Low cost fire alarm circuit.


When there is a fire breakout in the room the temperature increases. This ultra compact and low cost fire alarm senses fire breakout based on this fact.

Transistor BC177 (Q1) is used as the fire sensor here. When the temperature increases the leakage current of this transistor also increases. The circuit is designed so that when there is an increase in the leakage current of Q1, transistor Q2 will get biased. As a result when there is a fire breakout the transistor Q2 will be on. The emitter of Q2 (BC 108) is connected to the base of Q3(AC 128). So when Q2 is ON Q3 will be also ON. The transistor Q3 drives the relay which is used to drive the load ie,light,bell,horn etc as an indication of the fire. The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switched.

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Circuit diagram with Parts list.



  • The Preset R1 can be used to desired temperature level for setting the alarm ON.
  • This is not a latching alarm,that is;when the temperature in the vicinity of the sensor decreases below the set point the alarm stops.
  • The circuit can be powered using  a 9V battery or a 9V battery eliminator.
  • All capacitors are electrolytic and must be rated at least 10V.
  • The load can be connected through the C,NC,NO points of the relay according to your need.
  • The calibration can be done using a soldering iron, and a thermo meter. Switch ON the power supply.Keep the tip of soldering iron near to the Q1. Same time also keep the thermometer close to it. When the temperature reaches your desired value adjust R1 so that relay gets ON.Done!
Show Comments

87 thoughts on “Low cost fire alarm circuit.

  1. Ravinath

    I am willing to learn about this project more deeply.Please help me.I have know about the schematic circuit diagram and also the PCB designs..Thank You…

    • seetharaman

      NO means normally open contact(when relay is de-energised this contact will be open when the relay in energised it will become close contact) Similarly NC means normally closed contact will go open when relay is de-energised. R1 os a variable resistance which can be varied from 0 ohms to 470 kilo ohms as per required temperature level. It will be a practical adjustment

  2. Aymen

    Can i use BC107 instead of BC108? i couldn’t find BC108 in my country.

    from the datasheet of both transistors the only difference seems in the current gain, is it gonna be ok if BC107 used in this configuration?

    Thank you so much in advance

    • wagnas

      hi, am taking this as my project. Please kindly send me the design and whole complete circuit diagram plz. And specification if possble. Thx in advance

  3. mohan krishna

    please help me with the circuit i still cant get the output even after changing the q3 transistor the dead line of the project is closing

    • seetharaman

      AC128 is wrongly shown as NPN transistor it should be pnp and its emitter should go to + and collector should go to the relay. You can use any switching pnp transistor like S8550, 2N2905, BD136. BC557 cannot give the required drive current.

  4. seetharaman

    Hi Alok yes you have to leave base open as the temperature increases its collector emitter leakage current will increase and that will trigger the alarm. The basic principle used here is the CE leakage current vs temperature characteristic.

  5. Seetharaman

    Q3 should be PNP collector to relay and emitter to +9volt DC. copy Q1 and paste on Q3.
    Set the alarm just to reset in shade with R1 and use direct sun light on Q1 to activate the alarm.

  6. mrinal

    sir,at how much resistance value should i keep the potentiometer at normal room temperature??and q3 should be npn or pnp?

  7. Emma

    Hi, am in Nigeria and I appreciate this project… Any mini soundless generator project available? I will use this project of yours as 1st alternative project but I need more just incase my supervisor doesn’t approve it. Thanks

  8. romans

    sir,its not working and i replace q1 with bc307.is it ok?what do u mean on adjusting the potentio sir,is it a full volume?pls reply

    • Anish

      If I am not wrong, the potentiometer here adjusts the sensitivity of the circuit. Since it biases the transistor Q2 from the output of Q1. It means that, by adjusting this potentiometer (a variable resistance) you can change the sensitivity of the circuit (not full volume). For example, if the circuit is made more sensitive, it will go on for a very less amount of fire etc.

  9. kiran R

    sir i choose this as mini project for TE electronics engg so please give layout of circuit and also tricks to run it

    • Anish

      May be you wired it wrong. Check if the transistor pins are in correct position. Misplacing the transistor pins is the most common mistake which goes out unnoticed. Adjust the preset properly.

  10. mark andrew

    is the low cost fire alarm effective?
    from the description: transistor Q2 will be on. The emitter of Q2 (BC 108) is connected to the base of Q3(AC 128).

    question:y is it that the emitter of Q2 is connected to the ground not on the base of Q3???

  11. Rogelio

    Sir how much do you think will I spent for this project?,,is Q3 here a PNP transistor?..specifically what transistor is that so that i can canvass if there is any,,,tnx sir

  12. kaushik

    I want the detailed explantn abt this and i liked it and i need it urgently!!!!!plz do it as soon as possible

  13. Qaiser

    Hello! Sir the circuit diagram shows to connect the connect the collector to the base of Q3, while in description it says to connect the emmiter of Q2 to the base of Q3. Will u kindly correct it

  14. Denz

    Hi guys.. the circuit may work but take note you can’t use it as a standard fire alarm in rooms especially ventilated one,you need to heat the room first or the room is already on flames to activate the alarm and you can’t control the fire anymore. For kitchen you can used heat sensing alarm like this and other fire alarm circuits using heat sensing component such as thermistor where smoke alarm will be not suitable but for rooms use smoke alarm which is cheaply available for safety reason.

    • venkat.s

      hey can u plz send a video clip. or detailed explaination on how to carry out the this project. urgent.. reply plz..

  15. swapnali

    hi!this is swapnali(TE E&TC)student..i want miniproject with report or with more ideas.so please sir help me…

  16. Seetharaman

    Hi Zack you can replace it with 2N2905. the symbol shown for Q3 is wrong it should be similar to Q1 as it is also a PNP transistor

  17. juls

    can i supply this ckt by 24 volts? and regarding the n.c – n.o output of this ckt is it respect to ground or to the com. terminal of the relay.

  18. seetharaman

    Hi Elgardo you are right Q3 is wrongly shown as NPN transistor it should be PNP with collector to relay and emitter to positive. otherwise the circuit is correct and will work as per the design.

  19. Edgardo Diolola

    I am studying the circuit diagram and I got confused.
    Is it the collector of Q2 will be connected to the base of Q3? (It says on the text :The emitter of Q2 (BC 108)is connected to the base of Q3(AC 128).)
    Or is this a typographical error?
    Please help.

  20. seetharaman

    Hi Sarah for fire alarm DC is preferable, so that they can work during power cut also. You can use rechargeable batteries with a suitable charger for 9 volt. this will be fail safe, whenever power supply is available it will be charging the battery as well powering the alarm. during power failure the battery will take over.

  21. Innocent

    sir i didn’t understand the terms NO,NC, C; and the component K1, and is R1 is a rheostat or resistior, waiting for your reply

  22. rupesh

    sir i tried your ckt…but after lots of attempts…still the ckt is not responding to the heat.when i connect to the supply the relay gets on (showing that current is flowing through the ckt)..but after that it doesnot respond to heat at all.please help!

    • admin Post author

      This circuit will not work properly as soon as you plug it. You need to calibrate the circuit before using. Calibration can be done by adjusting the POT R1.
      I tested this circuit and there is no problem with it.

      • mark andrew

        sir…wat is this preset r1?is a variable resistor or potentiometer?can you pls send me the correct diagram and detailed description..?pls

  23. rafi

    sir i am from bangladesh.your circuit is very easy to establish.but i face a problem to find transistor Q1(bc177).its unavailable.equivalents of this transistor bc178,bc179 are also unavailable.please help me to find another transistor.

  24. chandra sekhar

    hi the circuit was nice but i have a small doubt that is “what is the relation(formula) between Temperature and Resistance R1”.

    i mean how can we know the resistance(R1) value for our desired temperature.

  25. quennie

    hi sir..
    im quennie
    from the philippines and
    i just want to ask if you have
    a diagram of the ic you used..
    i mean if you have some diagram
    to guide on so that i can place the materials in their
    right position and place..

    thank you sir
    and god bless!

  26. deepak

    sir the circuit u provide for fire alarm is not working according to the circuit.
    actully I am a student and working on this fire alarm project. I joined all the connections according to ur circuit but it is not working plz help me…
    at place of Q3(AC128)i used another transister like the same of this.
    plz sir help me to complete my project ..
    waiting for reply..
    thank you.

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  28. al safi co

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    we wnt to ask abawt low cost fir alarm braes lik thts model in yor websitik
    may ourdr is:500 panel 2zon
    500 panel 4zon
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    5000 smok detecotor
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