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# Lie detector (TL072)

2017-02-01 14:02
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This article describes the Lie detector (TL072). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: TL072.

The circuit diagram of the Lie Detector is shown above. It
consists of three transistors (TR1 to TR3), a capacitor (C1),
tw1o lights or LEDs (L1 & L2), five resistors (R1 to R5), and
a variable resistor (VR1).

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

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.

Capacitor C1 functions as a smoothing capacitor and removes
the 50Hz induced mains hum that is found on a person`s body.

TR1 and R3 form a buffer circuit (called an emitter-follower).
The voltage at the emitter of TR1 follows the voltage at the
probe wire and is now able to drive transistor TR2.

Transistors TR1 and TR2 act as a voltage comparator. If the
voltage at the base of TR2 is higher than at the base of TR3 then
the green LED (L1) will come on. If the reverse is true then the
red LED (L2) will light.

To test the Lie Detector hold the probe wires. Adjust VR1
until the green LED is just on and the red LED is just off. This
is the point at which the voltage at the base of TR2 is just
greater than at the base of TR3. Now use moist fingers to hold
the probes. This lowers the skin resistance and causes the
voltage at the base of TR2 to fall. The voltage at the base of
TR3 is now greater and the red LED comes on.

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