This is a false capture circuit or Lie detector circuit. The basic principle of the resistance of human skin. While dry skin is a resistance of about 1 Mega ohm but if the skin moisture resistance is reduced. When the body are excited or scared or not sure the skin is moist than usual.
Operation of the circuit is a circuit of R1 and R2 divide the line voltage and probe used to measure the resistance of the skin, which is parallel with R2.So voltage drop across R2 is based on the resistance of the skin with is if the skin dry (people do not lie), the resistance of about 1 mega ohm to a strong drop across about 4.5 volt, but if the skin moist (liar), the resistance is less than 1 mega ohm to a voltage drop across less than 4.5 volts.It uses the pressure is on bias voltage to flow in Q1 – BC548, where Q1 and R3 buffer circuit to circuit.
Voltage drop across R3 will be pressure to bias Q2 to conduct.There will be Q2 and Q3 (BC548) compared to the circuit voltage. That if the voltage at pin B of Q3 (voltage across VR1) is higher than the voltage at pin B of Q1 (voltage drop across R3), the pressure from the line probe to measure the resistance of the skin will allow. LED2 lit. show that the truth. In contrast, if a lie is the LED1 is illuminated.
Parts will you need
Q1,Q2,Q3________BC548,2N2222,2SC1815________NPN Transistors______________ = 3 pcs
Resistors (All 0.25 watt,5% metal/carbon film,Unless stated otherwise)
LED1,LED2_______LED(Light Emitting Diode) as you like
VR1_____________100K Trimmer Potentiometer
I builds this circuit with breadboard :
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