor gatecircuit to calculate the voltages to 12 decimal places. A logicgate can be taken as a black box; given certain inputs, itgives a predicted output. with various but small anddependable time delays. The whole approach to modellingdigital circuits is different.

The completely SPICE -modelled gate has uses forsimulating mixed -mode circuits but the time that this takeslimits the number of gates that can be included in a circuit.And we are talking about gates. which is still a long wayfrom modelling logic circuits with counters, shrft-registers.arithmetic units, and even further from LSI and VLSI devices.There is another reason why SPICE is inherently unsuitablefor simulating logic circuits. Logic gates change state in afew nanoseconds but. once changed, may remain in thesame state for microseconds or milliseconds. These areperiods several thousand or million times longer than theperiod of change. It makes no sense to repeatedly calculatevoltages and currents for these static periods. The simulator

'Ma NapM Mop12/4 ?imp/114 ?Bop)114 214.

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7Xt X'1Y, O,'79 C/o4I'74/7

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KC

Figure 1: this version of the JK ripple counter uses a 7449decoder and a 7 -segment display to read the output states

must be event -driven, leaping from one voltage change tothe next. The iterating procedures of SPICE do not fit in withlogic gate timings.

Simulators such as SpiceAge, on which we based thefirst five parts of this series, minimise these disadvantagesby breaking away from SPICE in certain respects. SpiceAgehas recently added logic gate primitives and other functionblocks to its range of 'components'. It also has algorithmsfor automatically calculating the size of the time step, toproduce longer time steps when feasible and so decreaseanalysis time. Going even further, some simulators havestarted again from scratch and use a purely logic -basedapproach to digital analysis. One such simulator is describedbelow.

B2 LOGIC, V3.0B2 Logic 3.0 is published by Beige Bag Software. of AnnArbor. Michigan, who also publish the analogue simulator B2SPICE which we looked at in the previous article. B2 LOGICis available in Windows and Macintosh versions and is animpressive piece of software which is very easy to use.

The first stage in setting up a logic circuit is to draw itsschematic on the screen. As a simple demonstration, we setup a ripple counter, one of the logic circuits that we analysedlast month. Logic elements are selected from the devicelibrary (right of screen in figure 1, though it can be movedaround). There is a very extensive library including everythingfrom a plain inverter gate, through all kinds of registers and

ELECTRONICS TODAY INTERNATIONALBO

Elle Edit Zoom Simulate Output library Window

I 11121 Lit,It .CKI

.ow

Generic Libra1 -bit Input2 -bit Input4 -bit Input8 bit Input16 -bit In utflint pill Protir1 -to -2 Bus Connecta1 to 3 Bus Connedo1 -to -4 Bus Conned°1 -to -8 Bus Connect°2 -to -1 Bus Connecto

Tools d Time

Figure 2: a simulation of one of many possible simulations of a digital die. The operator has lust'thrown' a five

flip-flops to a 74646 octal bus transceiver and register. Thedevices are displayed as standard IEC (InternationalElectrotechnical Committee) symbols. There are severallibraries to choose from. including Generic devices, TTL.LSTTL and CMOS technology. For this analysis we selectedfrom the Generic library and built a three -stage ripplecounter (it was two -stage a few months ago, but we can bea bit more ambitious with a dedicated digital simulator). B2Logic has a useful set of input and output devices. The highvoltage device (Vcc) provides logic high to inputs that needit, such as the J, K and reset inputs. Another device. aclock, is set to have a period of 8Ons, with a pulse width(length of time for which it is high) of 4Ons.

A probe is a handy device for reading output levels. Weattached one to the clock and set it to display '0' for logiclow and '1' for logic high (you can also make it display 'L' or'H'). We could have done the same with the outputs fromthe counter but, if we have a ready -to -use 7449 decoderhandy, why not use this instead? The 7449 is a BCD to 7 -segment decoder and the library also has a 7 -segment LEDdisplay which is easily attached (by dragging it with thecursor - simulation has that big advantage over real life). Allwe have to do now is to run the simulation and watch thedisplay. The 'Tools and Time' window has but --r-:on a tape player for reset to zero time, stop. . . .

forward, and continuous run. Single -stepping (tor presettabletime intervals) shows the clock alternating between high andlow and the 7 -segment display goes through this sequencerepeatedly:

01 (0) 2 3 (2) (0) 4 5 (4) 6 7 (6) (4) 0 1 etc

With 1Ons steps, the numerals in brackets are displayed fora relatively short time. They are the glitches on a ripplecounter that we explained last month. A continuous runmakes the glitch periods so short that you can hardly noticethem and the counter appears to be running correctly. At the

end of the run we can ask for atrace. which plots the levelsdetected by probes. Toexamine these we first need toattach probes to the threecounter output lines. The 1Onsdelay in each flip-flop is thenclearly seen. Alternatively weCan call up a table of probelogic levels at preselected timeintervals during the run.

This circuit demonstrateshow a counter can be built upfrom flip-flops and thenanalysed. If you need countersin other circuits, you would diginto the Library and usecomplete counter devicesranging from the 7493 binaryripple counter to the 74169synchronous up/down binarycounter with parallel load.

The ultimatesimulationIn the world of simple circuitsthere are probably more

designs for egg -timers than for any other function. Secondon the list comes the digital simulation of rolling a cite, withLEDs displaying a random count from 1 to 6 when you pressthe stop button. Figure 2 is a simulation of one of the manydigital die circuits. This is a simulation of a simulation - but itworks. The circuit is driven by the clock we used in figure 1but this time its output goes to a counter, a model of the74LS92 counter wired to count repeatedly from 0 to 5.

The output is decoded by vanous logic gates, to whichseven probes are connected. We have arranged these sothat, if you look for the '1's. they produce the conventionalpatterns of spots on the face of the die. The number ofgates is fewer than would usually be the case because itwould be uneconomic to use four different kinds of gate,when building a real circuit. We would probably manage itlust with NANDs and NORs and the simulator could be usedto check out the slightly more complicated logic involved.

To start the die rolling, click on the continuous run arrow.When you click on stop the display shows the score. In thefigure, the pattern of 'ONs' displays 'five' . Here we mustreluctantly leave B2 Logic, which has in fact shown us thelast simulation of th,c seripc and move on to somethingentirely different

MEXPRESS 1.1finis Is one of tne newer maths packages, and has a lot tooffer the electronics designer. The simplest way to use it isas an on -screen calculator. For example, type 25 sin (1.2)+ 3.27 at the prompt, press ENTER, and back comes theresult, 26.570. It can handle imaginary numbers too. Forexample 3.4 + 0.2i multiplied by 2.1 - 5.67i gives the result,8.274 - 18.858i, although entering the calculation is slightlymore involved. The package has a range of over 250functions, including matnx algebra, statistical functions andequation solving. Another strong feature of this package isits flexible system of plotting graphs, both 2D and 3D. andits ability to produce real-time animations. Before we look at


its graphics we will consider how the mathematical functionscan be of use to the electronics designer. AlthoughMExpress probably could simulate whole electronic circuits.we are not likely to use it for this purpose because it wouldtake too long to set up the equations and plotting routinesrequired. Regular simulators have all the required functionsbuilt in already. But there are numerous instances of designcalculations that we need to perform over an over again, yetare that bit more complicated than can be handled easily onan ordinary calculator.

To take one very simple example as an illustration, weoften need to find suitable resistor values for a potentialdivider. For instance, there is a need for this when we aretrying to bias a transistor. You can, of course just make thisa one-off calculation, type in the variables according to theformulae and obtain the results. But MExpress goes onebetter (actually two better, as we shall see later). We can setup the calculation plus a few frills in a special .X file. Then allwe do it type the name of the .X file at the prompt and theroutine is automatically called into action. Figure 3 showsthe notepad with the routine entered. The Input statementsask the user to key in the three values needed for thecalculation. Then the routine calculates 'outres', the value ofthe output resistor, and also the current through the network.After printing a heading, the calculated values are displayed.The contents of Notepad are then saved in a file calledpotdiv.X.

Figure 4 shows the outcome when, back in MExpress.we type the file name 'potdiv' and press Enter. We are askedto enter the values of input and output voltage and also theintended total resistance of the chain. The displayed resultsshow that the output resistor should be 2.2k and the otherresistor 7.8k. Probably the nearest E24 value. 7.5k?would beacceptable. Current through the network is 9mA. To recallthe routine for another calculation, simply press the cursor'up-arrow' key a few times until the original 'potdiv'command reappears at the prompt. Key 'Enter' and repeatthe calculation with new values.

This is only the barest routine. Several refinements arepossible, including routines to select the nearest E24 values.As an illustration of what can be done wehave added a 'message screen'. Thiscomes up on the screen as a typicalWindows message. to warn you if byaccident you have typed in the input andoutput voltages the wrong way round. Themessage screen can be tailored to fit yourneeds. This one has a bold exclamationmark on it and an 'OK' button foracknowledging the message.

We said we could go two better, and thenext step could be to compile the potdivroutine to produce a free-standing program.You need a C++ compiler for this.MExpress translates your .X file into C++,to which you can add any C++ code ofyour own, and compile the lot to produceyour faster -running and more sophisticatedpotdiv software. Whether you compile ornot, you can quickly write a number ofvariants of the potdiv routine. Anothervariant could ask for voltages and networkcurrent and calculate both resistor values.Another could ask for both resistor values

and input voltage and calculate output voltage. Then, turningyour attention to the 555 timer ic, there are several moreroutines which would be very useful to have handy.

GraphicsA picture is said to be worth a thousand words (or perhaps athousand data values) and so the 3D graphics of MExpressare specially useful for portraying the results of certain typesof circuit analysis. As an example, take the response of asimple RC lowpass filter network, consisting of lust oneresistor and one capacitor. Their values are selected to givethe filter a cut-off point of lkHz. The transfer function of thefitter is given be:

F(s) = w0/(s + wo)

where s is the complex frequency variable, which isequivalent to (a + kw). These two variables are measures ofsignal frequency (w) and the rate at which amplitude ischanging (a). For a signal of constant amplitude, a= 0, andwe can substitute ko for s in the equation.

MExpress is based on matrices. so the first step is to setup a matrix s containing the values of s for which we want toplot values of F(s). This will be a 2 -dimensional matrix withrows for each value of a and columns for each value of jw(actually iw because MExpress uses the symbol i instead ofj). Values of a are 0 to -8000 in steps of 2000, and values ofico from Oi to 3i in steps of I i. We then form a matrix t.holding of values of F(s) by using the command:t=abs(6283/(s+6283)):. The term 'abs' gives the absolute -value of the expression, since we are concerned here onlywith the amplitude of the output signal, not its phase. Thevalue cog = 6283. is obtained from:

cog = 2itfg

when f0=1 kHz. All that remains is to type MPlot(t), to obtainthe 3D graph shown in figure 5. The axis running off to theleft is frequency (kw) and the axis running to the right is the

File Edit Windowx Help

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// Potential divider

uin-Input(-Supply voltage? ");uout-Input("Output uoltage? ");total-Input("ldtal resistance? ");outres-total/vineuout;othres-total outres;currt-uin/totalx111118;if imut>vin then ilisglicix(-frror-, "Output exceeds input -;20);end if;head (IS, -RESULTS OF CALCUlATIONS-,111);Display(head);Display("Output resistor - -);Display(outres);Display(" ohms");Display("Other resistor - ");

Display(othres);Display(- ohms");Display( -Divider current - -);

Display(currt);Display(" RA");

Figure 3: in MExpress, users can type often -used routines into Notepad and savethem for instant use later. This is 'potdiv', a routine useful for calaculating values fortransistor biasing resistors


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Figure 4: the results of a calculation using the potdiv routine

rate of change of amplitude (a). The vertical axis is thetransfer function, or gain, (F(s)). At the ongin (centre front) thegain is 1, as is expected for a DC signal. Running along theleft-hand edge of the plot we have the response of the filterto increasing frequency at constant amplitude - the usualplot of gain against frequency. Gain falls with increasingfrequency, as would be expected in a low-pass filter. but thisdoes not show up on the graph because the vertical scale istoo large. The reason for this is the tall peek we havediscovered at the point where s se -8000 + Oi. Substitutingthis value into the equation gives F(s) = 6283/283 = 22.2. Ifwe made s equal to 6283. the result would be infinitely large.and project infinitely upward. We call this a pole. The poleand its surrounding region represents a set of conditions

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0100I."'20.011

Figure 5: the gain of a lowpass RC filter plotted against frequencyand rate of change of amplitude

at which the filter enters a resonant state, perhaps briefly,and its output voltage exceeds its input voltage by aConsiderable amount. Looking for poles is an importantaspect of design because, if there is a chance of thecircuit operating in the region of a pole we can expectsevere distortion. Plots such as figure 5 can help inlocating poles.

This ends our brief survey of software useful to theelectronics designer. New programs are coming onto themarket all the time, but we hope that the examples wehave used here will help you think clearly about simulationand to choose one most suited to your needs.

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ELECTRONICS TODAY INTERNATIONALso

TIC TAC TOE(NOUGHTS AND CROSSES)

A game -in -a -box with a PlC, by Rose and Andy Morel!

his is an electronic game t,t Noughts and Crosseswhich allows you to play against the 'computer'.that is, the PIC 16C64 microcontroller.

The PIC contains a long program designed toallow one or two players to play the game. The

source code is available (see below). The 16C64 is the-intelligence" of the unit, and the playing board consists of nine5mm tricolour leds (LED1 to LED9). In this way. the lights canbe either green or red when selected, depending on whoseturn it is. as the colour changes automatically. There are alsotwo further leds. LED10 and LED1 1). which indicate whoseturn it is. The eventual winner is heralded by a tune. whichplays when a game has been won (or lost!). while the winningline flashes on and off. There are ten pcb-mounted pushswitches. including one reset switch. The position of thebuttons corresponds to the position of the tri-colour leds, thatis, rt is a 3 x 3 matnx.

How to playWith the power on, select either push -switch 1, push -switch 2,or push -switch 3. To play against an opponent, select switch1, and to play against the computer, select switch 2 or switch3. Switch 3 will allow the computer to make the first move, andSwitch 2 leaves you to make the first move. After that, you canpress any one of the nine switches to commence play andmake your move. Should you opt to play against the PIC, beprepared for a rapid response!

If there is stalemate in the game, you will have to pressReset to start a new game. If, on the other hand, you or thePIC should win a game, you will hear a tune (exactly which onedepends on your choice of the sound generator Chip duringthe construction stage). No further moves will be pnssible oncethe winning move is made. To stop the tune and start a newgame. press 'reset'.

How the software worksThere are nine memory locations representing the nine lads. Onreset or power -up. the nine memory locations are filled with thenumber '8'. The software uses this information to determinewhat move to make. Now the game is being played. and youhave just made the first move. Next, it's the PIC's turn, so thecomputer scans an three rows of !eds, followed by the threecolumns, and lastly, the two diagonals. looking for a 'goodmovel

It does this by looking at the nine memory locations thatrepresent the nine leds. Once a move has been made, the PICputs a'1' in the chosen location for an 'X', or a '2' for an '0'.while a space remains an '8'. (The microcontroller is always the'0'.) On subsequent moves, the computer scans the memorylocations and adds up the numbers in each row, column, anddiagonal, looking for a line that contains two 'O's to win ortwo 'X's so that it can block the third 'X' and at least not lose.To summarise:

1) A win in one move: the PIC only has to put its '0' in theappropriate space to win.2) A lose in one move: the PIC must block the 'X's' next moveto avoid losing.3) Neither is possible yet: the PIC will check the centre led tosee if it has been occupied. If it has. rt will look for 'any space'.and if not it will bag the centre led!

For the 'computer' to find a win in one move, there must bethree consecutive locations resulting in a total count of '12decimal', that is. two 'O's and one space: 2 + 2 + 8 = 12.Therefore, if any row, column and diagonal totals 12. a win in

ELECTRONICS TODAY INTERNATIONALSO

SW1

SW2

SW3

SW4

SW5

SW6 __.SW7

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SW9 __61=16_.

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Figure 1: the circuit of the Tic Tac Toe

36

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SwIORESET

one move is possible. and the computer only needs to put its'0' mark in that space to win - and celebrate by turning on theappropnate led. Flashing the winning line, it then loops awayuntil Reset is pressed, or the power is interrupted.

Should no total of '12' be found in any one of the eightpacc,ibilities, then it will scan again, this time looking for a countof 1 + 1 + 8 = 10 decimal. This is 'may lose in one move'. andas before the corresponding space will be taken up with thecomputer's 2, to block that move.

if the computer cannot find a place to put the first '0'. it willcheck the address that represents the middle led, to see if ithas been taken. If riot, then it puts its 2 or 0 in the centre or, ifit has already been taken, it just takes any space it can for thefirst move only.

A check is made at the end of every move to find a possiblewinning row, column or diagonal.

ConstructionThe following should be sufficient for any hobbyist to build theirversion of Tic Tac Toe with the pcb and software provided.

There are a number of links, which should be fitted first.After that, frt the 40 -pin dil socket, taking care not to bend thepins. Next, fit the single -in -line resistor (SIL1). taking care to putthe 'dot' at the 5V pin on IC1, followed by R2 -R8. This in turnis followed by the ten key -switches. SW1-SW10, and the ledsLED1 to LED11, ensuring that the side with the flat edge/shortlead (cathode) is inserted correctly. Next, avert:11y insert the

51.0 IN

PUT '0 ON COOTIE LED

Figure 2: a flow diagram shows how the game programprogresses


SW6 SM3 SWIVERSUS RESET

Figure 3: the component layout ou

sound generator. IC2. the correct way round with the 'flat' sidefacing the keys. There are three caparitors. of which one (C1)is polarised (and optional). The voltage regulator (also optional)is fitted next with the 'curved' side facing the leds. A batteryclip or power socket can be fitted to the Veropins, perhapswith a 9V battery providing the power. A good dc powersupply can be used instead. if required, but please consult thesection 'Using the unit'.

Crystals are not used for the timing components. To keepcosts down, a resistor/capacitor network (R10,C3) is usedinstead. C3 is from 20 pF upwards. but it is not definitelyneeded. The oscillator operated quite happily with a 100kresistor on its own on the prototype.

Finally, a pezo speaker is fitted. A suitable one can beobtained from Maplin.

Casing the unitThe pcb was found to fit neatly in a small box, with the twohalves held together by a 'hinge' made of adhesive electricaltape. Real hinges could have been used, if they are smallenough. We stuck the pcb down with double sided tape insideone-half of the box, with the other half holding the batteries(side by side), and the piezo sounder stuck there also. So thecomponents are then protected from damage, and the box

BATTERY -

BATTERY .

ResistSIL1R2,R9R3 -R7

R8

R10

easily opens when it is required. Thebox we used was from Maplin andmeasural 145mm x 80mm x 32mm.

Using the unitProgrammed

and the hardware constructedsatisfactorily, the game can be played.At the start of the game select onepush -switch 1, 2 or 3. This determinesplay (see above). Press the buttonsgently. The game provides endlesshours of enjoyment for children oradults.

Battenes can be used. but we foundthat while the unit (including the PIC)worked quite happily on a 3 -voltbattery, without the voltage regulator.the display is not so bright.

CapacitorsCl

C2C3

4k7300R270R100k

(see optional regulator100 nF capacitor, ceramic.See text

Semiconductorslc, PIC 16C64 (see below) Write for

pricesIC2 Sound generator chip: choice of

music chips from Maplin (M66TSeries, eg GX55K)

LED1-LED9 5mm tri-colour LEDs (commoncathode)

LED10 3mm red ledLED11 3mm green led

Miscellaneous71 Piezo sounder PCB, battery clip, battery

box, solder, wire, Vero -pins.

Optional (for regulator)Cl luF 16v elect radialIC3 7805 100mA voltage regulator

A 3.5 -inch disk containing the source code isavailable for £5 (inc. postage and packing) payableto A J Morrell, 12 Boscobel Road. Winchester.Hants 5022 6RY. Postal only - please do not call.Programmed P$Cs may be available if demand issufficient - please write (enclosing an SAE) and


RS

Line -Up Oscillatorwith Glitch

A glitch is a good thing if it tells you which channel you are in,according to Tony Sercombe.

hen sending an audio signal to a remotedestination, or to a tape recorder, it is necessaryto know the average sending levels, and also, inthe case of a stereo signal, to know which leg isstereo right and which is left. The following circuittut% both these criteria.

The circuit is intended to be included within existing audioequipment, and as such a separate power supply is notprovided. since its requirements are very modest, and it can berun from venous voltages.

The circuitThe oscillator consists of a single transistor, working at 1 kHz.The output is fed to 02, which is an N -channel fet connectedas a buffer. This is necessary to prevent loading of theoscillator. which would lead to severe distortion of the

waveform. The output of the buffer is fed to a resistor networkconsisting of three resistors. The two series resistors have thesame total value as the single resistor in parallel. However, atthe junction of these resistors is the point at which the glitch inthe output of one leg takes place. The 220 uF capacitor inseries with a P -channel fet and a 1k2 resistor form a switchfrom this output feed to earth, controlled by IC1. Under non -glitch conditions, the two output resistances are equal. When aglitch is applied, the resistance of the fet 03 is reduced,effectively reducing the signal by the potential divider action ofR12 and R11, the resistance of the fet being negligible.

IC1 operates in the astable mode. R7 and R8, connectedbetween pins 8, 7 and 6. charge C6, which is connectedbetween pins 2 and 6, and ground. When this charge reachestwo-thirds of the supply voltage, C6 discharges via R8 into pin7. The standing voltage on pin 3 feeding the gate of Q3 now

-C2

RVI229

I= C3:22p

C4NM 15,,

Figure 1: the circuit of the Line -Up Oscillator with Glitch

022943819

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Figure 2: the component layout of the prototype

ramps low -switching 03 to the low (resistance) state. thusproducing the glitch. The IC is a standard NE555 timer. Thesomewhat heavy decoupling on pins 4 and 8 preventsdisturbance on the supply line. The component values shownat IC1 provide for a haff-second glitch every 10 seconds. Ifdesired. these times may be modified to suit indmdual needs.and the values changed accordingly. However, the valuesshown seemed to be about right in the prototype. It isimportant that the glitch should not take place too frequently,otherwise the frequent repetition may interfere with the line-upprocedure.

In the prototype the signal output level was 150 millivolts.measured as 146 mV with a 15 -volt supply, dropping to 130my with a 9 -volt supply. The outputs should be fed to a load 0410k. Much less than this will produce a disturbance of the un-glitched output. If this is unavailable. you may have to resort tobuffer amplifiers.

A circuit layout is provided. When construction is complete.it is a good idea not to fit the ic immediately, so that theoscillator may be adjusted more conveniently. I alwaysrecommend the use of ic sockets, so that this is a minormatter.

Testing:ter cnecking tor any construction mistakes, dry LantS, etc.,set the two preset controls to their midway positions. Ifpossible, connect an oscilloscope to one of the outputs. F,ithis, a pair of medium to high impedance headphones --suffice. Connect a voltage of between 9 and 15 volts.switch on.

At this point, oscillation may not take place. Slightadjustment of the presents RV1 and RV2 will remedy thissituation.

Remember here that these two controls are interdependentand since RV1 sets the frequency, and RV2 the purity of thg-waveform, there may be a small compromise in frequencyaccuracy to be made. However, the prototype provided arextremely good waveform at a frequency of 1.02 kHz. In anyevent, a precise value is not of paramount importance. butrather a well-defined waveform. If listening on headphones.check for the purest -sounding note.

Once this has been completed, the glitch can be checked.

Remove the supply and put the is into its socket. Afterrestoring the supply, the glitch should be noticed as a sharpdip in output of half a second, every 10 seconds.

RealR1,R2,R3R4

R5

R6,R11,R15R7

R8

R9,R10 100kR12,R13R14RV1

RV2

CapacitorsC1,C2,C4C3,C7C5C6C8C9C10

10kIOR

1k21M

47k

12k

24k22k470R

15n poly22u10n polyIOu

100n

220u100u

SemiconductorsICI NE55501 BC549 (or 108/9, etc .IQ2 2N381903 J176*

MiscellaneousThis project is designed to be fitted in existingequipment. PCB, ic socket, wire, solder, etc.

'The J176 can be obtained from Cirkit DistributionLtd.. Park Lane. Broxbourne, Hertfordshire EN107NC1. Tel 01992 448899 stock no. 59-02176. or

other suitable P -channel device may be used.

ELECTRONICS TODAY INTERNATIONAL.64

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JPG ELECTRONICS

ELECTRONICS Tot AY INTERNATIONAL72

ETI REVIEW

Peak Electronic DCA50Component AnalyserA neatly made analyser than can

identify transistors, diodes and LED andgive the pin -outs quickly and without

fuss.

he DCA50 is an analyser designed to identity discretesemiconductors and their connections. It can test avariety of two -terming or three -terminal devices.chiefly diodes, mosfets and bipolar transistors.Three -terminal diode networks can be tested for the

presence of diode junctions, and the incividua junctions can beidentified.

The analyser has three leads - n red. green and blue for quickidentification - each terminated n a miniature crocodie dip. Thereadout is a two-line OW crystal dot matrix display. The heat ofthe instrument is a PIC16C64.

Components are tested simply by ocnnecting each of the twoor three connections of the component to the crocodile cipsany order and operating the one button on the front panel. For acouple of seconds, the unit displays the message "DCA950Component Analyser (R1010)". and then the analysts. such as-NPN Transistor RGB=>BEC Hfer-.460", appears. This messagewas displayed when testing a BC109 transistor. The second time Itested the same device. the He reading cane up as 445.

The tester can identify signal ciodes, rectifier diodes. zenersand also identify the terminus of a light ()ratting diode, and flashthe component for a visual ncication that it is functioning(regardless of which way round it is).

At more length, the main components that can be tested areP- arid N -charnel trirahurrrerrt mosfets: NPN arid PNPtransistors: diodes and diode networks, and LEDs. It can indicatelife for bipolar transistors in the range 5 to 955, with an error of 5life and +1- 4 percent of the reefing. Darlington transistors can betested, but are likely to exceed the maximum range ofmeasurement, n which case a reeding of 955 is displayed.

The DCA50 cannot. apparently, test depletion nude mosfetsand junction fats (which are inevitably depletion devices, andtherefore not supported. They are. in any case. not in suchwidespread use as they used to be) but it will indicate the anodeand cathode of any diode junctions. which effectively identifies thegate, arid determines whether the device is N -channel or P -channel. The same method is used to read faulty transistors Ofthey are readable at all). If the current gain is very low or non-existent. the DC50 should be able at least to identify one or bothdiode junctions. Some diode -protected transistors carrot beaccurately identified.

The anaryser's crocodile clips are small, but they are not smallenough to make easy contact with the thin wires of 1092transistors. Surface mount devices are even harder to connect to.

It is very useful in that it can identify the pin connections ofunknown devices, including the determination of which is thecollector and which the emitter in junction transistors. It is easyenough to identify the base by using an ordinary multimeter on the

diode test range, but to identify collector and emitter automatic*saves a lot of effort.

Meek and blueThe black case feels robust enough for general use. and fits neatlyin the hand. The stylish metallic blue lettering on the CARP is moreeye-catching than easy to read, but this may actually be anadvantage, as it does not distract from the display.

The unit is specifically for examining components out of cira.it,and includes a warning aganst trying to test components in apowered Gait, as damage to the analyser and possibly the testcircLirt may occur.

tt will be a useful aid in replacing components in egtipmentwhich incur mates discrete semiconductors, particularlysemiconductors with ndecipherabls makings. lt vii also help inmaking sure that semiconductors are connected up correctly inprototypes.

In operation, it is handy and trouble -free. You simply connect Itup to the component. press the single button, and it grves you areadout. The readout is dear, and as long as you have read theinstruction booklet to make sue that you understand thecorrespondences on the readout, you should know at once whichleafs are the collector, base and emitter. or cathode (k) and anode(a). depending on the kind of readout given.

The instructions give a table of testable component types andthe parameters within which they can be tested accurately. Themakers say that the analyser Is optimised for the vast majonty ofsupported component types; a minority will fall outside therequired operating conditions.

Maintenance on the component analyser amounts to makingsure that the PP3 or equivalent battery is changed at least every18 months to guard against inadvertent battery leaks. A batterywar appears when the battery is nearing the end of its life.

Although it may be more than the hobby user would spend inorder to identify the occasional re LalLitrait transistor, the DCA5OSbasic list price of £49 is good for a PIC-driven instrument with thismuch functionality. and is well worth considering if there is muchcomponent identification to be done. We found it particulartyuseful for quick -testing diodes from the diode box before using indisplays or prototypes.

The 8 -page instruction booklet is easy to follow and dearlyillustrated, and includes the parameter list for components capableof being tested accurately.

For nformation, prices etc. contact Peak Electronic Design, 70Nunsfield Road, Buxton, Derbyshire SK17 7BW, UK.

ELECTRONICS TODAY INTERNATIONAL7$

Further.

193-nanometre herthan the much finer beams used in moreexperimental research. It had beenexpected that technology still in very earlystages of development, such as extremeultraviolet or x-ray lithography, would beneeded to achieve such small lines, butinstead it has been done with the same sortof optical technology used for the currentgeneration of 0.25 micron semiconductors.

It is not yet known whether this will turnout to be a commercially viable process,and it is likely to be several years before thisis determined. One interesting things is that,using the 193-nancxnetre wavelength oflight, the features etched are effectively finerthan the etching beam used.

But beside this dramatic progress, in theUSA. as in the UK, there is a shortage oftechnical skills. American companies haveasked the government to increase thenumber of foreign workers who can begiven temporary visas. Companies asreputable as Microsoft. CypressSemiconductor, and Texas Instruments areall reported to be lamenting the skillsshortage.

Texas Instruments' Stephen Leven isreported to have found evidence that theimage of high tech workers is distinctlyuncool with American youth. "Recently, Iwas shown a drawings by children asked to

r images of ther r noanies are

icket protectors,impled clothing ..."

that the engineeringmore comic. The

American response hasher salaries to attract theavailable, while in Britain,

are not too bad forpeople, they are not in the same

As an example, $40,000 (about£25.000) was offered for graduates with avery modest bachelor's degree from anaverage college.

Britain's attitude is compounded bysome of the great and the good who thinkthat the ability to understand anythingtechnical means an inferior intellect. Therel!,certainly a difference in outlook betweenengineers and politicians. As we know,engineers tend to regard facts asreasonably fixed and verifiable. whilepoliticians sometimes seem to regard themas infinitely mutable.

The attitude is typified by an MP on thefirst day TV cameras were used in theHouse of Commons. The interviewer askedwhether he would watch his performanceon video afterwards, and he replied, withpride ringing in his voice, that although theyhad a video recorder, neither he nor his wifeknew how to operate it.This was an extreme indication of theignorance of the true economic significanceof science and technology. Despite, orperhaps even because of this, governmentsthe world over react slowly to mosttechnological advances.

Speaking of which, we hops to have anEll page on the World Wide Web soon. Theuri will be published as and when it is setup. If you try a search engine, you may geta preview before the magazine containingthe url is published.


S

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Nexus Special Interests Limited 1998All nets reserved

ISSN 0142-7229

The Publisher's written consent must be obtanabefore any part of this publicsoon may be

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All reasonable care is Warn in preparanon ofmagazine contents. but the publishers. editors and

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The Ultimate 8051Microcontroller Programmer

Supports Atmel FLASH 89C +895, 1,4Generic 87C51/52/FA/FB/FC

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Support! Parallel Programmingmode in ZIF socket

Supports In -System Program*via ribbon cable (provided)Equinox FAST ISP Programming

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SYSTEM CONTENTSProfessional Device Programmdf

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