Here is a simple little circuit that I found interesting. It is basically a comparator circuit. It measures changes in resistance. Supposedly if a person is lying then they start to sweat. If a person sweats then the resitance through their skin goes way down. This can be measured by this circuit, and will turn on a “False” Led. If you are not lying then it will turn on a “true” LED.
This circuit is based on the fact that a person’s skin resistance changes when they sweat (sweating because they’re lying). Dry skin has a resistance of about 1 million ohms, whereas the resistance of moist skin is reduced by a factor of ten or more.
Resistors R1 and R2 form a voltage divider. They have resistances of 1 000 000 ohms (1 mega ohms) and, because their values are equal, the voltage at the upper probe wire is half the battery voltage (about 4.5 volts).
A person holding the probe wires will change the voltage at the upper probe wire depending on their skin resistance. The skin resistance is in parallel with R2 and, because it is likely to be similar to or smaller than R2, the voltage at the probe wire will fall as skin resistance falls.
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