It is desirable for the pilot light in a gas boiler to be always on, so that water is heated by a timer, or turning on water. For this reason, a circuit is needed to switch on an alarm when the pilot light goes out. A light dependent resistor may be used to sense the pilot light being on, When the light goes out, the light level will fall and the resistance of the resistor will increase.
The circuit is shown above. The inverting input is P (
)and the non – inverting input is Q (
). When the pilot light is on, the resistance of the LDR is low so the voltage across it and the voltage at Q is low so
and the alarm is off. If the pilot light goes out, the resistance of the LDR goes up, sogoes up so that
and the alarm switches on.