Fire Alarm Using Thermistor

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.


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


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


TH1 = Thermistor 10 KΩ

LS1 = 8 Ω, 1W speaker