Lie detector circuit

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.

Lie detector 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)
R4______________470 ohms
LED1,LED2_______LED(Light Emitting Diode) as you like
VR1_____________100K Trimmer Potentiometer

I builds this circuit with breadboard :


This Post Has 19 Comments

  1. I need Funclock Diagram For this lie detector circuit.

  2. Sorry Functional block diagram

  3. if these circuit will work or not

  4. if these circuit will work or not. replay

  5. Hi, @hhh
    It was my pleasure.

  6. its a wrong circuit…. Q3 collector, led2 and R5 should not be connected and there will be an overlap…. my circuit led spoiled coz of the above lie detector…

  7. Hi, all
    Now, This circuit worked.
    Please see in video.


  8. can i make this circuit with atmega8 microcontroller.please answer as soon as possible.

  9. is it possible???? the ckt work properly???

  10. how it sensed??? in ciruit how probe can be made ??

  11. it is not work friends

  12. Its not working, any thing must change in circuit tell me fast

  13. Hi,All

    Thank for your feedback.
    This circuit works.
    Please try again.
    The wire of R5 not connected to a collector lead of Q3.

  14. This circuit works,but the sensitivity is low.instade, I thing we can use comparator circuit(opamp)and increase the differential comparisin.

  15. what u have used as a probe in this video…plez reply as soon as posssible!!!!!

  16. which material have u used to connect it with probe!!!!…m not getting it by watching the video…plez tell me fast……..

    1. i will surely help you.
      my contact number is mentioned below
      u can call me
      number – 9011157145

  17. This Circuit WORKS,,Please Check Your Wiring!!!!

    1. Hi MR OHM 1970
      Thanks for your feedback. I am happy because your friendship.

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