build a brain wave feedback monitor

8/2/2019 Build a Brain Wave Feedback Monitor

1/6

Build an

You

through

may be able tolearn how to relax

electronics BY MITCHELL WAiTETHERE is nothing quite so pleasant asheing able to relax completely wheneveryou want to. Unfortunately, today's quickpace rarely leaves us the time to truly relax.Perhaps for that reason, scientists havecome up with ,111 electronic approach to relaxation that might revolutionize the ar tof "calming c l o W l ~ . " Drawing on knowledge

of general psychology, eastern meditationtechniques, and, in particular, clinical clectroencephalographv, researchers in thefield of alpha-wave feedback have progressed rapidlv in the last few years andmade manv significant gains.Unlike the older forms of meditation,alpha-wave feedhack requires neither anavatar or guru. Researchers have found thatthe minute brain-wave frequency band between 7..5 and 1:3 Hz is continuously produced in meditative stages of Yoga and Zen.This is called the "alpha state." The assumption is that the length and intensity ofalpha-wave production is an impartialmeasurement of the ahili ty to reach a special state of "relaxed awareness," found incertain types of meditation.40

People who produce continuous alphaseem to experience a generally heightenedsense of well-being, with a parallel increasein clarity. Thus, alpha feedback allows oneto prepare for demanding mental tasks bypreviously clearing the mind of distractingthoughts and ideas. I t is precisely for thisreason that some businesses are investigating alpha feedback. Researchers are alsosuggesting that the "pain" of education canhe lessened if these procedures are used inattention control. There is the possibility,they say, that recall can he improved andmental blocks avoided during examinations,by the use of alpha feedback.

Basic Approach. In alpha feedback. highgain, low-noise amplifiers detect the microvolt signals of the brain and use them tomodulate a sound or other stimulus. The

"person training for increased alpha completes the feedback loop by listening to therise and fall of a tone as the brain wavescome and go. Thus, by learning to producejust the elusive 7.5-to-13-Hz modulation, aperson can experience the alpha state.Actually, all brain waves have charac-POPULAR ELECTRONICS Including Electronics World


2/6

teristic mental correlates. For example, deepsleep produces the long slow waves between2 and 4 Hz; problem solving and daydreaming give rise to the theta rhythms (3.5 to7.5 Hz); while tension, worry, or surpriseproduce the beta frequencies (13 to 28Hz). There is also evidence that creativeand spontaneous moods occur most oftenwhen the frequencies between alpha andtheta are active. This has led some researchers to speculate that creativity andinsight might be facilitated by learning howto increase frequencies.The important thing is to find out more ofall this for yourself. With the circuit described, you may be able to influence andenjoy all of the brain-wave states. In addition, the project can be used to listen tosuch body Signals as scalp tension and heartrate.

About the Circuit. Because of the rapidincrease in the popularity of biofeedback,a large selection of feedback monitors haveappeared on the market. Their complexityranges from a device for alpha feedbackusing only one IC to research laboratoryequipment costing thousands of dollars. Thelatter include such features as strip chartrecorders, multi-channel amplifiers, highlycontrollable filters, percent time indicators,etc.

The circuit shown in Fig. 1 incorporatesfunctions usually found only in more sophisticated equipment. For example: becausethe different brain waves are verv close infrequency. a switchable 4-pole' bandpassfilter is used. Each filter is tuned to thecenter f requency of the theta, alpha, andbeta bands. These filters obviouslv makerecognition of a particular brain wave mucheasier and faster.Another critical parameter of a feedbackmachine is its ability to reject strong common-mode interference-such as 50-Hz humor erroneous signals from electrode movement-while presenting a high input impedance. An inexpensive solution to thisproblem i:, to use a single low-noise op ampin the differential mode. This solution is notcompletely satisfactory because of the inevitable tradeoff between input impedance,balance, and common mode rejection. Herewe use an instrumentation amplifier for thefront end, with two low-bias op amps (ICland lC2) providing an almost infinite inputimpedance and excellent common mode rejection.JANUARY 1973

Electrodes, which couple the microvoltsignals to the amplifier, are critical in DM?respects. They should not generate shortterm voltages (tiny noise spikes) or longterm voltages (offset or drift). A numberof low-cost commercial machines use aninert material such as stainless steel forelectrodes. The difficulty with these electrodes is that they produce some noisespikes and (more seriously) generate aslow voltage offset, which (if the inputstage is direct coupled) can eventuallysaturate the output. A better approach isfound in laboratory applications wheresilver electrodes coated with a layer ofchloride are used. Though these electrodesare free of noise and have no long-termvoltage drifts, the chloride surface musteventual ly be replaced so the electrodesare disposable types. However, with propercleaning, they will last for some time. Theleast troublesome approach is to use pellettype Ag/ Ag-Cl electrodes which, due totheir special construction, last indefinitely.Another more general consideration indesigning an EEG monitor is the type ofmodulation used to produce the audiofeedback. Most models use the amplified,filtered brain wave either to amplitude- orfrequency-modulate a fixed tone. In themonitor described here, a unique combination tone-threshold control can be adjustedto produce either AM, FM, or a combination of the two.

It is also necessarv to determine whataspects of the b r a i n ~ w a v e envelope shallvarv the tone. The two most common methods' use either a direct or integrated waveform to modulate the audio. With the modeselector switch, S2, in the DIRECT position,the instantaneous waveform passing throughthe filter frequency modulates an adjustabletone. This mode creates an effect in whichone seems to be tuning directly to thethought of the brain. I f the continuous toneis objectionable, the oscillator can be setjust below its threshold point so that onlythe peaks of the filtered waveform triggerthe tone. The latter method integrates thefiltered waveform over a fixed period oftime.In this monitor, depending on the settingof the threshold control (R42), the tonecan be made absent when no signal ispresent. When the threshold is exceeded,the frequency of the tone is proportional tothe envelope of the Signals. This mode isbetter for biofeedback training since the

41


3/6

Fig. 1. Brain wavesare amplified andused to drive themultivibrator thatprovides acousticoutput to the ear.

~ _ - - - ~ L + 9 V

1 - - - 4 ~ " " - - - " " ' - 9 V

)--


4/6

Fig. 2. Actual size foil pattern isat right with component layout above.tone gives a direct indication of the desiredresult.This monitor also has an audio amplifierwith speaker and volume control (R43), sothat a group can listen or the volume canbe reduced to a quiet level.How It Works. Integrated circuit lCI and

lC2 amplify the differential Signal between

R32,R37,R38-22,000.ohm, 1J4-watt, 5%resistorR34,R35-1O,OOO-ohm, %-watt, 5% resistorR40-1000-ohm trimmer potentiometer(PC type)R41-100,000-ohm miniature potentiometerR42-50,OOO-ohm miniature potentiometerR43-lO,OOO-ohm miniature potentiometerwith attached switch for 5351-4-pole, 3-position 'shorting rotary switchS2-Spdt switchS3-Spst switch on R43SPKR-Miniature 8-ohm speakerMisc.-Fonr feet of 2-conductor shieldedflexible cable, metal enclosure. battery connectors (3), knobs (4) , rubber grommet,

JANUARY 1973

the two input leads while providing unitygain for the common mode Signal. The residual common mode signal is removed bylC3 and can be nulled to zero by trimmerR40. The signal is then coupled throughCI to lC4 and further amplified. The gainof this stage can be varied from about 5 to95 by the setting of R41.Integrated circuit rcs forms a two-pole

headband. electrode cream. electrodes. ear-clip, mounting hardware. .Note-The following are available (postpaid,but insurance extra) from Extended Digital Concepts, Box 9161. Berkeley, CA 94709:#PE2, etched and drilled PC board at $4.49;#PE3. ICI and IC2 at $6.49; #PE4, set ofstainless steel electrodes at $1.49; set of disposable Ag]Ag-Cl electrodes at $3.49: #PE6,set of reusable Ag/ Ag-Cl electrodes at$14_95: #PE1. complete kit of parts including sets o] disposable and stainless steefelectrodes, drilled and etched board, drilledand painted enclosure. elastic headband,electrode cream, and earclip at .858.95. California residents. add 50/0 sales tax.43


5/6

SPKR

52 R43(53) R42 R41

active filter which rejects signals lower thanthe frequency determined by capacitorsC3 and C4 and Rll through RiB. Conversely. ICB removes signals higher thanits seiected frequency. The net effect is afilter which passes only a narrow band oflow frequencies.With DI as a shunt rectifier and C8 andR28 as a smoothing filter, the signal is

passed to Q1, a FET operating asa s?urc.efollower with unity gain. Integrated circuitIC7 is connected in a multivibrator circuitand is normally saturated with the outputvoltage near t'he positive supply voltage.When e.g charges through R.30 to a voltagehigher than the level provided by thevoltage divider made up of R31, R33, R42,and R34, IC7 saturates due to positivefeedback. Capacitor C9 then dischargesthrough D2 until IC? flips back to itsprevious state. The signal from Ql variesthe charge on e.g and thus modulates thetone.Transistor Q2 is a source follower whichprovides a low impedance to drive thespeaker without overloading the multivibrator. A separate hattery (B1) is used forthe speaker to avoid feedback.Transistor Q3 is a source follower whichcreates a low-impedance ground about halfway between the plus and minus supplyvoltages. This also permits the use of asingle-pole switch (S3) to tum the monitoron and off. It is not necessary to disconnectB1 because its drain is negligible with S3open.

Construction. The use of a PC board(foil pattern shown in Fig. 2) makes con-44

Photograph of prototype shows howcomponents were assembled in box.

struction easy. Mount the components asshown, observing the notch and dot codeof the IC's. Also make sure that the twodiodes and three transistors are properlyinstalled. The lettered terminals correspondto those on the schematic. The resistors associated with Sl are connected llirectlv tothe terminals on the switch. Use fine solderand a low-power soldering iron.The circuit board and batteries can heinstalled in any small enclosure. The threepotentiometers (R41, R42, and R43) andthe two switches (S1 and 52) should bemounted on the front panel , with a smallgrommeted hole also on the f r O l ~ t panel forthe shielded cable. The speaker IS cementedto the front panel with a few holes drilledin the panel for the sound to come through.

Prepare the electrode cable by removmgabout 12" of the outer insulat ion from thecable. Unwind the shield and twist it intocable form. Solder this shield lead to theearclip. Remove about W' of insulation fromthe two insula ted leads and carefully solderthem to the electrodes. When soldering tostainless steel, first lightly sand the metalsurface with fine sandpaper.

Testing. Install fresh batteries, turn thecircuit on, and adjust the tone/thresholdcontrol (R42) until a tone is heard in thespeaker. Set the bandpass switch (S1) to itslowest range (3.9-7.9 Hz) and the modecontrol (52) to direct. Using a small amountof electrode cream, clip the ground lead toan earlobe. Saturate the electrodes withcream, and steadily hold one electrode ineach hand. The circuit should pick up yourheartbeat , amplify it, and send it throughthe speaker. This is a noticeable beep, aboutone a second. The pulse Signal is about 1millivolt (10 times greater than alpha-wavelevel) so turn the gain control down. I f youcannot hear your pulse, check the wiring.If you have a signal generator and scope,the circuit may be further analyzed byclipping one input and the ground lead to'the signal generator ground and feeding anattenuated signal into the other input lead.

POPULAR ELECTRONICS Including Electronics World


6/6

The de output of all op amps should benear zero.Balancing the Amplifier. Potentiometer

R40 is used to trim the gain of one side ofthe differential amplifier to make both gainsexactly the same. When they are equal,common mode rejection is maximum. Thebest procedure is to feed a common modesignal of 3 to 4 volts into both inputs tiedtogether, across a 1O,000-ohm resistor. Puta scope or ac VTVM on the output of IC4and adjust R40 for the smallest signal. I fyou do not have a scope or Signal generator,hook the electrodes through the 10,000-ohmresistor to ground and touch the commonleads. You will hear 60-Hz noise from yourbody. Adjust R40 for minimum noise or theclearest tone.Use of the Monitor. First, a note ofcaution. The monitor, like most commercial machines of this type, is battery operated. This is to prevent a shock in the rareevent that the 60-Hz power line shorts to theinputs. Therefore, for complete safety, avoidhooking the monitor to any ac-operatedequipment such as scopes, battery eliminators, etc. When ac devices are hooked up

to an EEG monitor in a laboratory, lightcoupling devices or fused fail-safe systemsare used. 'If you are sure the monitor is picking upEKG and properly balanced, you are readyto try EEG feedback. Place a small bit ofelectrode cream on the earclip and attachit to either earlobe. Wrap an elastic or softcloth band around the head, aliened sothat it is over the eyebrows anl ' at thewidest part at the b;ck of the head. Pinthe cloth to hold it on. Put a small amountof cream on each electrode and place oneunder the band just above the left or riuhteyebrow. Place the other in line with thefirst at the rear of the head. Spread the hairapart and add a little more cream. Theelectrodes will function best when thevfloat above the scalp with electrode creambridging the gap. With the electrodesplaced in this manner, you should be pickmg up mostly what is called OCcipitala ~ p h a . In more advanced stages of meditation, alpha production increases in thefrontal areas of the brain. You can experiment with this by placing both leads on theforehead.Sit or lie down in a quiet, comfortableplace. Turn the monitor on, place the band-JANUARY 1973

pass switch in the alpha range (7.9-13.0Hz), with mode in DIRECT, turn the gain allthe way down, and adjust the tone andvolume to a pleasing level. Blink your eYesand listen for a beep. Slowly turn the gainup. I f the electrodes are correctly placed,no hum will be heard. Now, with the eyesopen and focused on an object, adjust thegain for a fairly steady tone. Because youare producing mostly beta and the bandpass is on alpha, you should not hear thebeta frequencies. Now close the eyes andlisten for a rhythmic modulation of the tone.Do not try to produce this rhythm; let themind go and just listen for it. The occasionalfluttering of the tone will be the alphawaves.Notice the types of thoughts that blockthe alpha. After you are sure you are producing alpha, switch 52 to INTEGRATE andadjust the threshold/tone control so that,when the eyes are open, there is no tone.Shut the eyes and practice increasing thenumber of times the tone is on (percenttime training). Later try increasing the frequency of the tone (amplitude training).In laboratory training, a usual alpha session lasts 10 to 15 minutes a day for abouttwo weeks. If you stick to it, you may eventually notice a feeling of well-being andrelaxation after each session. To experimentwith the other brain-wave bands, simplyrepeat the procedure with the filter switchedto the desired band. Try lowering the dominant alpha frequency toward theta in thedirect mode and notice if spontaneousthoughts or ideas come more easily.When you have finished using the monitor, carefully wipe the cream off the electrodes. If you are using stainless steelelectrodes, sand them lightly and clean themwith alcohol.One final note: alpha-wave feedback hasproduced results similar to meditation. butit works much faster. It is still, however, asubtle effect and requires diligence andexperimentation to obtain worthwhile results.

Editor's Note: This art ic le , which follows last month's story on principlesof biofeedback training. descr ibes aneasily constructed project fo r experimentation. There have been manyclaims made fo r brain-wave monitors- some highly exaggerated. We makeno such claims, other than that thecircuit operates properly.

45

build a brain wave feedback monitor

Documents