Fire alarm circuit using IC 555 – Mini Project

Hello friends, today we are going to make one simple mini project on fire alarm. In this mini project we will make a fire alarm circuit using 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 thermistor (i.e. thermistor changes its resistance as temperature changes). Thus when temperature of thermistor increases because of fire its resistance also changes.

Circuit diagram:

Following figure shows the circuit diagram of fire alarm using IC555 mini project:

Fire alarm circuit diagram
Fire alarm circuit diagram



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

  • VR1 = 10 K ohms Variable Resistor which is used for varying the sensitivity of circuit.
  • R3, R7, R8 = 470 Ω
  • R2 = 33 K Ω
  • 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 third one is 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 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 temperature of thermistor increases it causes a low resistance path for a current flowing towards base of transistor.

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


1) Thermistor do not have polarity.

bead type thermistor
bead type thermistor

2) Following figure shows the pin out diagram of timer IC 555:

IC 555 Pin configuration
IC 555 Pin configuration

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Tags: Fire alarm circuit using IC555 pdf, simple fire alarm circuit,fire alarm project using ic 555.

Published by

Mayuresh Joshi

Mayuresh Joshi a web geek and founder of MyClassBook, a popular educational blog since 2012. He has completed his B.Tech (Instrumentation Engg.) from SGGSIE&T, Nanded. He loves to write on electronic projects, SharePoint and other current technologies.

8 thoughts on “Fire alarm circuit using IC 555 – Mini Project”

  1. how can we increase the sound of buzzer? is dt possible . we hve rigged up d ckt but d sound made by buzer is very less

  2. what if I cancel D!.. no need for it.. and why did you use two transistor !!! one is enough
    I suggest you to justify the use of each component.

    nice work

  3. I have made a fire alarm using germanium diode instead of thermistor but it is not working, I’ve done all the connections and also checked them..what will be the fault? Pls help me!

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