lie detector

[seminar class]

PRESENTED BY:
HEMALATHA.T
SUDHA.S
SUSHMITHA.K

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[attachment=10521]
ABSTRACT
 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'.
Block Diagram
DESCRIPTION
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
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.
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.
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 transistor, the LEDs will come on.
POWER SUPPLY :
A 9v dc power supply is given using battery.
LED :
Depending upon the output from the voltage comparator either green or red LED will glow.
ALARM :
The alarm is connected parallel to red LED to produce alarm when the red LED glows to indicate the person is lying
CIRCUIT DIAGRAM
ALARM CIRCUIT
CIRCUIT DESCRIPTION
 The circuit diagram of the Lie Detector is shown above. It consists of three transistors (TR1 to TR3), a capacitor (C1), two lights or LEDs (L1 & L2), five resistors (R1 to R5), and a variable resistor (VR1).
 This 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.
 Resistors R1 and R2 form a voltage divider. They have resistances of 1 000 000 ohms (1 mega ohms) and, because their values are equal, the voltage at the upper probe wire is half the battery voltage (about 4.5 volts).
 A person holding the probe wires will change the voltage at the upper probe wire depending on their skin resistance.
 The skin resistance is in parallel with R2 and, because it is likely to be similar to or smaller than R2, the voltage at the probe wire will fall as skin resistance falls.
 Capacitor C1 functions as a smoothing capacitor and removes the 50Hz induced mains hum that is found on a person's body.
 TR1 and R3 form a buffer circuit (called an emitter-follower).
 The voltage at the emitter of TR1 follows the voltage at the probe wire and is now able to drive transistor TR2.
 Transistors TR1 and TR2 act as a voltage comparator.
 If the voltage at the base of TR2 is higher than at the base of TR3 then the green LED (L1) will come on.
 If the reverse is true then the red LED (L2) will light.
ALARM DESCRIPTION
 This small 3 transistors audio amplifier circuit will be suitable for small battery powered devices.
 The 3.3 ohm resistors connected to the emittters of PNP-NPN transistor couple, and the purpose of this resistors is to stabilize the transistor gain, so the temperature change won’t affect the performance much.
 Any PNP-NPN transistor couple with identical performance that capable of handling 100mA collector current should be suitable to replace the final transistors.
 The voltage swing of the output will be 2 Volt at maximum (at 9V DC supply), so the current at 8 ohm speaker will be 0.25A, and the maximum power would be the 2V*0.25A=0.5Watts.
WORKING
 To test the Lie Detector hold the probe wires.
 Adjust VR1 until the green LED is just on and the red LED is just off.
 This is the point at which the voltage at the base of TR2 is just greater than at the base of TR3.
 Now use moist fingers to hold the probes.
 This lowers the skin resistance and causes the voltage at the base of TR2 to fall.
 The voltage at the base of TR3 is now greater and the red LED comes on.
WORK SCHEDULE
 JAN 4th- Abstract Preparation.
 JAN 7th- Submitted Abstract for approval.
 JAN 17th-Necessary components were decided and block diagram was prepared.
 JAN 20th-Review.
 FEB 5th-Purchase of components.
 FEB 6th- Implementation of the circuit.
 FEB 9th- Circuit to be submitted for the approval.
 FEB 15th- Gathering information regarding our project.
 FEB 20th- Alterations in the circuit diagram.
 FEB 28th- Implementation of the Modified circuit diagram.
 MAR 3rd- Implementation of the circuit in PCB board.
 MAR 12th- Final verification of the output and our project model will be ready to be demonstrated.
 MAR 15th-Preparation of the complete paper.
 MAR 23rd- Presentation of the paper with demo for the approval.
 MAR 25th- Report preparation.
 MAR 30th- Report Submission.