this circuit senses excess temperature of around 70 degree celsius and gives an audible alarm. in the circuit, the sensor is a germanium diode DR25 connected in reverse bais. reverse resistance of the diode drops at higher temperature that triggers the timer IC 555. gain of the sensing circuit can be set by adjusting the resistance of the preset VRL. The circuit can be used as a low cost fire alarm in apartments , houses and offices etc.
3)ic and ic base