LIE DETECTOR PROJECT REPORT

                                                               CHAPTER – 1

INTRODUCTION

1.1  SUMMARY

Lie detection, also referred to as deception detection, uses questioning techniques along with technology that records physiological functions to ascertain truth and falsehood in response. It is commonly used by law enforcement and has historically been an inexact science. There are a wide variety of technologies available for this purpose. The most common and long used measure is the polygraph, which is considered by the U.S. National Academy of Sciences to be unreliable.

For our final project for experimental engineering we will design, calibrate and implement a polygraph test. We will sample data from two sensors collecting two distinct types of data.

The first sensor is a galvanic skin response sensor, which will measure the perspiration rate of the subject’s hand.

The second sensor, which is an electrocardiogram, measures movement of the person who is being tested. After the detector is assembled, we will initially gather data from a group of volunteers. Their movement and perspiration will be recorded while they are asked a series of test questions.

Afterwards they will be asked which questions they answered false. Based on these we will calibrate the data and determine how much of a change indicates a lie. From these determined values, a LABVIEW program will be assembled that will acquire future readings from subjects and compare these new values to the calibrated data.

If the new readings are above our pre-determined level, the LABVIEW program will activate a light indicating that the subject is most likely telling a lie.

1.2 OVERVIEW

The detector circuit which we are going to do is for detecting a Lie. The lie detector circuit diagram consists of three transistors, a capacitor, two lights or LEDs, five resistors, and a variable resistor. The Lie Detector circuit is based on the fact that a person’s skin resistance changes when they sweat (sweating because they’re lying).

Dry skin has a resistance of about 1 million ohms, whereas the resistance of moist skin is reduced by a factor of ten or more.

We have also added an alarm circuit which will indicate when the person is telling lies so that it will help us as an alternate when there occurs a problem with the LED.

This lie detector is a simple one and detects lie when the person sweats based on the fear when he lies so that the resistance level goes down when compared to the dry skin. We can also call this as a sweat detector.

The Lie Detector has a number of other uses, detailed below, and it could perhaps more accurately be described as an 'Experiment Machine'.

 National Academy of Sciences issued a report named “The Polygraph and Lie Detection”. In this report, the accuracy of polygraph was reexamined and proved to be “short of perfection”. Since then, many countries started to ban polygraph testimony.

The reason polygraph testimony is banned by some countries is that lie detection lacks of credibility, because people are able change the test results consciously. In order to find out the accuracy of lie detectors and prove that people can change the polygraph results consciously, we are trying to make a lie detector. In this project, we will examine polygraph in 3 different aspects including blood pressure, heart beat and skin conductivity.

We will buy wrist-type pressure transducer to get input analog signals from human body and use amplifiers, A/D converters and high-pass filters to filter out the noise (about 60Hz). Then we will use D/A converters to get the analog signal so that we can examine the polygraph change. 

                                                                 CHAPTER – 3

PROJECT DESCRIPTION

3.1 OVERVIEW

This electronic lie detector circuit will give two readings: one for difficult questions for the subject and another to show its emotion astatine general. The emotional states are detected not only by heart beat acceleration and trembling hands but also an increase in skin humidity whose resistance decreases causing the entry into operation of the lie detector.

Two electrodes can be used as a flexible wire, bare, wrapped around fingers or wrist. In order to not influence the measurement result the device must be powered from two 9 Volts batteries.

Each change in resistance, and therefore the voltage at the input circuit will be amplified by operational amplifier A1, which also serves as separator. The output signal will determine, by R3, a deviation of the measuring instrument.

General emotional state of a person can be assessed by measuring the average resistance of the skin over a period of time. The indication is provided by an indicator instrument connected to point B of the circuit. Operational amplifier A2 is connected as an integrator and allows the circuit to automatically adjust according to the average resistance of the skin.

Length of time to measure the skin resistance is determined by R5, C2 and C3. Until such time elapses, the lie detector gives no indication although diodes D1 and D3 provide a rapid response of the circuit.

                                                CHAPTER – 7

DESCRIPTION

7.1 Voltage Divider

 A voltage divider (also known as a potential divider) is a simple linear circuit that produces an output voltage that is a fraction of its input voltage.

 A voltage divider referenced to ground can be created by connecting two electrical impedances in series.

7.2 Low pass filter

A low-pass filter is a filter that passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cut off frequency.

Capacitor can act as LPF and removes the 50Hz induced mains hum that is found on a person's body.

7.3 Buffer Amplifier

A voltage buffer amplifier is used to transfer a voltage from a first circuit, having a high output impedance level, to a second circuit with a low input impedance level.

Unity gain voltage buffer is used.

7.4 Voltage Comparator

A comparator is a device that compares two voltages or currents and switches its output to indicate which is larger.

Two transistors act as a voltage comparator. Depending upon voltage at the base of two transistors, the LEDs will come on.

7.5 Power Supply

A 9v dc power supply is given using battery.

7.6 LED

Depending upon the output from the voltage comparator either green or red LED will glow.

7.7 Alarm

The alarm is connected parallel to red LED to produce alarm when the red LED glows to indicate the person is lying

CHAPTER – 8

USE OF LIE DETECTOR

8.1 USE OF THIS PROJECT

The Lie Detector needs tuning before it can be used, and it needs tuning for every person that uses it as everyone has slightly different skin.

Touch the two probe wires against the palm of your (dry) hand; such that the metal ends are a couple of centimeters apart (the metal ends must not touch each other). Adjust the tuning control (VR1) until the red light (FALSE) just goes out. The Lie Detector is now tuned for your skin. If you lick your palm and touch the wires against it again, the red light should come on brightly.

You should now understand how to use the Lie Detector to detect a real lie. Touch the two probe wires against the palm of the subject's hand and adjust the tuning control as before until the red light just goes out. When the subject tells a lie, and begins to sweat, the red light will get brighter.

It must be emphasized that the Lie Detector won't detect every lie, as it is really only a sweat detector. It only detects lies that have consequences to being told, lies that cause the subject to sweat (with fear). Pretend or 'joke' lies won't have any effect.

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