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