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  • In this fire alarm circuit project, a thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature ncreases beyond 100°C. The circuit uses readily available components and can be easily constructed on any general purpose PCB. Check out the fire sensor circuit.

    Schematic of the Fire Alarm Circuit

    fire alarm circuit circuit schematic

    Timer IC NE555 (IC1) is wired as an astable multivibrator oscillating in audio frequency band. Switching transistors T1 and T2 drive multivibrator NE555 (IC1). The output of IC1 is connected to npn transistor T3, which drives the loudspeaker (LS1) to generate sound. The frequency of IC1 depends on the values of resistors R5 and R6 and capacitor C2. To prevent the thermistor from melting, wrap it up in mica tape.

    The home fire alarm system works off a 6V-12V regulated power supply. LED1 is used to indicate that power to the circuit is switched on.
    We have not tested this fire alarm systems project.

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    60 Responses to "Fire Alarm Circuit"

    1. very small pixel picture. please enlarge properly.

    2. Will you please give me more information about this ckt diagram……

    3. gabriel barje shaibu says: on February 24, 2011 at 5:47 pm

      pls what is the function of D1 and C1. i implemented the circuit without the two and it still worked. are all thermistors having the same resistance?

    4. sir,
      iam doing this fire alarm as the miniproject.so, i want brief explanation in this aspect.

    5. olalekan says: on March 18, 2011 at 1:26 pm

      please i need more information about
      this fire alarm circuit diagram and the function D1 an C1

    6. Asams Pere says: on April 10, 2011 at 11:08 pm

      Please, I will like to write a project on fire alarm, can you feed me with information.

    7. Charles Ebiteh says: on July 25, 2011 at 5:09 pm

      Please sir, before the buzzer i thought there is supposed to be a relay.

    8. Emmanuel E Offiong says: on July 26, 2011 at 10:13 am

      Please sir what hels can we reprecent instent of thermistor

    9. I maked this circuit and this is works

      • i m also made this ckt bt it will not work can u plz help me. what problem arises on this ckt … its urgent plz help me ……

    10. ASAMS PERE says: on August 9, 2011 at 8:37 am

      I was able 2 carry out this project and it worked.

    11. sir,
      iam doing this fire alarm as the miniproject.so, i want brief explanation in this aspect.

    12. John Olateju says: on August 14, 2011 at 10:26 pm

      what else can i say?…..a very big THANK YOU!…this pretty small thing did saved my Electronics research lab from fire hazard. cheers!!!!

    13. sir,

      iam doing this fire alarm as the miniproject.so, i
      want brief explanation in this aspect.
      pls send it to my id.

    14. please give me the pcb layout and its working information on my id.

    15. this circuit is working perfectly,but changing the value of the thermistor into 15k will increase the sensitivity of this circuit,thanks…:-)

    16. i need more information to complete my project.plz snd it to my id.

    17. plz give me the pcb layout at my mail id…

    18. Manish kumar singh says: on March 20, 2012 at 5:23 pm

      i required more informatn bout it pls sir send it to my id sir please.

    19. Sir,as I am going to do this for my project,could you please help me in details of writing process and how does it work. As soon as possible

    20. amrutha says: on June 19, 2012 at 9:28 am

      can i get the full details of the project…

    21. Hai…..
      Can i get more information about this project.like discription,component rage,working process…..anybody plz send to my id….

      • vipul tomar says: on November 1, 2012 at 2:54 pm


        if u hav any information about this project
        plz… send me fast becoz my project summit date is 28 navamber……… plz help me

    22. Hey. I’m in need of a circuit in addition to a fire alarm to warn of smoke and carbon monoxide as well. PLEASE. Help? Thanks

    23. hello sir i need a complete discription abt tis prjct…. can we use this same circuit in AC rooms… what type of heating element should i use to explain…

    24. Aniket Ingale says: on September 14, 2012 at 3:28 pm

      A have made this circuit but it not works

    25. it is possible to make a special fire alarm basically for more marks in my project.

    26. its really help me doing finish my project ,thank you and sir i want to learn more

    27. sir can u send me the pcb layout of this circuit… because i dont know how to make it and it is my project …
      if snd it i m so thankfull to you..

    28. please ask me. i make this project but not work. use bc549 and bc556 an replace sl100 with 2n2222 this can be my problem? i can measure thermistor with ohmmeter can i test it?

    29. i made this circuit bt after giving suply it will not works . plz help me ….

      • This is a repeat of a reply made in Sept.

        Welcome the club of disappointed hobbyists. Remember that unlike humans, electrons never make mistakes.

        First recheck your wiring avoiding assumptions like: I checked that detail before so it is OK.

        Verify that T1 is NPN and T2 PNP. Measure cold resistance of thermistor.

        Divide circuit in half and test part of it. For instance, tie pin 4 of the 555 to +6V to see if the audio annunciator functions.

        Measure input voltage to D1 with voltmeter and determine that it changes with temperature.

        To set the temperature threshold more accurately, it may be necessary to replace R8 with a 5K pot.

        Above all, do not give up easily because what you eventually learn will improve your future trouble-shooting skills.

      • everything is correct still its not working…

    30. lizgrace says: on August 23, 2013 at 5:22 pm

      can the speaker be replaced by a buzzer??

    31. ghanshyam sharma says: on September 6, 2013 at 6:01 am

      sir I doing this fire alerm as mini project so I must to sme information……plz send to my id

    32. selvamthenmozhi says: on September 27, 2013 at 8:36 pm

      Sir..I need the detailed explanation abt this working principle..can u pls send me to my mail id on or before 28thsept

    33. selvamthenmozhi says: on September 27, 2013 at 8:36 pm

      Sir..I need the detailed explanation abt this working principle..can u pls send me to my mail id on or before 28thsept…pls sir

    34. This is one of the best circuits which I like very much. This circuit is mainly helpful for normal engineering students and as well as for general people also.

    35. i’m going to do mini project of fire fighting system using sensor and sprinklers plz help me….

    36. Muhammed Shareef says: on February 4, 2014 at 8:46 pm

      Sir,how to include sprinkler system with this circuit?how can i add smoke detection circuit wit this?

    37. Somanath Nayak says: on August 19, 2014 at 12:37 pm

      Respected sir,
      its a good circuitry and I have done it practically its working well,pls can u send me like circuit to my mail

    38. sir
      i need the detailed explanation of this project as i am doing this as my mini project.Thank u

    39. where is the detail …..
      how its work ????????????????

    40. ziad kotb says: on December 6, 2014 at 4:03 am

      if any body have a PCB circuit for the fire alarm
      send my or guide for me where is he

    41. Thank for information,give more information about fire alaram

    42. can someone send me some more information on this circit and how the role of each component works in order to make the circuit work.

    43. whay type of thermistor should be used in the circuit ptc or ntc plzzzzz hlp me

    44. N(V)E(A) says: on April 8, 2015 at 10:13 am

      I can’t find the SL100B,
      could you please give me an equivalent transistor?

    45. Merewai Benidito says: on July 16, 2015 at 5:11 am

      Sir, I bn attempting this project for couple of weeks and I’m still finding it hard bcoz it still doesn’t work. Please help asap. It’s due nxt wk

    46. shine chitimbe says: on August 4, 2015 at 6:40 pm

      Please send me the PCB layout of the fire alarm circuit using thermistor.Its urgent please!!!

    47. prachipatel says: on August 28, 2015 at 7:59 am

      I want more description of this project because i am designing this project in my 5th sem so i want more information about this project

    48. I tried this circuit. it worked! pm me if you need my pcb layout.

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