The Electronic Lie Detector

Desperate times call for desperate measures. I direct all my 7th grade students to assemble and solder together an electronic lie detector...and submit to it! - SciencetoyMaker

Group Project

This is a good electronics project for groups: technology classes, camps, etc. If you just want to make one or a few electronic lie detectors, here is a source for a very similar kit. Electronic components like transistors, resistors and capacitors are expensive when you just buy one or a couple. But when you buy supplies for a few dozen people, the price drops to about a couple of dollars in materials for each kid's lie detector. That's why I say it is a group project. Because I developed the project for classes of young middle school students, it has an alternative to the typical printed circuit board. Etching a printed circuit board uses a messy, corrosive chemical like ferric chloride. Instead, this design uses stripped wire for the copper paths.

Does it or doesn't it?

A "lie detector" test--more correctly called a polygraph test-- is really a whole battery of tests. It measures physical changes in our bodies when we undergo psychological stress. Measurements include pulse (how fast your heart beats), blood pressure, respiration (how fast you breath) and how much your hands perspire. I think they say something about "galvanic skin resistance", but it comes down to sweat. The more you sweat, the better your skin conducts electricity. That is what this project does: it measures how good you, or anything, conducts electricity. Here is a good video about polygraph tests and the controversy surrounding them.

Fortunately for the person being tested, the project does not send shocking jolts of electricity to measure this. However, it does send a tiny bit of electricity through their fingers--so small they can't feel a thing. The trace that gets through is amplified and the output measured. And that is exactly what this simple "electronic lie detector" does. In our version the output is measured as a sound--the higher the pitch, the better the conductivity. On a real polygraph the conductivity is graphed.

Note: lots of students e-mail me about using this "lie detector" as the basis for a science fair project or science class assignment. This project will work very well as a macro-demonstration: People touch the wires with dry hands and they get a certain sound. Then they wet their fingers and touch again. The pitch is dramatically higher! But then they want to conduct real sessions to determine if people are lying. I understand that people want to apply knowledge, but you cannot get the accuracy of polygraph units that cost tens of thousands of dollars with a project that costs a couple of dollars. Other sites that make it seem like you can actually use these simple electronic projects to conduct sophisticated, sensitive tests are being dishonest. Furthermore, to get someone to sweat through their hands, you would probably have to ask very embarrassing questions. The subject matter and invasion of privacy would not be appropriate for a school setting. So what can you do? Contact a polygraph practitioner. Be polite, be clear that you are embarking on an educational project and ask for their help. If you can find a polygraph practitioner who cares about education, perhaps they could demonstrate the operation to a class or on your video camera. Start with your local police department and see if they can tell you where to begin your search.