The instrument used to find whether a person is speaking the truth or not is called a lie detector or polygraph. It records the heart beats, blood pressure, and body temperature and tension level of the person undergoing the detector test. General questions along with other relevant questions are asked to the subject. When the person tries to speaks lies, his body physiology changes. These changes are recorded and compared with the results that are obtained for normal questions. Such sophisticated instruments can however give wrong results. Hence the results obtained through the lie detectors are not taken as evidence in court cases.
Our electronic lie detector is a low tech version of a real polygraph or lie detector. Emotional stress is not only reflected by heart beats or trembling hands, but also by an increase in skin moisture.
When the skin becomes moist, its resistance decreases. Thus the skin resistance is a good conductor of the stress level of the subject. The lie detector presented here has been designed to detect skin resistance of the subject and to give visual indication through bar arrangement of different color LEDs.
The circuit for the lie detector is shown here. It is very simple to construct and use.
The circuit consists of a bridge, an amplifier built around IC1 and a display stage around IC2. The electrodes, fixed resistors R1 and balance control VR1 make up a bridge circuit. The bridge network is connected to operational amplifier IC1. In this circuit, popular CA3140 op-amp is used as it is readily available and inexpensive. Much better op-amp is available in the market and these can be used for precision results. As this lie detector circuit is only for entertainment purposes, engaging such costly op-mps is not necessary.
The output of IC1 drives IC2. This display driver is the popular IC LM 3914, which measures the voltage from IC1 and converts it into digital signals that light appropriate LEDs.
The output of IC1 is extended to pin 5 of IC2. This is the input voltage of IC2, according to which it acts. The IC has an internal stable 1.25V reference voltage source. The reference voltage is available at pin 7 and may be varied by preset VR2. This IC is wired to give a bargraph mode which lights all LEDS successively and keeps them ‘on’ to display a bar of light. The length of the bar will vary in accordance with the value of input signal available at pin 5 of IC2.
1.5V cell is provided to supply power to the bridge.
Place the palm of your hand on the electrodes. The skin resistance across the palm becomes part of the bridge. Since the skin resistance and fixed resistance R1 are not normally equal, there will always be some error in the positive or the negative direction in its output voltage. This condition is corrected by the use of balance potentiometer VR1, which allows the output of the bridge circuit to be corrected for variation. Also, the general emotional condition of a person can only be ascertained by measuring the average resistance of the skin over a period of time. Hence, it is necessary to adjust VR1 a few times for balance until a relaxed state is reached. Once this is done at the start of the session, there is no need to adjust VR1 thereafter.
Also, as the skin resistance varies from person to person, it may be necessary to adjust VR1 for each participant. Once the bridge is balanced, the lie detector reading will vary in accordance with the changes in body resistance. The circuit amplifies any minor changes in the subject’s resistance and cause LEDs to light. Any LED that lights up thereafter indicates that the skin resistance is changing under stress.
For calibration, connect a resistor with the same value of R1 in the place of electrodes. Bring potentiometer VR1’s wiper to the center of the rotation. Now the differential voltage at the input ports of the op-amp is near zero. Thus the output of the IC1 assumes its midrange value of 4.5V.
Now slowly adjust preset VR2 so as to light fewer LEDS.
Remove the fixed test resistor and connect the electrodes. Touch the electrode with your palm. Adjust the setting of the balance control of VR1 such that the approximately midway. Note down the number of the glowing LEDs. Hold the electrodes very tightly with the forefingers and the thumb. Now the electrodes will get heated up and consequently their resistance will reduce, and this will cause more LEDs to turn on. Once you get the above mentioned indications from the initial test, your circuit is ready. Preset VR2 need not be adjusted further.
Now ask the participant to touch the electrodes with his palm and set the balance control so that some LEDs light up. Stress causes a decrease in body resistance which results in more LEDs lightning up in the display.
Two bare flexible wires can be used as electrodes. If regulated power supply is available, the LEDs will give a stable output.
The lie detector circuit can be used as a relaxation monitor for monitoring relaxation and reduction of tension. Set the balance control to light more number of LEDs. As you relax or reduce tension, your body’s resistance increases. This will cause more LEDs to extinguish.
Resistors (all ¼-watt, ± 5% Carbon)
R1 = 68 KΩ
R2 = 4.7 MΩ
R3 = 1 KΩ
VR1 = 100 KΩ
VR2 = 10 KΩ
C1 = 0.02 µF
IC1 = CA3140E
IC2 = LM3914N
LED1 – LED3 = Green
LED4 – LED7 = Yellow
LED8 – LED10 = Red