Using an Oscilloscope as a Lie Detector

Originally printed in Understanding and Using Your Oscilloscope, by Allied Radio.

Polygraphs (lie detectors) are complex devices that measure a number of physiological characteristics of a person who is being questioned.  In this introductory discussion, we can describe only one basic characteristic that is measured.  Most polygraphs are actually oscillographs that make a permanent record of the waveforms.  However, an oscilloscope can be used when a permanent record is not required.  Lie detectors are not infallible, by any means, and many persons are able to beat the test by tensing their muscles and various other dodges.

One of the most basic polygraph tests is a waveform that shows the variation of skin resistance versus time.  We know that when a person experiences fear or other stresses, his perspiration glands increase their activity and, in turn, his skin resistance becomes lower.  Normally, a person may have a resistance from one hand to the other hand of 10,000 ohms.  Under stress, this resistance value will decrease considerably.  An arrangement for displaying variations in skin resistance is shown in the below figure.  One electrode is taped to the palm of each hand; the electrodes are taped in place so that a constant contact pressure is maintained.

A 6.3 Volt, 60-Hertz source is suitable for energizing the circuit.  The scope displays a 60-Hertz sine waveform.  Variations in skin resistance cause more or less current in the circuit.  In turn, the height of the displayed waveform varies up and down.  In operation, the person under test is asked various questions which are known to be true.  Thus, a reference waveform amplitude is determined.  Next, an occasional question is inserted which is known to be false.  In turn, the variation in waveform amplitude can be determined.  As the interrogation proceeds, questions are asked to attempt to determine whether the answer is true or false.

Thus, John Doe might be asked whether his name is John Doe.  Later, he might be asked whether his name is Richard Roe.  After a number of such questions, the operator might ask whether he stole the money that is missing from the First National Bank.  Results obtained from lie-detector tests vary greatly from one person to another.  Hence, most courts of law do not regard the results of such tests as convincing evidence.  However, the results may be taken into consideration with other evidence that is presented.

Block Diagram



Be sure the incoming AC power is passed through a filter to remove any interference or voltage spikes.

How Lie Detectors Work

The Lie Behind the Lie Detector  (1MB PDF)

Simple Lie Detector - 1

Simple Lie Detector - 2

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