According to some sources, when a person is telling a lie, he can’t look at your eyes directly; changes or diverting the topic; and behaves with some unexpected physical gestures and reactions. Another interesting thing about the person who is lying is his body usually perspires rapidly (normal when frightened or over excited). When body perspires, your body or skin resistance drops.
The circuit illustrated below is a simple lie detector that measures changes in body resistance of the person who is under lie detection test. When the person is lying, the meter needle will move. To use, attach the pads 1 to 2 inches apart in the palm. Adjust the potentiometer R3 until the reading of the meter M is zero.
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| lie detector circuit |
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| 2N2222 pins |
R1 - 100 kilo ohms 1/4W
R2 - 120 ohms 1/4W
R3 - 2 kilo ohms potentiometer
R4 - 1 kilo ohms 1/4
R5, R6 - 4.7 kilo ohms 1/4W
Q1 - 2N2222 or similar NPN transistor
M - Analog or digital ammeter (10mA)
Bat - 9V battery
Pads - any conducting pads
Note: The circuit is for educational purposes only. Though the circuit is working, it does not guaranty its accuracy. Keep in mind also that there are many factors that can affect to the sudden drop or change of body resistance.
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