Lie Detector
Électronique / Electronic

This project is a good one to use at a party to play "truth or consequences". It is a sensitive resistance-change detector. You have heard about (and probably seen on TV) lie detectors being used to help determine if a person is telling the truth. Technically such an instrument is called a "psycho galvanometer".

They are used in criminology, security work and for psychological tests. A "lie detector" detects very small changes in resistance of the skin. Normally skin resistance is affected by emotional reactions, so that when a person lies his skin resistance drops slightly (small amount of body sweat reduces normal skin resistance) and the "lie detector" senses this.

The procedure is as follows:

1. Tape two bare wires or metallic conductors to a person's skin. The back of the hand is a convenient location.

2. Ask the person questions that require an honest answer and adjust the 50K Control for a reading of about 5 on the Meter.

3. Now play the game.

Ask each other questions to which you might give false answers. If you are able to get him "emotionally involved", the Meter reading will increase when the skin resistance decreases. This may be caused by his lying. Commercial lie detectors also include measurements of breathing rate and heart pulse rate and are still not considered 100% accurate, so don't expect too much from just a skin resistance test.

The circuit is a two-stage DC amplifier. The current that is amplified is the current that flows over the surface of the person's skin from the 9V Battery.

The Control functions by shunting (bypassing) excess current around the input to the amplifier.

The 1K prevents Transistor leakages from having an adverse effect on circuit operation.

The 470 ohm Resistor also helps to stabilize the circuit from changes due to Transistor leakages, temperature changes, etc.

The 10K provides protection against burn-out of the Meter due to excessive current.