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
Circuit diagram for Fire Alarm 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
bead type thermistor

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

IC 555 Pin configuration
IC 555 Pin configuration

You may also like:

I hope you liked this article. If you have any queries, please use below comments section for the same. Please like our facebook page and subscribe to our newsletter for future updates. Have a nice day! tada…

How To Build Home Made Electric Fan Regulator – Mini Project

Hi friends, in this article we will see one simple mini project for electronics and electrical engineering students. We all are familiar with electric fan regulator which we use for varying the fan speed. The basic principle behind regulator is a change of resistance.

This article is written by Mohamed Maricar & Modified by Mayuresh Joshi

Cheap and Best Electrical Fan Regulator

Electric Fan Regulator

Fig (A): shows the full view of connection diagram of the product
Fig (B): shows full view of the product and external side
Fig (C): shows the PCB without resistance


Components required:

  1. One Resistance = Rs. 0.15
  2. 28 Resistances = Rs.4.20
  3. One PCB = Rs.5.00
  4. One Rotator Switch = Rs. 8.00
  5. One Plastics Cabinet = Rs.5.00
    Total Cost = Rs. 22.20 Rupees Only

Steps to build electric fan regulator:

  1. First, we have to take PCB as shown in Fig (C)
  2. Then we put the Resistance and solder in PCB holes as shown in fig(A)
  3. Now we connect PCB ends with rotator switch as in fig (A)
  4. We connect rotator switch with AC supply
  5. Now if we vary the rotator switch we can get the different speed of motor
    1. The Resistance network is used to drop the voltages in five steps that are level
    2. The first step 13 volts dropped second steps 26 v third step 39v fourth step 53v
    3. By dropping the five level used to vary the speed of motor
    4. By dropping voltages we can vary the speed of electrical fan motor
  • What is my idea?
    My idea is to design a new circuit to reduce the cost and increase the performance of the product
  • What does it do?
    It regulates the speed of electric fan motor by dropping a.c. voltage in different level
  • How does it work?
    It is working by Ohms law and circuit theory principles
  • What makes my idea Unique?
    Cheap and best

Advantages of my product

  1. Cheap cost
  2. Low care
  3. Low power loss
  4. Low heat loss
  5. Good performance

If you like this article, please share it with your friends and like or facebook page for future updates. Subscribe to our newsletter to get notifications about our updates via email. If you have any queries, feel free to ask in the comments section below. Have a nice day!