A burglar alarm circuit uses a NOT gate and a thermistor to detect fire. When the temperature increases, the thermistor’s resistance changes. As a result, the circuit turns on a buzzer to give an alert. This simple project shows how logic gates can be used in fire detection and safety systems.
Apparatus
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NOT gate
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Thermistor (for example, GM472 or VA 3404)
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Variable resistor
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6V power supply
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Buzzer
Theory
A thermistor is a resistor whose value changes with temperature. Usually, its resistance decreases as the temperature increases.
In this experiment, the thermistor replaces the LDR in a potential divider circuit connected to a NOT gate. At normal temperature, the thermistor has high resistance. Therefore, the input to the NOT gate is at logic 1, and the output stays at logic 0, keeping the buzzer off.
However, when the temperature rises, the thermistor’s resistance drops. Because of this change, the voltage across it also decreases. Once the voltage is low enough to make the input logic 0, the NOT gate output becomes logic 1, and the buzzer turns on. Hence, the circuit works as an automatic fire alarm.
Procedure
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Connect the components as shown in the circuit diagram.
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Adjust the variable resistor so that point A has logic 1 (5V).
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Gently heat the thermistor using a lighter or warm air.
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As the temperature rises, the resistance decreases, lowering the voltage.
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When point A reaches logic 0, the NOT gate output becomes high, and the buzzer sounds.
Precautions
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Keep the circuit connections clean and tight.
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Adjust the voltage across the thermistor slowly and carefully.
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Always assemble the circuit on a breadboard for safety.
Viva Voce
Q1: What is the function of a NOT gate?
Ans: It changes a high input into a low output and a low input into a high output.
Q2: Can a NOT gate have more than one input?
Ans: No, it always has one input and one output.
Q3: What happens when two NOT gates are connected in series?
Ans: The output becomes the same as the input.
Q4: How can we make a NOT gate using a NAND gate?
Ans: By joining both input terminals of a NAND gate, it works as a NOT gate.