Wiring Schematic Diagram

In this fire alarm circuit, a thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the thermistor is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100°C. This phenomenon is employed here for sensing the fire.

The NE555 is configured as a free running oscillator at audio frequency. The transistors T1 and T2 drive IC1. The output of IC1 is couples to base of transistor 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.