The circuit diagram of the Lie Detector
is shown above. It consists of three transistors (TR1 to
TR3), a capacitor (C1), two 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 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.
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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