Fire Alarm Circuit

The fire alarm circuit makes use of an op amp, thermistor and a buzzer. A thermistor is a resistor that’s resistance varies as the temperature varies. A 10kΩ NTC thermistor is used for the fire alarm circuit, which means that at 25 degrees Celsius the thermistor has a 10kΩ resistance. As the temperature increase, the resistance decreases. It can be said that the resistance is inversely proportional to the temperature, unlike a PTC thermistor that’s resistance is directly proportional to temperature.

The op amp is configured as a comparator. Therefore, when the temperature increases past a point, the resistance of the thermistor will drop and trigger the buzzer to sound.


Components required

  • 1 x 10 K NTC Thermistor
  • 1 x LM358 Op amp
  • 1 x 4.7 KΩ Resistor
  • 1 x 10 KΩ Potentiometer
  • 1 x 5V Buzzer
  • 5V Power Supply
Fire Alarm Circuit