Here’s a cheap fire alarm circuit for building one. When there is a fire breakout in the room the temperature increases. This ultra compact and low-cost fire alarm senses fire breakout based on this fact.
Transistor BC177 (Q1) is used as the fire sensor here. When the temperature increases the leakage current of this transistor also increases.The circuit is designed so that when there is an increase in the leakage current of Q1, transistor Q2 will get biased. As a result when there is a fire breakout the transistor Q2 will be on. The emitter of Q2 (BC 108)is connected to the base of Q3(AC 128). So when Q2 is ON Q3 will be also ON. The transistor Q3 drives the relay which is used to drive the load ie,light,bell,horn etc as an indication of the fire. The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switched.