The thermistor offers a low resistance at high temperature and high resistance at low temperature. This phenomenon is employed here for sensing the fire. The IC1 (NE555) is configured as a free running oscillator at audio frequency. The transistors T1 and T2 drive IC1. The output (pin 3) of IC1 is couples to base of transistor T3(SL100), which drives the speaker to generate alarm sound. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2. When thermistor becomes hot, it gives a low-resistance path for the positive voltage to the base of transistor T1 through diode D1 and resistance R2. Capacitor C1 charges up to the positive supply voltage and increases the time for which the alarm is ON.
Transistor T1 = BC548, T2 = BC558, T3 = SL100B
R1 = 470ohm, R2 = 33K, R3 = 470ohm, R4 = 560ohm,
R5 = 47K, R6 = 2.2K, R7 = 470ohm
C1 = 10uf, C2 = 0.04uf, C3 = 0.01uf
D1 = 1N4007
6volt battery or (9v battery + 7806 voltage regulator)
Speaker S1 = 8ohm 1watt