Fire Alarm Circuit using IC555 Mini-project

Hello friends, today we are going to make one simple mini project which is a fire alarm. In this mini project, we will make a fire alarm circuit using a timer IC (IC555) and thermistor. Here we are using a thermistor sensor to sense the fire. The working principle of fire alarm circuit depends upon working principle of a thermistor (i.e. thermistor changes its resistance as temperature changes). Thus when a temperature of thermistor increases because of fire its resistance also changes.

Fire Alarm Circuit using IC555 Mini-project

Circuit diagram:

The following figure shows the circuit diagram of fire alarm using IC 555 mini project:

Fire Alarm Circuit using IC555


Following components are required for making fire alarm circuit (mini project).

  • VR1 = 10KΩ Variable Resistor which is used for varying the sensitivity of the circuit.
  • R3, R7, R8 = 470Ω
  • R2 = 33 KΩ
  • R4 = 560 Ω
  • R5 = 47 KΩ
  • R6 = 2.2 KΩ
  • C1 = 10 µF/16V
  • C2 = 0.04 µF
  • C3 = 0.01 µF
  • IC1 = NE555 (timer IC)
  • T1 = BC548
  • T2 = BC558
  • T3 = SL100B or (NPN transistors like 2N4922 , 2N4921,2N4238)
  • D1 = 1N4001
  • TH1 = Thermistor 10 KΩ
  • LS1 = 8 Ω, 1W speaker

Working of fire alarm circuit:

In this fire alarm mini project three components are very important i.e. thermistor, IC 555 (which is timer IC) and a third one is a transistor which is used to drive the circuit.

The timer IC used in this circuit works as an astable multivibrator oscillator which is used to oscillate in the audio frequency band. As stated earlier timer IC is driven by two transistors T1 and T2. The frequency of timer IC 555 depends upon the values of resistors R5 and R6 and capacitor C2. The loudspeaker which is used for fire alarm is connected to the pin no 3 of timer IC through transistor T3.

When the temperature of thermistor increases it causes a low resistance path for a current flowing towards the base of the transistor.

For this type of fire alarm circuit, we can use a power supply of 6V to 12V.


1) Thermistor does not have polarity.

bead type thermistor

2) The following figure shows the pinout diagram of timer IC 555:
IC 555 Pin configuration