Transistor BC177 (Q1) is used here as a fire sensor.As the temperature rises, the leakage current of the transistor also increases.When designing the circuit, the transistor Q2 will be offset as the leakage current of Q1 increases.As a result, when there is a fire, the transistor Q2 will conduct.Q2 emitter (BC 108) is connected to Q3 base (AC 128).
