Here is a simple fire alarm circuit based on a Light Dependent Resistor (LDR) and lamp pair for sensing the fire. The alarm works by sensing the smoke produced during fire. The circuit produces an audible alarm when the fire breaks out with smoke.

When there is no smoke the light from the bulb will be directly falling on the LDR. The LDR resistance will be low and so the voltage across it (below 0.6V). The transistor will be OFF and nothing happens. When there is sufficient smoke to mask the light from falling on LDR, the LDR resistance increases and so do the voltage across it. Now the transistor will switch to ON. This gives power to the IC1 and it outputs 5V. This powers the tone generator IC UM66 (IC2)  to play a music. This music will be amplified by IC3 (TDA 2002) to drive the speaker. Resistor R6 is meant for protecting the transistor when R4 is turned towards low resistance values. Resistor R2 and R1 forms a feedback network for the TDA2002 and C1 couples the feed back signal from the junction of R1 & R2 to the inverting input of the same IC.

The diode D1 and D2 in combination drops 1.4 V to give the rated voltage (3.5V ) to UM66. UM66 cannot withstand more than 4V.

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Circuit diagram with Parts list.

fire alarm
Fire alarm circuit


  • The speaker can be a 32Ω tweeter.
  • POT R4 can be used to adjust the sensitivity of the alarm.
  • POT R3 can be used for varying the volume of the alarm.
  • Any general purpose NPN transistor(like BC548,BC148,2N222) can be used for Q1.
  • The circuit can be powered from a 9V battery or a 9V DC power supply.
  • Instead of bulb you can use a bright LED with a 1K resistor series to it.

Power supply for the circuit.

A well regulated power supply is essential for this circuit because even slight variations in the supply voltage could alter the biasing of the transistor used in the fire sensing section and this could seriously affect the circuit’s performance.

9V power supply
9V/500mA power supply circuit

A regulated 9V/500mA power supply that can be used for powering the basic fire alarm circuit and its modified versions is shown above. Transformer T1 is a 230V primary, 12V secondary, 500mA step down transformer. D1 is a 1A bridge which performs the job of rectification. Capacitor C1 filters the rectifier output and C2 is the AC by-pass capacitor. IC1 (7809) is a 9V fixed positive voltage regulator. The output of the rectifier+filter section is connected to the input of 7805 and a regulated steady 9V is obtained at its output. S1 is the ON/OFF switch. F1 is a 500mA safety fuse.


Relay version of the circuit.

Here the above fire alarm circuit is modified to operate a relay when the fire breaks out. The usage of relay makes the circuit able to switch high power warning devices like alarms, bells,beacon lights etc that operates from the mains.

Relay version of the circuit

Two additional transistors are used with the basic fire sensing circuitry (consisting of Q1, R4, R5 and L1) to attain the target. Whenever the fire breaks out the transistor Q1 is switched ON. The collector voltage of Q1 drops to 0.2V and the transistor Q2 gets switched OFF. This makes the collector voltage of Q2 rise towards 9V and this result in the switching ON of transistor Q3. The relay connected at the collector of Q3 is activated and the load connected through the relay contact is driven. Resistors R7 and R8 limits the collector current of Q1 and Q2 respectively. D1 is a freewheeling diode which protects Q3 from the voltage spikes induced when the relay is switched. Resistor R9 controls the base current of transistor Q3 (2N2222).




  1. that is very nice circuit diagram infact iam doing aproject which more related to it and i would to request you to send me “a fire alarm system with a sprinkler water circuit diagram” based on LDR,urgent pliz

  2. please can you help me for the relay version of the circuit through the email.



  4. plz tell about the connections through mail am waiting

  5. what the apparatus of this circuit or give detail how can connection between the apparatus

  6. why capacitor is connected in series with speaker while circuit is run by dc input…..

    • Seetharaman

      The TDA2002 IC pin no4 will be at half the power supply (say around 4.5 volts in the above circuit) to avoid this DC flowing through speaker a blocking capacitor is used such that only the audio signal present at Pin no4 alone is fed to the speaker.

      • thanks a lot…. but what is the function of capacitor c2 which is in series with speaker

        • Seetharaman

          Function of C2 alone explained in my reply.

  7. Talaviya Mehulbhai Balubhai

    I like this information and my problem why rasister connected in circuit example:-R6 1K but why not connected 5K

  8. I like this project.I suggested this one to my partner in mini project lab.But I also need the project report

    • Smith Watson

      Hey Bini did u get the project report for this circuit. It wil b great if u help me with it.. Thanks..

  9. hi, can anyone tell me what to use as a substitute for um66?? It doesnt exist in proteas…

  10. can u suggest any other ic to replace TDA2002 ,as TDA2002 does not exist in proteus software for my circuit simulation …thank u for ur help

  11. Hi. I just want to know what would happen if you remove the um66? or if there are any substitute for um66

  12. ee student

    how if i still want the speaker as the output but at the same time want relay in my circuit?i’m doing this as my project

  13. sufian ismail

    sorry i am ask about the function of relay of this alarm circuit…it will effect if we do not make the relay for the alarm circuit

  14. Michael Zurg

    Thank you so much for this amazing circuit .
    This site is breathtaking.

    I want to use a Silicon Controlled Rectifier (SCR) instead of
    NPN Transistor . What changes should be made to the circuit in this case?

    Email me ur recommendation or reply to my comment .

    Thanks guys.

  15. harikishan

    Guys i am going to make the fire alarm project.and i have no large idea for make this project. so i requested to all the internet users and you have any information of this topic so please share me yours information and help me this project.

  16. sulthana parvin


  17. Prosenjit

    For the power supply circuit, what is the model of the diode that can be used.

  18. S.A.Warsi

    Wow,its awesome,they have to mention steps of construction with the help of video.

    • Thank you Warsi. We will try to develop videos soon.

  19. vikki yadav

    Guys i am going to make the fire alarm project.and i have no large idea for make this project. so i requested to all the internet users and you have any information of this topic so please share me yours information and help me this project.

    • John terry

      vikki am also doing my project on fire alarming .. if you have any information share with me .. thanx sis

  20. doreen iortsor

    Please i need an alternative for the um66 since it is difficult to find. I wil appreciate if you can help me out because this is my project topic. Thanks.

    • seetharaman

      Hi Doreen you can use electronic peizo alarm from Pin 3 IC1 and 2 by observing the polarity through 100 ohms resistance feed the junction of Piezo buzzer and 100 ohms resistance to Pot R3.

  21. ashish butola

    i am going to make a project on the fire alram so i want that you should guid me to make this project. i am in confusion that what requriment is for the fire alram so please help. i need to submit this project on next week. i hope that you will short out my problem.

  22. Abdulrahman f.c.e(t)bichi kano

    Helo guys am this project now.and i ned detail information on this components use in this project? Thanks alot it urgent pls

  23. Seetharaman

    Hi John you can use 4nos of 1N4007 in bridge formation. (use 1N4007 for each diode shown in the bride circuit with correct polarity)

  24. alternative for 1A bridge in power supply ??? thanks

  25. Olawale kabir

    please i dont no if there will ba an alternative for UM66 in fire alarm circuit please help me it is orgent please. this my mail ( )God bless u

  26. Iam not able to locate a circuit which tells me the wiring of an industrial smoke and heat detector system
    can all detectors be wired in parallel/series or what tis the single line diagram illustrating this.
    how many detctors can we install in one zone and how are they connected to each other ??

  27. abbas rezaeian rad

    hi , thank you for this project , that’s very Useful ,

  28. Mohammed Afzal

    Hi Mr Qalvin,
    Am dealing with the fire alarm Installation and i have some Queries about connection of the Manual Pull station.
    In our Project we are using NBG -12x Addressable Pull station and the Wiring System in the Catalogue Describes About SLC -ve And +ve of the Fire alarm Panel And the next addressable Device.
    And the cable we Ordered is they have +ve and -ve wire i dont understand in the described cable which one acts a SLC Wire.

  29. Hi guys, am a student in Electrical & Electronic Eng. Post your questions on fire alarm systems, i’ll gladly answer.

    • pls send me the working principle of the fire alarm system

  30. gunashekar

    We have three cherrytech made fire alaram pannel which doesn’t work even one. please help regarding rectify the problem

    Thank u Regards


  32. abdulaqhil

    iam student of electrical engineering i just wanted the information of the working of smoke detector

  33. akash kumar meher

    how can it be done with the STARTER, SPEAKER & POWER SOURCE? pls reply

  34. I am a studen of electrical and electronics engineering.I need a fire alarm circuit with details .

  35. hi!i am electronic engineering student.
    i need details info and circuits diagram of ‘smoke detector with auto water sprinkler’.
    mail me.thank you.


    Am a final year student pursuing bachelor of science in electrical engineering. Am in need of a software which i will use in simulation of my final year project on June 2012. The project deal with designing of electrical installation and fire alarm system in a seven floor building. Please post Any suggestion or advice in Thanks

  37. nsumba geofrey

    i would like to inquire if there can be another replacement for the IC UM66 because i have failed to find one please in need urgent help


    I am an electrical engineer working with COMA TRANS S.A.DOUALA CAMEROON.Thank you for this program very much god bless you. will be great full if the diagram of an electronic rectifier of an alternator of about 120kva can be sent to me thanks

  39. Abumbe francis

    Hi am a student of electrical electronics engineering with power/machines as my option,pls i need workable power project topics,to construct as my final project.pls u can aid me by sending the topics to my email,,

  40. fire alarm can be made by using only the starter of the tube light…..its the simplest fire alarm ckt
    it reqire 1 starter,speaker and a source of power supply

    • rashid kilongola

      correct,that is best way to design simple fire alarm

  41. seetharaman

    Hi Ajay you can use standard good quality 8 ohms tweeter.

  42. sir,
    iam doing this fire alarm as the, i
    want brief explanation in this aspect.
    pls send me on my id.
    i need it urgently

  43. i m from the stream electronics & communication. It was my mini project. Thanxxxxxxxxxxx a lot , bcz i hv understood completely this circuit. Now i m very happy.

  44. i would like to have more information on this circuit as in the schematic circuit and how the circuit is connected…thanks

  45. fire alarm circuit.

    Hi! i am an electrical technology student
    i need some innovated projects to help…
    so please i request you to send me the projects via the e-mail which given below and thank you so much!

  46. Seetharaman

    Hi Kart it is just a smoke detector not the temperature detector. the proverb is where smoke is there fire is there. when the smoke passes through the light path between lamp and LDR, it resistance value changes it is detected by the electronics and through relay it can give alarm or the relay contacts can operate a solenoid which sprinkles water during fire.


  48. please, i need the project report on fire alarm system with automatic water sprinkler

  49. padmakumar

    iam an ECE student plz mail the ckt diagram of “FIRE DETECTOR WITH AUTOMATIC WATER SPRINKLER SYSTEM” with project report .. urgent plz (mail not publish)

  50. Seetharaman

    Hi Hanister any audio amplifier IC capable of delivering around 2 to 3 watts of power will do. This IC is a simple 5 lead package with heat sink tab similar to TO220 casing, easy to fit heat sink.

  51. is there any replacement for TDA2002 IC for the fire alarm?

  52. siva sankar

    hi i am an ece student
    i want some new innovative projects….
    so i request you to send me thr projects via email given below

  53. hi I am electronics engineer want to know working priciple and circuit diagram of fire alarm system.Also let me know
    components used in this system .Thanks

  54. hi?i’m called Lacoste and i’m in final year electronics and telecommunication engineering.i want to do my final project on fire alarm detector,can u plz send the report for fire alarm,and some informations.thx God bless u.


  56. hello, thanks 4 ur efforts. pls i need you 2 send me the details of this Fire alarm and 2km fm transmitter:how they work and all the principles involved. i want to do it for my final year project

  57. Seetharaman

    Hi Ban it is a pentawatt package IC. But you can use any audio IC here with suitable PCB. TBA810, LM386, TDA2822, TBA820, any chip which can work with 9 volt and gives an audio output of 1 to 5watts to 8 ohms speaker.

  58. is their any circuit for fire alarm using only one ic..

  59. hiiiiiiii
    plz upload pcb layout of this ckt plzzz…
    i need it urgently…..



  61. sayyed muzaffar

    Hello I am an Electrical Eengin I need some Projects especially Automatics, sensing & control circuits please. P.O.Box STAR ELECTRONICS BHIWANDI DIST THANE M.S INDIA
    Pin 421302,

  62. seetharaman

    Hi Shubham you can use. the light should be always falling on the LDR R5 whenever smome passes between light and LDR the light ray will get interrupted and the alarm will get activated.

  63. is it possible to use light activated switch for this purpose. .

  64. seetharaman

    Hi Timmy you can use NE555 oscillator in place of UM66.

  65. which Ic or component can i make use instead of UM66 in a fire alarm circuit please I need the reply B=before 24hrs.
    thanks God Bless

  66. which Ic or component can i make use instead of UM66 in a fire alarm circuit please I need the reply before 24hrs.
    thanks God Bless

  67. which Ic or component can i make use instead of UM66 please I need the reply B=before 24hrs.
    thanks God Bless

  68. please i need a circuit diagram and guidelines on fire alarm system

  69. pls i need a good crcuit diagram and guild lines on how to build it

  70. haii am an ectronics student i want to know more about fire alarm so plz send me detail report of this subject .plz heip us.

  71. seetharaman

    Hi Syarief this fire alarm is based on smoke detection principle. A light beam falls on LDR, so its resistance is low hence Q1 cannot conduct. once smoke obstructs the light, the intensity of light falling on LDR reduces hence its internal resistance increases. Q1 gets the bias inturn regulator IC1 gets the supply. Regulator IC feeds the supply to tone generator IC its out put is amplified by TDA2002 ic Hence the loud music from the smoke detector indicates that there is smoke. but smoke is normally due to fire.

  72. hye…im a student of electrical and electronic engenering..can u send me your fire alarm report..i need the detail..tq..

  73. yannick noah

    hi,i m verry happy for your support provided ,so ,i’ll like to get all notes concerning fire alarm circuit ,in oder to improve my reseach .thank u

  74. saheed shittu

    please,i need a digital calendar circuit diagram project that show days, date,months and years, using discrete components with its operation principle and detail about circuit diagram.send it to my e-mail.thank you very much.

  75. helo i am a std electronics eng pls send me the whole description of fire alarm so i can built the project pls

  76. hey i implement this circuit today but i was failed to do it properly..!there was a little beep in the beggning but then it stopped.can any body send me schematic diagram of this Circuit?

    • admin

      You need to calibrate this circuit before using. Adjust the POT and try again.

  77. om prakash

    hi,i am a student of electronics . please send me your project report on fire alarm .i really want 2 study that plzz..e-mail me ..

  78. sir/ madam,

    I would like to know abt the names & authors of any text book that contains all electric/electronic circuit concepts.


  79. hello sir, i m electronics engineer from pakistan, i want to make a circuit for ups who’s capacity should be atleast 1000watts.
    Regards……..kaleem butt
    cell # 00923457866765

  80. demo lynn

    hi, i m an electronic student, i m finding the fire alarm system using IC CD4049, so i would like you to send me some information on how to design a smoke sensor, i m doing my project now & wanna do better fire alarm system, pls send me some details,it’s very important bcauz my presentation is on Nov 11, 2009, waiting ur reply

  81. olaiya shola

    Hi, am a student and i want to do this project . plz i want all the details about this project.thanks

  82. hi this is mubasahr i whould like to make a circuit which is time dependent like when we turn on the electric supply alarm will turn on only for 20 seconds and after 20 seconds alarm turn off

  83. Is it possible for me to have some more and relevant informations about the two transister siren, pliz

  84. hi, am a student and i want to do this project so plz i want all the details about this project


  85. Dear Sir/Madam
    I am an electronic student and i would like you to send me some information on how to design a smoke alarm sensor.
    Very Important ( Please do not publish my email)

  86. M.Sri Lisha

    Industry or home already has lights. So does the LDR gets affected and what is the role of LDR. Smoke alarm using LDR and bulb and preset what is the significant role of ecah. Please mail to my email-id.

  87. can u please provide me with the circuit diag. of the detector which includes diff another alarms along with the fire alarm….plz mail me…

  88. Dear Sir/Madam
    I am an electronic student and i would like you to send me some information on how to design a smoke alarm sensor.
    Very Important ( Please do not publish my email)

  89. Pls I need 1000watts and 2000watts comprehensive circuit diagram for an inverter/ups. Thanks. God bless you.

  90. twits4twats

    Yes you can if you so desired. It would be a bit pointless though as the purpose of using a pot and not resistor is so that you can adjust the signal.

  91. thanks sir i want’s that it’s my requared for my project.

  92. Plz i want to build a 2000Watts inverter pls send me the circuit diagram and the rating of the components required to my email address.which is thanks.

  93. Nice easy fire alarm circuit and its operation is simple.I like the circuit and expect i do it.

  94. what should be the value of the ldr?? or its range??

  95. i am an electrical/electronic student,and i am very good at reading circuit and constuction,please i want you to send the fire alarm circuit that made by micro .

  96. matjopile Tseliso

    hi there! i want 2 build a magnetically levitation train. can u send me more details about the construction. i want 2 use solenoid and permanent bar magnets. hoping 2 recieve it in my mail provided 2 u. thanx

  97. Christian Ehifoh

    i am an electrical/electronic student,and i am very good at reading circuit and constuction,please i want you to send the fire alarm circuit and its working principle,so i can construct one.thanks.hoping to recieve it in my mail provided to you.

  98. Yahuza Umar Adam

    Hello I am an Electrcal /Electronics Eengineering Student, Ineed some Projets espeially Automatics, sensing & control circuits please. P.O.Box 7136 Bompai, Kano Nigeria.

  99. is there any new upgraded circiut on fire alarm circuit