Build Your Inspiring 240VAC Mains Smoke Detector Alarm Circuit

Description

This fire detection circuit was conceived to supervise a power-driven smoke detector situated in a workshop (which is utilized for the housing of canine enclosures). It furnishes complete segregation from the electrical supply so that low-voltage (12V) cabling could be routed to the alarm circuit, which is positioned within the residence. During operation, the alarm signal (I) originating from the smoke detector is converted utilizing a bridge rectifier, BR1, and subsequently supplied to the optoisolator, OPTO1, via resistor R3. This, in turn, activates the gate of SCR1, causing it to switch on and trigger a piezo buzzer with a built-in oscillator.

Power for the circuit is obtained through a mains transformer, T1. This drives a full-wave rectifier based on diodes D1 & D2, producing approximately 9V DC, and this is then applied to the alarm circuit via switch S1. Capacitor C1 smooths the power supply rail, while LED1 provides a visual indication of power-on status. When the alarm is activated, it remains engaged until reset by S1 (i.e., the switch must be opened and then closed again). Furthermore, a relay could be connected between pins 1 & 2 to control larger loads than the piezo buzzer – for example, to activate emergency lighting.

Circuit diagram:

Note:

This circuit is advised for individuals with substantial construction experience only. Specifically, it must be noted that all components to the left of the MOC3021 optocoupler, including BR1 and R3, operate at mains (240VAC) voltage.

More about components

The MOC3021 is a versatile optocoupler, often referred to as an optoisolator, which is a semiconductor device designed to transfer electrical signals between two circuits while maintaining electrical isolation. This isolation prevents ground loops and protects the control circuit from voltage spikes or transients present in the power circuit. The MOC3021 typically operates with a DC input voltage between 3V and 12V, providing a high-speed switching capability. It features a gallium nitride (GaN) light emitting diode (LED) and a silicon phototransistor, enabling fast switching speeds and robust performance. The optocoupler’s primary function is to electrically isolate the signal source from the load, which is crucial in applications such as motor control, relay switching, and instrumentation where safety and noise reduction are paramount. The MOC3021 is commonly utilized in applications requiring high-speed switching while providing protection against electrical interference, making it a reliable choice for a diverse range of electronic circuits.