Power for the circuit is derived via mains transformer T1. This drives a full-wave rectifier based on diodes D1 & D2 to produce around 9V DC and this is then applied to the alarm circuit via switch S1. Capacitor C1 filters the supply rail, while LED1 provides power-on indication. When the alarm is triggered, it latches on until reset by S1 (ie, the switch must be opened and then closed again). Finally, a relay could be connected between pins 1 & 2 to switch larger loads than the piezo siren - eg, to turn emergency lights on.
Circuit diagram:
Remote Alarm Circuit Diagram For Smoke Detector
Editor's Note:
- This circuit is recommended for experienced constructors only. In particular, note that all parts to the left of the MOC3021 optocoupler, including BR1 and R3, are at mains (240VAC) potential.