Here's a simple lie detector that can be built in a few minutes, but can be incredibly useful when you want to know if someone is really telling you the truth. It is not as sophisticated as the ones the professionals use, but it works. It works by measuring skin resistance, which goes down when you lie.
33K 1/4W Resistor
1.5K 1/4W Resistor
1uF 16V Electrolytic Capacitor
2N3565 NPN Transistor
0-1 mA Analog Meter
Case, Wire, Electrodes (See Nots)
1. The electrodes can be alligator clips (although they can be painful), electrode pads (like the type they use in the hospital), or just wires and tape.
2. To use the circuit, attach the electrodes to the back of the subjects hand, about 1 inch apart. Then, adjust the meter for a reading of 0. Ask the questions. You know the subject is lying when the meter changes.
Source: Electronics Lab