Fire Alarm using Thermistor

Published  May 28, 2015   66
J Jayant
Fire Alarm Project

Fire alarms are prime necessities in modern buildings and architectures, especially in banks, data centers and gas stations. They detects the fire in ambiance at very early stage by sensing smoke or/and heat and raise an alarm which warns people about the fire and furnish sufficient time to take preventive measures. It not only prevents a big losses caused by deadly fire but sometimes proves to be life savers. Here we are building one simple fire alarm system with the help of 555 Timer IC, which will sense the fire (temperature rise in surrounding), and trigger the alarm.

The key component of the circuit is Thermistor, which has been used as fire detector or fire sensor. Thermistor is temperature sensitive resistor, whose resistance changes according to the temperature, its resistance decreases with the increase in temperature and vice versa.

We have built the circuit using, mainly three components that is, Thermistor, NPN transistor and 555 Timer IC. You can find more such simple circuits here in this electronic circuits section.


Working Concept

Here the 555 timer IC has been configured in Astable mode so that Alarm (Buzzer) can produce an oscillating sound. In Astable mode, capacitor C charges though resistance R1 and R2, till 2/3 Vcc and discharges through R2 till it reaches to 1/3Vcc. During the charging time OUT PIN 3 of 555 IC remains HIGH and during discharging it remains LOW, thats how it oscillate. We have connected a Buzzer to OUT pin, so that it produce beep sound, when 555 is high. We can control the oscillation frequency of the alarm by adjusting the value of R2 and/or capacitor C.



555 Timer IC

NPN Transistor BC547

Thermistor (10K)

Resistors (1K, 100K, 4.7K)

Variable resistor (1M)

Capacitor (10uF)

Buzzer and Battery (9v)


Circuit Diagram and Explanation

Fire Alarm Circuit Diagram using Thermistor

You can see the circuit diagram of fire alarm in above figure. When there is no FIRE, thermistor remains at 10k ohm. And transistor remains at ON state because there is sufficient voltage across the base-emitter of transistor, which makes it ON. When the Transistor is ON, Pin 4 (RESET) is connected to the Ground, and when Reset pin is Grounded, 555 IC doesn't operate.


Now when we start heating the Thermistor through Fire, its resistance starts to decrease, and when its resistance decreases, the voltage at the base of Transistor starts to decrease and when the voltage becomes less than the operating voltage (base-emitter voltage VBE) of transistor, then transistor becomes OFF. And when transistor becomes OFF, Reset pin of 555 timer IC, gets positive voltage through R3, and 555 IC starts to work and buzzer beeps.


In transistor, usually 0.7v voltage is required across the Base and Emitter, to turn it ON. So we have to carefully adjust the value of Variable resistance RV1 and Thermistor, to make the circuit work properly. To do this remove the thermistor and let RV1 be the grounded, now adjust the value of RV1 to that point, where even slight turning of the RV1 starts the Buzzer. Means from this point, if we decrease the resistance, even very little, Buzzer starts to beep. Now at this point, connect the thermistor again.


We should also note that we can also build a Fire alarm circuit, using DR25 germanium diode, as it works as heat sensor. When DR25 germanium diode is connected in reverse bias, it has a very high reverse resistance and it only conducts at more than 70 degree of room temperature.


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Please be specific, what kind of siren you are talking about? Are you trying to generate siren using some IC? You can connect any siren but circuit will be modified accordingly.

Submitted by ganeshraj on Wed, 11/25/2015 - 16:36


i did this circuit and my speaker comes on as soon as i connect the power. ive looked through it and i dont know what i'm doing wrong. please could you suggest likely errors i would have made. Please it's urgent!!!!!

If your circuit connections are proper, then you surely need to adjust the Variable Resistor RV1. It has been clearly explained in second last paragraph of the Article above: "remove the thermistor and let RV1 be the grounded, now adjust the value of RV1 to that point, where even slight turning of the RV1 starts the Buzzer"

Submitted by kumar on Sat, 02/06/2016 - 21:27


we have tried a lot to make this project succesfull... but we did not got any output ....about this project can you plzz.. tell that where was the problem in this project

before connecting the circuit check the resistance under normal temperature and check its resistance when it exposed to heat, now adjust the variable resistor in such a way that its value lies between the thermistor resistance in normal temp and during heating. Generally it will be aroung 3.5K ohm

Submitted by akki on Wed, 02/17/2016 - 20:54


sir ,we had made our circuit but as soon as we connect battery siren is in on state even though thermistor is not heated,can u suggest what correction to be made in circuit

Submitted by akhila on Thu, 03/03/2016 - 10:48


can u pls send me block diagram of fire alarm using ic555 my mail id.....

Submitted by katty on Mon, 04/04/2016 - 22:44


we hve tried the circuit several times out evryting but itdoesnt operate i.e as soon as we connect the battery the buzzer starts beeping evn without buzzer

Submitted by Moni shankar on Thu, 05/05/2016 - 01:55


i am using this circuit as it is..only replaced 1mohm with a 10kohm...and ckt working but only prblem is that output beep sound is not continuous..its beeping in regular (approx after 1sec gap ,just on and off)intetval of time...pls suggest me what to do..

Submitted by Pooh on Thu, 05/12/2016 - 07:38


Hey, I have tried many times but connection doesn't really work. I grounded my variable resistor but after adjusting the buzzer still not beep. Can i ask how many pin that you use for variable resistor? Because it have three pin, and i do not know where should connected

Submitted by Brian O Callaghan on Tue, 07/05/2016 - 04:05


Hi there,
Thanks for the circuit. I am trying to trigger a relay using a 33k Ohm @ 25 degrees C thermistor. I have attempted this with a 555 timer as a comparator, but have found that the variation in resistance of the thermistor does not allow the 1/3 - 2/3 voltage range to be covered in order to get switching on and off. The relay should trigger at only 30 dec C, thus the low range (from 33k Ohm to approximately 20k Ohm at trigger temperature.) Also, is it possible to leave out the capacitor and R1 and R2 since only the relay must be switched on and off?
Thanks a million,

Resistor R1, R2 and capacitor C has nothing to do with Fire alarm circuit, they are just to set the Frequency of 555's output in astable mode, so that Buzzer can beep periodically, according to that frequency.

Submitted by nive on Sun, 07/31/2016 - 09:16


shall i check the fire alarm circuit using multisim software and how?

Submitted by Ankita on Sun, 07/31/2016 - 17:31


Can LM35 be used as the thermistor here?if yes then where to connect the 3pins exactly..?thanx!☺

Submitted by sam on Fri, 08/26/2016 - 02:29


Pls you stated Capacitor (10uF),what is the exact voltage of the capacitor

Submitted by irfan on Thu, 09/15/2016 - 13:28


sir what will be the output variable resistor?????

Submitted by umesh on Thu, 09/22/2016 - 18:33


I have found you best website for how to design a circuit from starting to every component is working in circuit ..It is very helpful for freshers like me.
I am very impress..
And highly recommend it.

Submitted by milos on Fri, 10/28/2016 - 04:18


Hi . I have variable resistor , and i don't know how to adjust it( since they are not the same ) . So , is this type of RV usable ? Can u give me some advice how to do that properly , since my sound starts as soon as i connect the circuit . Why is only 1 pin of RV used ? What are those 2 for ? Thanks in advance .

Submitted by reubenct on Wed, 11/09/2016 - 18:17


Hi, I tried the circuit with all correct values for components and what I observed was that the transistor was never switching on, i.e, the voltage at base (Vbe) never went below 0.7V. I calculated and got that for Vbe=1V, value of RV1 should be 1M/8 = 125k. My RV1 can go only to minimum of 200k. Please suggest a solution. Thanks

Submitted by Ram on Sun, 12/04/2016 - 13:00


Sir, I have done every thing as shown in the diagram but still it does not works. Please help sir. When i adjust the variable resistor the buzzer beeps continuosly.

Submitted by vijay on Sun, 02/19/2017 - 12:44


hi sir,
i made all connection as per block diagram shown but even i connect that way it is not working properly .i didn't have 1M variable resistor so i used 1M fixed resistor& instead of 10k thermistor i used 10k fixed resistor (before fire thermistor resistance is 10k so used it ,actually the buzzer need to beep at this time but it is beeping even i use 100k instead of 10k thermistor(i dont have thermistor) ) .thermistor resistance will reduce when fire comes so i replace thermistor with short ckt .so sir tell me neccesary modificatication to work my ckt

Submitted by Aaron on Sun, 03/05/2017 - 16:49


Sir, i have done all the connections made in the diagram, but when i put the power on the buzzer was beeping even though i haven't fire the thermosistor

Submitted by CHERRY on Tue, 03/14/2017 - 17:56


Sir, I have to ask. Will it be possible to have both buzzer and LED outputs? I have tried connecting a LED parallel to the buzzer. The LED did work but the buzzer didn't.

For both LED and buzzer u have to use also a 1k resister with it otherwise LED will be fuse.
Then first connect one terminal of resister to the pin 3 thats output terminal then the positive terminal of led to the other terminal of resister and then ground the negative terminal to the ground and then the positive terminal of buzzer connect with the positive side of LED n then groung the negative terminal of buzzer to the ground. Then it will work properly.

Submitted by Eggroll on Thu, 03/23/2017 - 22:39


Sir,I just made the circuit above.But there is a problem,the buzzer keeps beeping as soon as i ground the circuit.The 1M variable resistance is not showing any effect even i changed the variable resistance.Please give me the solution as i need to submit it within a week.
Thank you

Submitted by Eggroll on Fri, 03/24/2017 - 00:48


This circuit is just a fine circuit and works well.With all the connections being made at correct places,it works. Applying fire to the thermistor , it tkaes around 25 secs to turn the buzzer on.

Submitted by Charlie Catindig on Fri, 03/24/2017 - 09:54


if you don't mind, is it advisable to use capacitor instead of resistor in a conventional fire alarm and detection system (FDAS)? what is the effect using capacitor? why not a resistor?
please reply in my personal email, not here.. thanks.

Submitted by duncan on Tue, 05/09/2017 - 16:56


good project mine is working well just needed to play around the variable resistor

Submitted by Mushfiqur Rahman on Wed, 11/01/2017 - 00:21


How long will this circuit last on a normal 9V battery (non-rechargeable)?

Submitted by Omer on Mon, 11/13/2017 - 17:39


I have connected all the circuit same as it is but it doesn't work but when i short the transistor buzzer starts

Submitted by Abush on Tue, 11/14/2017 - 10:37


Please can help me I use this circuit but its not run what the problem

Submitted by Manthan Sirsikar on Thu, 02/15/2018 - 23:11


i tried to adjust the variable resister lots of time by putting its one end ground but as i replace it by thermistor without giving any fire it starts beeping plse give me some solution i read out all the content u have written above ........ Thank you...
waiting for your reply

Submitted by khaled on Fri, 06/08/2018 - 15:39


How much is the frequency and duty cycle for this circuit because i want to now how to design the component value.pleas replay me as soon as possible

Submitted by khaled on Sun, 06/10/2018 - 00:29


How much is the frequency and duty cycle for this circuit because i want to now how to design the component value.pleas replay me as soon as possible