Fire Alarm using Thermistor


ByJayant 66

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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  • aimen's picture
    Sep 05, 2015

    Can i replace 9v buzzer with siren

  • Jayant's picture
    Oct 06, 2015

    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.

  • ganeshraj's picture
    Nov 25, 2015

    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!!!!!

  • Maddy's picture
    Nov 28, 2015

    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"

  • kumar's picture
    Feb 06, 2016

    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

  • Maddy's picture
    Feb 08, 2016

    Check the circuit connections and value of components. Adjust the variable resistor properly, otherwise circuit won' t work. Carefully read the second last para to set the variable resistor

  • venkatesh babu's picture
    venkatesh babu
    Oct 18, 2017

    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

  • akki's picture
    Feb 17, 2016

    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

  • Jayant's picture
    Feb 24, 2016

    This is a very common problem and can be easily corrected, just adjust the variable resistor according to the second last paragraph of the article.

  • abhishek pratap's picture
    abhishek pratap
    Mar 01, 2016

    why have you used Resistors (1K, 100K, 4.7K) and npn transistor and what happens if we change the value of resistance.

  • Kapil's picture
    Mar 02, 2016

    sir, what is the use of the transistor in the circuit ?

  • Maddy's picture
    Mar 11, 2016

    Transistor here is working as a Switch, it is clearly explained in the description. Please read the two paragraph below circuit diagram.

  • akhila's picture
    Mar 03, 2016

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

  • easin's picture
    Apr 01, 2016

    what valu may be perfect for Variable Resistor

  • Gatumwa's picture
    Apr 03, 2016

    555 ic there we may change to anather kind of componant which ones?

  • katty's picture
    Apr 04, 2016

    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

  • Maddy's picture
    Apr 17, 2016

    Its only the matter of adjusting the Variable Resistor.

  • Moni shankar's picture
    Moni shankar
    May 05, 2016

    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..

  • Maddy's picture
    May 21, 2016

    This is what it meant to be. 555 is in Astable mode so it beeps in intervals, use monostable mode for continuous beeping.

  • Dan's picture
    May 05, 2016

    What kind of thermistor did you use
    is it NTC, and what is the voltage reading on the thermistor

  • Pooh's picture
    May 12, 2016

    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

  • Maddy's picture
    May 21, 2016

    two pins of Pot have been used, one middle pin and one side pin (any)

  • Rahul's picture
    Jun 29, 2016

    can i replace variable resistance with simple resistance

  • Abhishek's picture
    Jul 09, 2016

    After finding the threshold resistance, where you want to set the sensitivity of Fire alarm, you can replace Variable resistor with simple one with the value of Threshold.

  • Brian O Callaghan's picture
    Brian O Callaghan
    Jul 05, 2016

    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,

  • Abhishek's picture
    Jul 09, 2016

    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.

  • zeedic's picture
    Jul 22, 2016

    the block diagram of fire alarm system using themistor

  • nive's picture
    Jul 31, 2016

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

  • Ankita 's picture
    Jul 31, 2016

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

  • sam's picture
    Aug 26, 2016

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

  • Cherish's picture
    Sep 09, 2016

    At what specified temperature should the device detect before it alarms?

  • Abhishek's picture
    Sep 20, 2016

    You cant exactly define the Temperature threshold without using any Microcontroller, but you can use Variable resistor to adjust the sensitivity. You can check this article to define temperature range with LM35 sensor: Arduino Based Digital Thermometer

  • irfan's picture
    Sep 15, 2016

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

  • umesh's picture
    Sep 22, 2016

    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.

  • milos's picture
    Oct 28, 2016

    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 .

  • sindhu's picture
    Oct 30, 2016

    Hi.can u please clearly explain the connenctions of thermistor and variable resistor please.

  • reubenct's picture
    Nov 09, 2016

    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

  • Ram 's picture
    Dec 04, 2016

    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.

  • Somanath's picture
    Feb 06, 2017

    How this project is different from the other fire alarms

  • vijay's picture
    Feb 19, 2017

    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

  • Maddy's picture
    Feb 27, 2017

    1M is the max value of RV1, and it is set much lesser than 1M, better use Variable resistor as shown.

  • shrinjit's picture
    Feb 25, 2017

    is it sufficient to use a single npn transistor int the above circuit becoz i checked other links wherein they have used 3 transistors ?

  • Aaron's picture
    Mar 05, 2017

    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

  • CHERRY's picture
    Mar 14, 2017

    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.

  • RAJBBER NRGI's picture
    Apr 22, 2017

    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.

  • francis's picture
    Aug 22, 2018

    Have complete the circuit and its working but, what else can I do to the circuit to produce siren sound instead of beeping.

  • Eggroll's picture
    Mar 23, 2017

    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

  • Eggroll's picture
    Mar 24, 2017

    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.

  • Shehryar's picture
    May 27, 2018

    can you please send me the correct connections.

  • Charlie Catindig's picture
    Charlie Catindig
    Mar 24, 2017

    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.

  • duncan's picture
    May 09, 2017

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

  • adebiyi muyiwa's picture
    adebiyi muyiwa
    Sep 15, 2017

    please can i get the circuit for all the alarm system we have

  • mesloy's picture
    Oct 31, 2017

    The pin 5 of the ic, is it open?

  • Mushfiqur Rahman's picture
    Mushfiqur Rahman
    Nov 01, 2017

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

  • Omer's picture
    Nov 13, 2017

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

  • Abush's picture
    Nov 14, 2017

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

  • Manthan Sirsikar's picture
    Manthan Sirsikar
    Feb 15, 2018

    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

  • khaled's picture
    Jun 08, 2018

    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

  • khaled's picture
    Jun 10, 2018

    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

  • Hiro_Hamada's picture
    Jun 12, 2018

    This circuit produces a pulse only to keep the buzzer beeping. It is not important to design this pulse since it will only vary how fast the buzzer is beeping. 

  • Mohamed Rahamathulla abdul Gaffoor's picture
    Mohamed Rahamat...
    Jun 14, 2018

    nicely explain

  • Mounika's picture
    Jan 29, 2020

    At what temperature the buzzer starts beeping??

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