Introduction: Fire Alarm Circuit Using Operational Amplifiers

Fire alarm

circuit is a simple circuit that activates the circuit and sounds the buzzer after the temperature of the surrounding has increased to a certain level. These are very important devices to detect fire in the right time in today’s world and prevent any kind of destruction to life or property.

Nowadays almost all the important industrial and commercial buildings have been installed with fire and smoke sensors in order to prevent any damage to the building and avert a possible crisis.

Step 1: Components Required

-Components required –

1 x 10 K Thermistor

1 x LM358 Operational Amplifier (Op – Amp)

1 x 4.7 KΩ Resistor (1/4 Watt)

1 x 10 KΩ Potentiometer

1 x Small Buzzer (5V Buzzer)(one can also use a 12 volt buzzer)

Connecting Wires

Mini Breadboard

5V Power Supply

Step 2: Circuit Diagram

The above image shows the connections of the various components used in the project...

Step 3: Design Issues


Maximum supply voltage should not exceed 15V

Humidity should not exceed 85% relative humidity.

Step 4: Approach/methodology

The design of the Fire Alarm Circuit with

Siren Sound is very simple. First, connect the 10 KΩ Potentiometer to the inverting terminal of the LM358 Op – Amp. One end of the POT is connected to +5V, other end is connected to GND and the wiper terminal is connected to Pin 2 of Op – Amp.

We will now make a potential divider using 10 K Thermistor and 10 KΩ Resistor. The output of this potential divider i.e. the junction point is connected to the non – inverting input of the LM358 Operational Amplifier.

We have chosen a small, 5V buzzer in this project to make the alarm or siren sound. So, connect the output of the LM358 Op – amp to the 5V Buzzer directly.

Pins 8 and 4 of the LM358 IC i.e. V+ and GND are connected to +5V and GND respectively.

We will now see the working of the simple Fire Alarm Circuit. First thing to know is that the main component in detecting the fire is the 10 K Thermistor. As we mentioned in the component description, the 10 K Thermistor used here is a NTC type Thermistor. If the temperature increases, the resistance of the Thermistor decreases.

In case of fire, the temperature increases. This increase in temperature will reduce the resistance of the 10 K Thermistor. As the resistance decreases, the output of the voltage divider will increase. Since the output of the voltage divider is given to the non – inverting input of the LM358 Op – Amp, its value will become more than that of the inverting input. As a result, the output of the Op – Amp becomes high and it activates the buzzer.