![]() The transistors Q3 and Q1 are connected as a Darlington pair hence the small variation of base voltage Q3 will affect Q1. Variable resistor VR1 decides the sensitivity level of the circuit. Green LED is connected with Q3 transistor this LED represents the detection of true. Here Q1 transistor base terminal takes the input from a finger, it is responsible for detecting the resistance changes in the finger, Red LED connected to the collector terminal of the Q2 transistor this LED represents the detection of a lie. In another hand, if the skin is moist (liar), the resistance is less than 1M. If the skin dries (people do not lie), the resistance of about 1M. So, the voltage drops across R2 depending on the resistance of the skin. Here the probe checks the resistance of the skin in parallel with R2. The simple lie detector circuit presented here has been designed to detect skin resistance, it works based on the skin resistance variations, consider a person who lies during the conversation his anxiety level will rise and it makes him sweat so his hands & finger becomes moisture by using this lie detector circuit, we can detect those moments by the way of conductivity and resistance variation then LEDs glows depends on the moisture & conductivity of the skin.Īs we can see in the circuit, the operation of the circuit starts with the R1 and R2 voltage divider circuits. Thus, skin resistance is a good conductor of the stress level of the subject. When the skin becomes moist, its resistance decreases. As we know that emotional stress is not only reflected by heartbeats or trembling hands, but also by an increase in skin moisture. Here we design a simple two transistor-based low-tech version of a real polygraph or lie detector. Hence the results obtained through the lie detectors are not taken as evidence in court cases. Such sophisticated instruments can however give wrong results. These changes are recorded and compared with the results that are obtained from normal conditions. When the person tries to speak lies, his body physiology changes. In recent years brain activity has also begun to be measured in this setting. ![]() The polygraph is a set of equipment that accurately measures various sorts of bodily activity such as heart rate, blood pressure, respiration, palmar sweating, body temperature, and tension level of the person undergoing the detector test. ![]() The instrument used to find whether a person is speaking the truth or not is called a lie detector or polygraph. It has been assumed that lying is accompanied by a change in the body’s physiological activity. In this tutorial, we are going to make a “Lie Detector Circuit”.
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