Fire Alarm Circuit using IC 555

We have seen many alarm circuits but Fire alarm was one of the most important among all.In many buildings,industries and houses fire alarm circuit have been installed in order to alert people them in case of a fire break out in the buildings.So now we can study how this circuit and what are the principles of this circuit briefly.

WORKING OF FIRE ALARM CIRCUIT:

This circuit uses IC1 (NE 555), Thermistor, Resistors, Capacitors and Transistors in it.The IC1 is configured as a free running oscillator at audio frequency.Timer IC (IC1) can produce continuous signal when used as a Astable Multivibrator.Thermistors are the special purpose ICs whose temperature varies significantly with temperature and here we are using NTC (Negative Temperature Coefficient) Thermistors.These Thermistors exhibit high resistance when temperature is low and low resistance when temperature is low.Thus in this circuit this was used to sense the rise in temperature in the surroundings.

Initially when there is a fire break out the resistance of the Thermistor decreases and this allows the voltage to the base of the transistor through the diode D1.The function of the Diode is to allows the current in one direction through it.In this process the capacitor C1 get charges up and increases the time for which the alarm in ON.The larger the value of C1 larger the positive bias applied to the Transistor T1.As the collector of the Transistor T1 is coupled to the base of the Transistor T2 the transistor provides a voltage to the pin 4 (reset) of the NE 555 IC.It is the function of R4 which keeps the IC inactive unless positive voltage is given through transistor.It should be selected in such a way to keep the IC inactive.So thus it makes the Speaker to give sound when there is a fire breakout in the building.

The same reverse process happens when there is reduction in temperature it cools out the Thermistor therefore it gives the high resistance path to the voltage flow thus inhibiting it from reaching the Transistor therefore it keeps the IC1 in inactive state therefore the alarm remains off.

JLCPCB - Prototype PCBs for \$2 + Free Shipping on First Order

10 PCBs Price: \$2 for 2-layer, \$15 for 4-layer, \$74 for 6-layer

The following two tabs change content below.

Frank Donald

Junior Network Engineer at Tata Communications
Frank Donald is an Electronics and Communication Engineer who loves building stuff in his free time. Video games, blogging and programming are the things he loves most

17 Comments on "Fire Alarm Circuit using IC 555"

Subscribe
Notify of
Guest

Hi! Can you please tell what is the value of thermsistor?

Guest

So when T1 is switched on, the collector goes High and switches on T2? If so , how dose T1 collector go high? Transistors are a hard concept for me.

Guest

Thanks for the help! I see that now. If the npn signal is too low by itself , could a 2 stage npn be used ? I’m just wondering if it’s by choice or a have to situation. are there advantages over either way?

Guest

circuit is not working…using 9v battery..
give me reason why..?

Guest

Frank I mead the current and the voltage

Guest

I need the theoretical part of this circuit if you have send me please

Guest

Boughella,
Have given enough theoretical part in the description. If you want some tweaks in it read Mr. Bill’s comment above you might get extra piece of info.

Guest

Hello..
.
I hv another question ..
I hv 9V battery ,i want it convert into 6V. so what value of RESISTANT should be use???

Guest

Use 7806 6V regulator IC for that purpose

Guest

Hello sir..

Are u tried this firealarm by own??

Can i use 0.01u or 0.22u in the place of C2 0.04.
1N4007 in the place of D1 N40001.?

Guest

Yes. you can use the cap value you specified, it only will alter the frequency of the output signal. And as far as diode is concerned you can use 1N4001 also, it was used to permit the current flow in one direction.

Guest
This is an “Over-Temperature-Alarm”, NOT A FIRE ALARM. A fire alarm typically detects combustion byproducts, not heat. Also, if your thermistor will drive a 470 ohm resistor, why do you have a two transistor circuit between it and the input of the 555? You are actually driving less impedance with the second transistor than the thermistor is… And, the transistors provides a double inversion, so the input to the 555 is the same phase as the voltage at the thermistor. If you are doing a voltage shift here, try doing it with a change in your thermistor/resistor pair.What is D1… Read more »
Guest
Am glad to see that you took some time to provide us extra piece of information here. Actually there are several ready made sensor modules which will do a lot better job than this one but this circuit was posted in the intention to help viewers to build this by their own from scratch. As you said design practices are necessary to hone the circuit designing skills. And about inviting the visitors to improve or add extra piece of information seems to be good idea and surely will try to implement it here. Many thanks for your suggestion regarding this… Read more »