# Simple fire alarm- thermistor circuit diagram

Here i-St@r comes with a simple and automatic fire alarm and detection system. This fire detection alarm circuit is based on thermistor. What is a thermistor? A thermistor is nothing but a variable resistor whose resistance changes with temperature. Fire detection is simply possible via this low-cost alarm circuit.

A simple potential divider arrangement using thermistor is capable of sensing the temperature (presence of fire) and alerting us with a warning signal. This is suitable for your home security systems. Also school students can do this fire detector circuit as their high school science fair projects. Here I will explain in detail about fire alarm circuit wiring with animation/ simulation of fire alarm.

## Components required

1. Power supply
2. Resistors (1kΩ)
3. Potentiometer (10KΩ)
4. Capacitor (10µF, 16V)
5. BC548 transistor
6. High resistance thermistor (NTC)
7. Diode (1N4001)
8. 6V Buzzer

## Working of fire alarm circuit

• The thermistor and R1 forms a potential divider network which drives the transistor.
• The heart of this circuit is a thermistor. Thermistors are low-cost, easily-available temperature sensors widely used for uncomplicated temperature measurements.
• Thermistors are temperature sensitive resistors. When temperature increases resistance offered by the thermistor decreases and vice versa. At normal temperature, the resistance of the thermistor is around 10kΩ.
• All resistors vary with temperature, but the semiconductor materials used for thermistors are especially sensitive to temperature.
• The transistor is turned ON by the voltage drop across the resistor R1.
• Consider the temperature of the atmosphere is around 25°C, and then the resistance of thermistor is high so the voltage across the thermistor is also high according to the basic ohm’s law V=IR.
• The threshold voltage can be set by varying the potentiometer (R1).

• At this situation voltage across resistor R1 is low and it is not sufficient to turn on the transistor.
• As temperature rises, the resistance of thermistor decreases so that the drop across the resistor R1 increases which turns ON the transistor.

• When the transistor is turned ON, the current from Vcc starts to flow via 6V buzzer which produces a beep sound.
• Diode is used for enabling unidirectional conduction and the capacitor removes sudden transients from the thermistor.

## Simulation of the fire alarm project

We have used an LED instead of buzzer for the simulation of fire alarm system.

## 108 thoughts on “Simple fire alarm- thermistor circuit diagram”

• Jaseem vp says:

Hi pankj khedekar
Please try a 10K potentio meter instead of 470 ohm resistor and set the threshold voltage by varying it.

• vijayalakshmi says:

Have u used the 1 k resistor.My buzzer is ON, even before fire.What are the corrections made in the circuit given here?

• Jaseem vp says:

Hi vijayalakshmi,
Sensitivity of this circuit can be adjusted using R1 potentiometer. Try to vary R1 and test the circuit.
Our circuit is 100% working and I added Video Demo also.
My recombination are
1. Adjust R1 Pot
2. Crosscheck the components

Please let me know your further improvements in the circuit

• kedar says:

@Jaseem vp
thank u 4 d fire alarm ckt,
pls keep on sharing d other electronic ckt.

• rajat says:

Sir plz u can give me IR module sensor circuit. ..

• SOURAV BISWAS says:

Even I m also having the same problem my buzzer is on before the fire. What 2 do?

• chandu says:

tku.sir for shareing ur knwolge with me……………

1. Prasanth says:

It was very simple for understanding………….

• Khaleel says:

Happy to hear that :).

2. wankhairri says:

do you have the truth table and k map for this

3. farhan says:

sir, i want to do this project. but i want the confirmation that this circuit is practically work or not? thanks

• Khaleel says:

Hi Farhan,
We’ll come up the working video soon, hope that would do the confirmation…Keep visiting……

4. rajesh says:

not working this project

• Khaleel says:

Hi Rajesh,
We had made a little changes in the circuit, so that you could get the output easily. Watch the video.

5. ravi says:

i want to know value of thermister..,current source of power supply…,full details of this circuit…,pls……..

• Khaleel says:

You can use any thermistor, but change the potentiometer accordingly (with respect to resistance). Power supply 6V 500mA DC.

• Adams Nigeria says:

If I use any Thermistor, how do I change potentiometer accordingly. I have been connecting the circuit, but it does not work at all, buzzer sound at all times. Please tell me something………Thanks

• Yaseen says:

Hi Adams
If your thermistor is 100K use 47K pot.

6. Carl says:

Hi sir , what thermistor should i use?

• Yaseen says:

Hi Carl
You can use high resistance thermistor.

• Carl says:

How much sir ? 10k ohms?

• shabir says:

Why we use 9v battery
why not 11 v

• Jaseem vp says:

Hi shabir,
It will work for 11V also

• Yaseen says:

Yes of course

7. Carl says:

sir which is the true value for resistor , in the schematic or in the components required?

• Khaleel says:

It’s 1k as in the schematic and it has been changed in the components too. Thanks for notifying.

• Yaseen says:

Hi Carl
You can use 10K thermistor.

• manish says:

how to use ldr. give me circuit diagram

8. shouvik says:

can i use ldr insted of thermister

• Yaseen says:

Hi Shouvik
You can use LDR too.

9. julian says:

is it not okay to use 100 ohm thermistor sir? i tried to use it and the speaker turned on even without presence of heat

• Yaseen says:

Hi Julian
If you are using 100 ohm thermistor you should use 22ohm POT (Here is a possibility to burnout POT, due to high current, to prevent it use high wattage POT)

• md. shahjalal says:

want to get total fire system design.

10. GHAB says:

Good Morning. What is the value of thermistor you used in this project? We can’t buy thermistor because we didn’t know the exact value of thermistor for this project. PLEASE REPLY AS SOON AS POSSIBLE. Thank you so much.

11. Shaurya says:

I would like to know what is the difference between 10uF/16V and 10uF/63V.

Also, can the latter one be used in this circuit?

• Yaseen says:

Hi Shaurya
Voltage rating indicates that the maximum voltage that the capacitor can operate, for 12V circuit you can use 16V and above voltage rated capacitors.

12. yamna shakil says:

itz working 100% i’ll made it

13. NISARG says:

hey, i think the page has been changed and the circuit diagram too. the last was the 470 ohm resistor used. my project worked on it. i need that old page urgently. please send me the old page as soon as possible.

• Khaleel says:

Both cases it will work, you can set the pot to 470 Ohm.

14. Adrian says:

Sir, what if I use a 9V battery?

15. shrutha jain says:

is it possible to replace buzzer with any other component?

16. tom says:

Thank u so much . It helped me a lot for my project. The circuit does work. It’s cool

• Khaleel says:

Glad to know that.

17. Rama Kumar says:

i want to know the function of all the equipment . And the k-map of the circuit.

18. Harsh Verma says:

Hello sir, my circuit is not working can you tell me why. I have used 10k ohm variable resistor and then also it is not working. Reply pls….

• yashwant shende says:

this cirucit is not workung model because it has no proper bising for transistor. you can remove first of all capacitor and diode. after that give proper baise to bc547. u wii get success.

19. Harsh Verma says:

Sir my circuit is not working. Sir, pls can you make FM receiver circuit so that I can make it. Pls Sir……….

• Khaleel says:

Check your components, try using a high resistance thermistor.

20. Qamar says:

Hi guys,
i followed this guide for a class mini project, and i encountered a problem, my buzzer will sound even when there was no heat.
the problem was the THERMISTOR itself, u see these are of two types, NTC vs PTC.
NTC works by dropping the resistance as the temperature rises, PTC does the oppositte.
so make sure u use the right type, here we need NTC.
cheers and thanks to the Author for this guide

• Khaleel says:

Hi Qamar,
Actually I forgot to mention it, thanks for reminding and best of luck for your project.

21. janine joyce siva says:

i tried constructing the same circuit as shown in the video. sir, if you permit,can you tellin here the steps you did in constructing the circuit.?thankyou.

22. shivangi says:

plzz sir this project mean for me alot…. plz understand my filling and send how to arrange on breadboard …..plzzz

23. Khaleel says:

Hi Shivangi,
At this point we are helpless, due to our busy schedule we won’t be able to prepare the video you requested. May be we could do it at a later time, it seems that you’re a beginner in electronics.

Please comment the problem your’e facing with the circuit so that we can figure it out. Never loose hope and keep trying. All the best.

• shivangi says:

sir i will sent you my circuit which i made… plzz give me your email id to send pic…plzz i tried so much but i fail so plzz he me and take out of me……..
if uh dont that only send me pic of joined circuit but plz take it clear….

• Khaleel says:

You can contact us at admin@circuitsgallery.com. Let us know your grade, and don’t post the same comments again and again.

• Khaleel says:

I analyzed your circuit arrangement, please make the following changes
1. Take preset connection from middle and anyone of the end points.
2. Interchange connections to the buzzer.

Now vary preset and check the output. Hope that helps.

• shivangi says:

i cant understand what you say ..plz explain mr in easy way…..which changes i do in circuit…tell me and where i use 1k resistance and 10k resistence??

24. mahunisha says:

give a brief explanation

25. Harsh Verma says:

Sir I have maded this fire alarm by removing 10 uf electrolytic capacitor and changed 10 kilo ohm variable resistor with 1 kilo ohm variable resistor thank you sir.

• Adams Nigeria says:

Pls……….can you send me the picture of your circuit arrangement

26. socrates says:

what will be the total cost for this

27. Shashank says:

Thank you very much.
Its simple,easy to understand and works well.

• santhosh says:

ny change u have made in that circuit? as mentioned above

28. ahmer says:

how we make the simplest circuit of alarm indicator for high temperature

pleased !!!!!

reply me

29. Marco Dlamini says:

That’s a simple circuit and operation to detect fire. may you guys explain the block diagram of this circuit.

30. Aliyunjobdi says:

I want to work on it this year 2015, But Sir is there any way of constructing it using the must least cost of materials,pls. i’m waiting because i need an advice through my email.Thank

31. himanshu says:

what specific type of thermistor should i use

• Yaseen says:

Hi Himanshu
use 10K ohm Thermistor.

32. nilesh says:

Is it possible to replace buzzer by anything else??

• Yaseen says:

Hi Nilesh
Yes, you can replace it by a LED or Relay.

33. banu says:

How to make the continuous beep sound in the buzzer….when i light the thermistor…….any help plz…..??

• Yaseen says:

Hi Banu
Use Mono stable multi vibrator

34. RAJESH says:

FROM WHERE WE HAVE TO GIVE POWER SUPPLY THERE VCC=6V
PLEASE CAN YOU JHELP ME FRND IF ANY REASON PLEASE

• Yaseen says:

Hi Rajeesh
You can use 12V too, but replace 6V buzzer with 12V buzzer.

35. Ali Haider says:

thanks for uploading project working.

36. Utkarsh Verma says:

Pls help me how to wire the 3 pins of pot

• Yaseen says:

Hi Utkarsh Verma
Shot circuit center pin and one side pin then you get two pin pot.

• Adams Nigeria says:

You are very helping Mr Yassen. I’am trying this circuit, but buzzer keeps sounding all the time without fire. How do I correct this error

• Yaseen says:

Hi Adams
Use one more diode in series to the base.

37. SMITHA says:

Thank u so much…it really worked…it helped us a lot…thank u once again:-)

38. akshay says:

i simulated this circuit using pspice and i’m new to pspice simulation software,so.. wanted to know how the output of this circuit will look like in a CRO,,
Thank you,

• Yaseen says:

Hi Akshay
You can see voltage variation when temperature increase.

39. Vaishnavi says:

I want to make this alaram for my science project… But I just want to confirm that it really works or not…???

40. Aml says:

Please can you write the analysis of this circuit

41. Asheesh rai says:

I am using 100ohm thermistor and 22ohm resistance in the place of potentiometer. My buzzer is sound continuously. And thermistor is also becomes hot after some time. Please help me.

42. surekka says:

sir,i use only 1okohm pot it does not give as variable type and thermister is very small in black colour this type of thermister can i use, and the problem is it produce light the thermister does not heat,pls reply me as soon as possible

43. surekaa says:

sir, i use 10k thermistenot and 10k pot also but it does work. the circuit is always on even i does not given a heat. can you given solution immediately.plssssssss.

44. Mahan says:

Hi friend
I just ask from you’re this ckt working or not?(100&working) and I want to if it working good
How much amp I need 6VDc & how much amp?

45. vidya says:

hi. this circuit is working. plz give
me idea about project report

46. Mahan says:

Hi friends
Can I change thermistor with it sensor that it can working
With infrared instead heat?

47. Mahan says:

Can I change thermistor with ir sensor ? If I can what should I do? I want that it working with it beam instead heat?

48. kajal sikdar says:

Can I use 9v battery in place of 6v battery???

49. Hitesh Patil says:

Sir,
Would you let me know the working temperature range of fire alarm using above components & circuitry?

50. vinodh says:

Hi sir the circuit even when the thermister us not heated pls tell what to do
For 10k thermister which value of pot should be used

51. vashini sharma says:

what is the set point of temperature for this fire alarm? I mean at what temperature this fire alarm will be activated because when a match stick will be ignited then also the thermistor will sense it and the circuit will be activated

52. santhosh says:

have you used resistor 1 kohm?

53. Samilou says:

Hi. I have a 12 v buzzer available and I want to use it in the circuit. What changes will iI make to my circuit?
Thank you

54. Prasanth T Paul says:

For 10k thermister which value of pot should be used.if i use 10 k pot them what abut thermistor rating. also tell me how they are related.

55. PRASANTH T. PAUL says:

For 10k pot which value of thermister should be used. also tell me hw they are related.

56. R says:

can u please take another clear picture of the circuit u built? Please it’s for a project and i need a reply as soon as possible.

57. Timie says:

hi guys… this is cool.

58. Pankaj says:

I want to know the use of R2, without R2, what happens..

59. Confidence says:

love this! but I am a newbie .please vcc stands for what? and what are d two terminals for d supply voltage?.thanks in anticipation

60. arash says:

hello,
my company needs a industrial fire control systems.
would you give us any information about your products?

61. Maverick says:

why do we need a capacitor in this project? please help.

62. Pradeep says:

Pls give the video link for this project….i need it

63. manjunath says:

It’s really working I made it..nd very useful