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    This fire sensor circuit exploits the temperature sensing property of an ordinary signal diode IN 34 to detect heat from fire. At the moment it senses heat, a loud alarm simulating that of Fire brigade will be produced. The circuit is too sensitive and can detect a rise in temperature of 10 degree or more in its vicinity. Ordinary signal diodes like IN 34 and OA 71 exhibits this property and the internal resistance of these devices will decrease when temperature rises.

    The fire sensor circuit is too sensitive and can detect a rise in temperature of 10 degree or more in its vicinity. Ordinary signal diodes like IN 34 and OA 71 exhibits this property and the internal resistance of these devices will decrease when temperature rises. In the reverse biased mode, this effect will be more significant. Typically the diode can generate around 600 milli volts at 5 degree centigrade. For each degree rise in temperature; the diode generates 2 mV output voltage. That is at 5 degree it is 10 mV and when the temperature rises to 50 degree, the diode will give 100 milli volts. This voltage is used to trigger the remaining circuit. Transistor T1 is a temperature controlled switch and its base voltage depends on the voltage from the diode and from VR and R1. Normally T1 conducts (due to the voltage set by VR) and LED glows. This indicates normal temperature.

    When T1 conducts, base pf T2 will be grounded and it remains off to inhibit the Alarm generator. IC UM 3561 is used in the circuit to give a Fire force siren. This ROM IC has an internal oscillator and can generate different tones based on its pin connections. Here pin 6 is shorted with the Vcc pin 5 to get a fire force siren. When the temperature near the diode increases above 50 degree, it conducts and ground the base of T1. This makes T1 off and T2 on. Alarm generator then gets current from the emitter of T2 which is regulated by ZD to 3.1 volt and buffered by C1.Resistor R4 ( 220K) determines the frequency of oscillation and the value 220K is a must for correct tone. To set the fire sensor circuit, keep a lighted candle near the diode and wait for 1 minute. Slowly adjust VR till the alarm sounds. Remove the heat .After one minute, alarm will turns off. VR can be used for further adjustments for particular temperature levels.

    Fire Sensor Circuit Diagram

    Diode based Fire Sensor circuit diagram

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    33 Responses to "Fire Sensor Circuit"

    1. dear sir please tell me which amount of impedence of speaker is perfect for this circuit.

    2. maa di lun ………………

    3. fuddu bna rahe ho………

    4. hello your web site is very good i,m from iran and thanks fram you

    5. hello sir your web is very good

    6. i am making a final year project on fire fighter robot. I got very important data for my project from your web sight. thank you very much.

    7. can u give more information of electronic componenets tutuorial…it will be every useful

    8. can we use 0a79 instead of oa71?????

    9. WHere CAn i Get IC um3561 FROm ORCad

    10. sir..what if IC UM3561 does not exist in the circuit, what will be a good alternative IC can be used in this circuit??

      is it ok to use can ZD 3V instead of ZD 3.1??

    11. your website is great…

      what other IC can be used in this project??
      is it ok to use ZD 3V instead of ZD 3.1V??

      please answer this… i really need it for my project..
      thank you very much

    12. Nilesh Trivedi says: on August 2, 2011 at 8:25 pm

      I have seen your circuit its good and
      very handy(easy available components).
      I would like to know more about germanium diode. (OA71 OA79)

    13. Is there any IC that can replace it.

    14. Hi Site Owner,

      Thank you for all of your efforts. I’ve been roaming around your site to look for a circuit that has no complaints from the users. And i found this.

      More Powers!


    16. haw can you make a fire sensor to open a door?

    17. Ayanangshu Banerjee says: on February 2, 2012 at 9:01 am

      Dear sir here u said that vr needs to be adjusted to get the optimim temparature. So in emergency purpose how will it work if we do not adjust vr. It is also impossible to know the the actual tempareture of fire in any place. So will it always require the adjustment to operate. CAn there be any arrangement by which it can operate in a matchstick flame also.

    18. Dr. I want to do this circuit in Proteus program but I couldn’t find UM3561 so please if there is any IC in same work of the previous one please tell me

    19. Ranajit Banerjee says: on April 11, 2012 at 4:10 pm

      if we use bc 547 instead of bc 548 will there be any difference?

    20. sir i am going to making my final year project about GSM base home security. which involved the fire sensor, IR sensore, lpg gase sensore gsm phone , microcontrooler so sir can you help me any thing about that project.. so plz reply me i m wating.

    21. can u give more information for this fire sensor circuit.. and this is reallu useful

    22. hello sir…your website too good..,
      i want full explaning of fire alarem circuit..of full description,working and logical qusns on that circuit..,
      sir please send that details…
      thankyou very much…!

    23. What is VR 1M on top left corner of the circuit diagram

      • Jim Keith says: on November 2, 2012 at 6:27 pm

        1M potentiometer wired as a variable resistor. Must have been added at a later date due to different drawing convention being used.


    25. i wants a new mini project ,
      this is very nice

    26. sir ,
      i am not able to find OA 71 Heat sensor :(

    27. can u tell me what is VR1M ??

    28. can u please tell me what is VR 1M used in circuit ??

    29. hello,your website us some how very good.so that ineed some temperature that can be detected by this circuit.am from Rwanda.

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