Simple Lie Detector

The circuit uses a two-transistor direct-coupled oscillator that has a frequency determined by C1, R2, and the (skin) resistance across the touch pads. Since C1 and R2 are fixed values, only the skin resistance across the touch pads can vary the sound of the oscillator. To sustain oscillations, C1 feeds a portion of the output from Q2 back to the input of Q1 through resistor R1. Transistor Q1 is an npn type and transistor Q2 is pnp type. The output of Q2 is fed into small speaker. The circuit relies on the fact that the human skin conducts electricity.

C1    0.01-uF capacitor
Q1    2N3904 Transistor
Q2    2N3906 Transistor
R1    4.7 kohm Resistor
R2    82 kohm Resistor