The Lie Detector

Project Number: 123

OBJECTIVE: To show how sweat makes a better conductor.

Description: Turn on the slide switch (S1) and place your finger across points A & B. The speaker (SP) will output a tone and the LED (D2) will flash at the same frequency. Your fingers act as a conductor connecting points A & B. When a person is lying, one thing the body starts to do is sweat. The sweat makes the finger a better conductor by reducing its resistance. As the resistance drops, the frequency of the tone increases. Lightly wet your fingers and place it across the two points again. Both the output tone and LED flashing increase, and the lamp (L2) may light. If your finger is wet enough, then the lamp will be bright and the sound stops - indicating you are a big liar (person who lays down a lot)! Now change the brightness of your finger and see how it affects the circuit. This is the same principle in lie detectors that are sold commercially.