What is a function of a transistor in a fire alarm circuit?

Answer 1

A special sensor or transducer called a thermistor is used to detect an increase in the temperature of the surrounding air whenever smoke is produced by a fire. A different type of sensor is used to detect an increase in the size and density of particles in the surrounding air after it has been contaminated by smoke from a fire.

Either sensor gives an output in the form of an increase in voltage whenever it detects an increased temperature and/or smoke particles. That increase in voltage is fed to the base of a transistor. When the voltage to its base increases, the current from the transistor's emitter to collector (or from its collector to emitter) increases and that increase in current operates the alarm. In other words, in a fire alarm circuit, the transistor acts as a switch which turns on the alarm whenever a sensor or transducer detects the presence of a fire, else it remains off.