Makezine recently posted a diy lie detector kit that uses your skin resistance (aka sweat) to detect a lie. The kit is well explained and consists of two probes you place on your palm with a red/green led readout.
Multimeter based feedback
This is all well and good, but sometimes a little more feedback is a bit more intuitive. We are trying to learn here.
Some time ago, Jason Bradbury created his own lie detector using only a resistor, a transistor, an led, and a multimeter. The led lights up if you are lying, but the multimeter provides more precise feedback of the skin’s resistance.
If you are really desperate, you can just clip the leads of your multimeter straight to your subjects palm. The ‘scary’ device may cause them to sweat and ruin the experiment, though. Just don’t tell your girlfriend you will shock her if she does not love you.
Finger straps and the LEGO RCX
The galvanic skin response sensor uses a LEGO RCX brick to detect lies and also has numeric feedback. The finger straps are the best part of this project. Your subject will know you mean business when you clip these puppies to your multimeter.