Best Tutorial on Lie Detector.

In this tutorial, we are going to learn about Lie Detector.

Lie Detector or Love-o- Meter-

This is assessment of a verbal statement that the is the goal to reveal a possible intentional deceit. It may refer to a cognitive process of detecting deception through evaluating message content as well as non-verbal cues; it also may refer to questioning techniques used along with technology which record physiological functions to ascertain truth and falsehood in response. 

Application of Lie Detector –

  • A further application of the circuit, as per below image for the Lie detector which is working showing how we can use in daily basis for our use in application.
  • In this application, a test person holds the two blank wire ends between thumb and index finger. Then the test person has to answer different questions. If the test person does not tell the truth, the LED will light up.
  • Similarly, the circuit can be used as Love-o- Meter. For this purpose a test boy and girl have to sit opposite to each other. Then they have to look deeply into one another’s eyes. If the LED light up, they surely have fallen in love.
  • In both cases the circuit relies on the effect that people tend to start sweating it they lie or fall in love. The additional moisture on the skin surface leads to an increased conductivity and analog to the last two experiments, leads finally to an increasingly bright LED.
  • OF course the results of this simple detector should not be taken too seriously. Even highly sophisticated lie detectors that are registering a whole lot of other parameters like blood pressure.

Use of Lie Detector –

 It is used for the purposes of getting accurate information from an unwilling subject Information obtained through publicly disclosed truth drugs which has been shown to be highly unreliable, with subjects apparently freely mixing fact and fantasy. Mostly it is the claimed effect relies on the belief of the subjects that they cannot tell a lie while under the influence of the drug and also it is use for heart beat rates additionally the skin surface moisture do not lead to reliable results. Therefore these devices are not accepted in court in many countries.

Working of Lie Detector –

As per below circuit for the lie detector which is very simple, It has a simple two transistor oscillator, which oscillation frequency depends on the values of capacitor C1, R2 resistance, as per below image of the circuit, the skin resistance between the two sensor terminals. In this circuit the values of capacitors C1 and R2 are fixed, but the skin resistance can vary and it is what will cause a variation in the sound coming from the speaker. (When a lie is detected).For this circuit to oscillate part of the output signal is taken from the Q2 transistor collector and it is feed back to the input at the Q1 transistor base through capacitor C1.

Parts Required For Lie Detector-

  • Q1 (2N3904) It is NPN transistor.
  • Q2 (2N3906) It is PNP transistor.
  • R1, 4.7K resistor
  • R2,  82K resistor
  • C1,0.01uF capacitor
  • SPK, 8 ohms speaker
  • Bat,1  9 volt battery

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