Simple fire alarm- thermistor circuit diagram

Here i-St@r comes with a simple and automatic fire alarm and detection system. This fire detection alarm circuit is based on thermistor. What is a thermistor? A thermistor is nothing but a variable resistor whose resistance increases as temperature decreases. Fire detection is simply possible via this low-cost alarm circuit.A simple potential divider arrangement using thermistor is capable of sensing the temperature (presence of fire) and alerting us with a warning signal. This is suitable for your home security systems. Also school students can do this fire detector circuit as their high school science fair projects. Here I will explain in detail about fire alarm circuit wiring with animation/ simulation of fire alarm.

Circuit diagram of simple fire alarm

Components required

1. Power supply
2. Resistors ¼ watt (470Ω; 33kΩ)
3. Capacitor (10µF, 16V)
4. BC548 transistor
5. Thermistor
6. Diode (1N4001)
7. 6V Buzzer

Working of fire alarm circuit

• Working is based on the switching property of transistor.

• The thermistor and R1 forms a potential divider network which drives the transistor.

• The heart of this circuit is a thermistor. Thermistors are low-cost, easily-available temperature sensors widely used for uncomplicated temperature measurements.

• Thermistors are temperature sensitive resistors. When temperature increases resistance offered by the thermistor decreases and vice versa. At normal temperature, the resistance of the thermistor is around 10kΩ.

• All resistors vary with temperature, but the semiconductor materials used for thermistors are especially sensitive to temperature. 

• The transistor is turned ON by the voltage drop across the resistor R1.

• Consider the temperature of the atmosphere is around 25°C, and then the resistance of thermistor is high so the voltage across the thermistor is also high according to the basic ohm’s law V=IR. 

• At this situation voltage across resistor R1 is low and it is not sufficient to turn on the transistor.

• As temperature rises, the resistance of thermistor decreases so that the drop across the resistor R1 increases which turns ON the transistor. 

• When the transistor is turned ON, the current from Vcc starts to flow via 6V buzzer which produces a beep sound.

• Diode is used for enabling unidirectional conduction and the capacitor removes sudden transients from the thermistor.

Simulation of the fire alarm project

We have used an LED instead of buzzer for the simulation of fire alarm system.

Pinout of BC548

• BC548 pinout

Related circuit

• Street light circuit