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Simple Lie Detector Circuit using Transistors

Enjoy making this circuit for detecting lies!

Real lie detectors are far more complicated so it will not be perfect but it is an an entertaining method to see how our bodies respond to stress.

This circuit uses differences in sweating to figure out whether someone is speaking the truth.

People who are worried or nervous sweat more.

Warning: This should not be used for actual lie detection it is only for entertainment.

What is a Lie Detector Circuit:

A lie detector circuit also known as a polygraph or truth verification system is an electrical device designed to monitor and record bodily signs that are believed to be connected with dishonesty or lying.

The basic idea is that when someone is lying or is under stress than certain changes in their bodies occur accidentally.

These emotions are what the lie detector looks for and examines to figure out whether someone is telling the truth or lying.

The construction method and how the circuits works are shown below:

Circuit Working:

Parts List:

CategoryDescriptionQuantity
Resistors4.7k CFR, 82k CFR1 each
CapacitorCeramic 0.01µF1
SemiconductorsTransistors 2N3904, 2N39061 each
Speaker1
ON/OFF Switch1
Battery 9V1
Blue Rings2

The circuit activates when one places his fingertips on the blue rings and turn on the power supply.

A low frequency sound produced by the speaker indicates that fingertips are reasonably dry in a normal non stressful condition.

On the other hand when one is stressed or afraid it release little volumes of mineral and salt rich fluid from its fingertips.

This shift is quickly detected by the lie detector circuit which makes the speakers frequency tone sharper and clearer.

The tone swing indicates a great deal of tension which raises the possibility that the speaker is lying or giving misleading answers to queries.

The lie detector circuit is a useful instrument for measuring emotional reactions and revealing hidden facts because of this interesting technique.

Formula:

The formula mentioned below describes the resonant frequency of an LC circuit.

f = 1 / 2π√LC

where,

The resonant frequency is inversely proportional to the square root of the product of capacitance C and inductance L according to the formula.

Therefore a lower resonant frequency will be the result of a bigger capacitance or inductance.

Thus a larger capacitance or inductance will result in a lower resonant frequency.

Note:

This formula works for suitable LC circuits that have no resistance.

Actual circuits will always include resistance which can modify the resonance frequency and cause damping effects.

Several fields of electronics such as oscillators, filters and radio circuit tuning use the idea of resonant frequency.

How to Build:

To build a Simple Lie Detector Circuit follow the below mentioned connections steps:

Get the transistors ready:

Attach the Blue Rings:

Connection to the Power Supply:

Put the Feedback Oscillator together:

Configuring the Audio Output:

Conclusion:

It is essential to remember that although lie detectors are used in many situations including law enforcement and job screenings but there is constant discussion and disagreement over their accuracy and dependability.

Personal differences and their ability to control how they feel are only two of the various factors that can affect how well lie detector tests work.

Also lie detectors are governed by laws and in many legal countries the findings of these tests are rarely accepted as evidence.

References:

Lie detection

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