## Fire Alarm Using Thermistor

Many fire alarm circuit is published in different website. But, here in this website is a simple and inexpensive project of fire alarm using thermistor. where thermistor is used as temperature sensor of fire alarm.Working principle of thermistor is same as LDR (change their resistance with change in heat where LDR change their resistance with change in light fall on it).

### Circuit Description of fire alarm using thermistor

The whole circuit of fire alarm using thermistor is build and fabricated around thermistor (TH1) and timer IC (IC1) with its driver transistor. The timer IC (IC1) used in this circuit is as astable multivibrator oscillator used to oscillate in audio frequency band. The two transistor T1 and T2 used to drive the timer IC (IC1). The output from pin 3 of IC1 is fed to loudspeaker through transistor T3 to generate sound. The value of resistor (R5 and R6) and capacitor (C2) determines the frequency of IC2.

The low resistance path of extend positive voltage to the base of transistor is provided when the thermistor TH1 become hot. Further collector of transistor T1 is connected to base of transistor T2 provides positive voltage to reset pin 4 of IC1 for reset. Fire alarm using thermistor circuit works on wide range of input power supply voltage i.e. 6v to 12V.

#### PARTS LIST

Resistors (all ¼-watt, ± 5% Carbon)

VR1 = 10 Kohms Variable Resistor for changing the sensitivity of the circuit.

R3, R7, R8 = 470 Ω

R2 = 33 K Ω

R4 = 560 Ω

R5 = 47 KΩ

R6 = 2.2 KΩ

C1 = 10 µF/16V

C2 = 0.04 µF

C3 = 0.01 µF

#### Semiconductors

IC1 = NE555 (timer IC)

T1 = BC548

T2 = BC558

T3 = SL100B or any Medium power general purpose NPN transistor like: 2N4922 , 2N4921,2N4238, FCX1053A

D1 = 1N4001

#### Miscellaneous

TH1 = Thermistor 10 KΩ

LS1 = 8 Ω, 1W speaker

## 18 thoughts on “Fire Alarm Using Thermistor”

1. Because we are using a thermistor and sensing heat ; the placement of the circuit plays a key role in detecting the heat and thud fire.
For better fire sensors we must use smoke sensors but this circuit should also work correctly in most cases and if the circuit does not work properly please change the value of VR1 to change the sensitivity of the circuit.

For the demonstration purpose please fire a simple candle or match and bring it near the thermistor.

1. Joshua says:

I tried to make it but the thermistor heats up after I hook it up with the battery.

1. Atreya says:

I’m not finding the transistor SL100B in the market so can you suggest any equivalent one?

1. Thanks for your comment the part list is now updates with some replacement for sl100 transistor.

1. Atreya says:

Thank you so much.

2. Anonymous says:

Which diode is this that you are using here? Is it a germanium diode or a silicon diode?

3. Anonymous says:

Which diode are you using here? Is it a germanium or a silicon diode?

4. tux119 says:

Hello,

This system has a dysfunction:

either there is fire, or no fire, whenever you connect the thermistor, between +6V branch and diode extremity the speaker continuously beeps.

it is not fire dependable, the sound from the speaker only stops when you open circuit the system in the place of TH1

????

5. Anonymous says:

what will happen to the circuit if IC stops working???
is there a chance for this to happen??

6. Physicslover says:

In my circuit, the speaker constantly makes a sound irrespective of whether the thermistor is heated or not. Till last month, it wasn’t so… what could be the problem?

7. vishnu says:

how do you find the resistors values?
why u are using 10 micro farad capacitor?
pls explain….

8. nameer ahmed says:

Speaker is continually producing sound even if we are not providing heat

9. yuvraj singh says:

this is amezing. really.
i join just and found a lot of thing which for i have been seeking for a month. thanks to admin to open a such plateform

10. koushik.k says:

We are trying to exibit it as a project …how much expensive is it…

11. whare set this circuit? How work this circuit? Please kindly tell me

12. arjun says:

Can I use 9volt battery as the supply voltage?