Fire Alarm Using Thermistor:-

Many fire alarm circuit is published indifferentwebsite. 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 CircuitDescriptionoffirealarmusingthermistor:-

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 T₁and 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.

circuit diagram:-

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

Capacitors

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