Resounding Truth an Acoustic Lie Detector （2）

Published:2011/7/29 1:16:00 Author:Amy From:SeekIC

Bukhard Kainka

Construction

For this project too, we have designed a printed circuit board. However, this time not quite as you have come to expect from us. Figure 2 shows a PCB layout for our Universal Prototyping Board size-1 (UPBS-1) which can be purchased ready-made through the Elektor Electronics Readers Services. A normal piece of prototyping board, stripboard or Veroboard can be used as well, of course.



We can be brief regarding the actual construction, the only thing that may need some clarification is how to measure the skin resistance. Very sim­ple: attach two wires, one to the posi­tive power supply and one to Rl. The stripped ends are then loosely wrapped around two fingers of one hand. That is all.

Component values

In the inset you can read how the com­ponent values in the circuit are calculated. If you are only interested in the result, then only the last equation is of importance. Normally, the resistance of the skin will be between 10 and 500 kfl. With the component values as shown, this will produce a tone from buzzer Bzl with a frequency between 500 to 3500 Hz. The frequency is mainly dependent on the dampness of the skin: the more sweat, the better the conduction and therefore the lower the resistance. When the resistance reduces, that is, the test subject becomes nervous, the pitch increases. Note that the pitch is not an absolute measure of ’the truth”. One person has a naturally better conducting skin than another. That makes no difference here, because it is the change that matters. Human hearing is actually quite sensitive to such changes in pitch. An additional feature is that the test subject can hear the tone as well. This often leads from bad to worse. Despite the best intentions not to break under duress, this psychological feedback is usually the last straw that breaks the camel’s back.

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