Simple Lie Detector Circuit

Have fun building this lie detector circuit!

While it wont be perfect real lie detectors are much more complex, it can be a fun way to learn about how our bodies react to stress.

This circuit uses changes in sweat to see if someone telling the truth people sweat more when they are stressed or nervous.

Disclaimer: This is for entertainment purposes only and should not be used for real lie detection.

What is a Lie Detector Circuit:

An electronic device called a lie detector circuit, sometimes referred to as a polygraph or truth verification system, is made to monitor and record physiological indications that are thought to be connected to lying or deception.

The basic premise is that some physiological reactions happen unintentionally when someone is lying or under stress.

In order to determine whether someone is being truthful or dishonest, the lie detector looks for and measures these reactions.

See below for the circuits working and the building process:

Circuit Working:

Parts List:

When you press your fingers onto the blue rings and switch on the power, the circuit springs to life.

In a normal non stressful state, the speaker produces a low frequency sound indicating that your fingers are relatively dry.

However, heightened stress or fear triggers the release of tiny amounts of fluid from your fingers rich in minerals and salts.

The lie detector circuit swiftly detects this change causing the frequency tone from the speaker to become shriller and sharper.

The altered tone signifies a significant amount of stress hinting that the person may be concealing facts or providing false answers to posed questions.

This intriguing mechanism makes the lie detector circuit a valuable tool for gauging emotional responses and uncovering hidden truths.

Formula:

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

f = 1 / 2π√LC

where,

• f: This represents the resonant frequency of the circuit, measured in hertz Hz.
• At this frequency, the inductive and capacitive reactances of the circuit cancel each other out, resulting in a maximum transfer of energy between the inductor and capacitor.
• L: This denotes the inductance of the circuit, measured in Henrys H.
• Inductors oppose changes in current by inducing an electromotive force EMF.
• C: This represents the capacitance of the circuit, measured in Farads F.
• Capacitors store electrical energy in an electrostatic field.
• π (pi): This is a mathematical constant with a value of approximately 3.14159.
• √: This symbol represents the square root.

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

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

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

Note:

For perfect LC circuits with no resistance, this formula is applicable.

There will always be resistance in real world circuits and this resistance can introduce damping effects and change the resonant frequency.

Resonant frequency is a notion with applications in many fields of electronics, including oscillators, filters and radio circuit tuning.

How to Build:

Prepare the Transistors:

• Begin by identifying and placing the NPN transistor 2N3904 and the PNP transistor 2N3906.
• Ensure proper orientation and connections for seamless circuit functionality.

Connect the Blue Rings:

• Attach the blue rings to the designated points on the circuit.
• These rings will serve as the contact points for your fingers facilitating the detection of physiological changes.

Power Supply Connection:

• Integrate the 9V battery into the circuit to power the lie detector.
• Establish the necessary connections, ensuring a stable power supply for optimal performance.

Construct the Feedback Oscillator:

• The core of the lie detector circuit is a two transistor feedback oscillator.
• Use the above formulas to determine the appropriate values for resistors and capacitors, ensuring the desired frequency response.
• Implement above formulas to configure the feedback oscillator creating a system capable of detecting subtle changes in physiological conditions.

Audio Output Configuration:

• Connect the 8 ohm speaker to the circuit to receive the audio output.
• The speaker will emit different tones based on the frequency generated by the lie detector, providing audible cues for interpretation.

Conclusion:

It is important to note that while lie detectors are used in various settings such as law enforcement or employment screenings their accuracy and reliability are subjects of ongoing debate and controversy.

Many factors, including individual differences and the ability of individuals to control their physiological responses can impact the effectiveness of lie detector tests.

Additionally, the use of lie detectors is regulated, and their results are generally not admissible as evidence in many legal jurisdictions.

References

Lie detection