Here is a little of the introduction on the page:
"A polygraph (often and incorrectly called a 'lie detector') is a machine which plots in real time several human biological signals such as pulse rate, galvanic skin resistance (GSR), blood pressure, and breathing rate. This machine, in conjunction with a certified examiner, is then used to analyze a subject's stress during interrogation with the intent of distinguishing truth from lying. These machines can cost anywhere from hundreds to thousands of dollars. We attempted to build one for $50.
"For our design project we constructed a polygraph which measured pulse rate, GSR, and breathing rate, and performed a discrete cosine transform (DCT) on the subject's voice in the hope of measuring a deviation in the fundamental frequency (another indicator of stress). The measurements were sampled, analyzed, and transmitted to a computer for further analysis using an ATMEL AVR Mega32 microcontroller."
Absorb it all!