Resounding Truth an Acoustic Lie Detector （1）

Published:2011/7/28 22:49:00 Author:Amy From:SeekIC

By Bukhard Kainka

There are those cool-headed types, who can lie through their teeth with a straight face. But in the end it is just a matter of asking the right questions. Sooner or later everyone will get sweaty palms.

Lie detectors are always a great deal of fun at parties. Who is prepared to face up to the challenge from technol­ogy? And also: who is able to defeat the machine! The latter is also very dependent on the actual situation — the questioner must not make it too easy for the test subject! When the ten­sion increases, even the best liar can’t avoid a subtle but inevitable physio­logical reaction: sweaty palms.

The operating principle of this lie detector makes use of this phenome­non. When the hands of the person in the hot seat become damp, the elec­trical resistance of the skin will reduce. This change is relatively easy to detect. In that respect we certainly don’t want to claim that the circuit presented here is a feat of brilliant innovation. However, it is the manner in which this device signals the result that sets it apart. That is, this circuit will emit a tone, the frequency of which depends in the resistance of the skin.

Five-five-five

The design of the circuit shown in Figure 1 is actually a relatively typi­cal application for the well-known 555. This timer-IC has been wired here as an astable multivibrator, that means that at the output there is a signal, the frequency and duty-cycle of which are determined by two resis­tors and one capacitor. In this case there are actually three resistors and one capacitor. The resistance of the skin is connected in series with Rl. In conjunction with R2 and CI, this series connection determines the duration that the output (pin 3 of IC1) is high. When the power supply is switched on, the capacitor is charged, through these resistors, to 2/3 of the power supply voltage. An internal comparator compares, via pin 6, the voltage across the capacitor with this threshold value. When this threshold is exceeded, the output of the IC will change state. This means that the inverting output of the internal flip-flop (Q in Figure 1) will become high. This causes an internal transistor to conduct which results in the dis­charge of the capacitor via pin 7 and R2. Note that the length of time that the output remains low is not depend­ent on Rl or the skin resistance.



Subsequently, the second internal comparator starts to play a part. Again, the voltage across the capacitor is compared with a threshold value {pin 2). When the voltage is V3 of the power supply voltage, the internal flip-flop is set, the output changes state again and the whole process begins anew.

Two pins of IC1 are not used in this configuration. Pin 4, the inverted reset input, can be used to interrupt the charging process of the capacitor pre­maturely. This can be very useful in other circuits but is not necessary in this design. Here, the reset input has been connected to Vcc, so that the internal comparator alone determines when to reset the flip-flop. That leaves the control input (pin 5). Via this input the upper threshold value can be changed. That function is not used here and the input is left open.

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