Lie Detector Circuit Explanation -with video

 

This circuit is a basic two transistor direct-coupled oscillator with a frequency determined by the capacitor and the resistance of the skin.
According to the theory; the lie detector, also called "polygraph"¹, can detect changes in the resistance of the human body. Is supposed that when someone lies, the resistance of the body is lower because the sweat and other factors. So the change in the frequency of the sound will indicate it.