If you want to test someone telling you the truth or not, you can use this simple lie detector. It’s easy to build. You need transistor, electrode, resistor, capacitor and analog ampere meter. This lie tester works by measuring skin resistance which goes down when you lie. For electrodes, you can use alligator clips, electrode pads, or just wires and tape.

To use the circuit, first, attach the electrodes to the back of the subjects hand, about 1 inch apart. Secondly, adjust the meter for a reading of 0. Ask simple questions that you know he lying or not for calibration. After that ask your main question. You know the subject is lying when the meter changes.

source : http://www.hackcanada.com/ice3/wetware/lie_detector_circuit_2.html