Emergency Light Alarm Circuit Diagram 220Vac

Warning! The circuit is connected to 220Vac mains, then some parts in the circuit board are subjected to lethal potential!. Avoid touching the circuit when plugged and enclose it in a plastic box.

This device is powered by two AA NI-CD batteries.

There are four selectable operational modes.

Circuit diagram

Components:

Device Purpose:

This circuit is permanently connected to a mains power supply and the NI-CD batteries are continuously charged at a low current. When there is a power interruption, the torch lamp automatically activates. Alternatively, an audible alarm can be triggered. Upon restoration of the power supply, the torch lamp or the alarm is automatically switched off. A switch is implemented to provide a “latch-up” function, allowing the lamp or alarm to continue operating even after power is restored.

Circuit Operation:

The mains voltage is reduced to approximately 12V DC across the terminals of C2, utilizing the inductive reactance of C1 and the diode bridge (D1-D4). This approach eliminates the need for a conventional mains transformer. A trickle-charging current for the battery B1 is supplied by the resistor R3, diode D5, and the green LED D6, which simultaneously monitors the presence of the mains voltage and controls the correct battery charging process. The transistors Q2 and Q3 form a self-latching circuit that becomes active during a power outage. In this scenario, the base biasing of Q1 becomes positive, activating the self-latching pair. If the switch SW3 is configured as shown in the circuit diagram, the torch lamp is illuminated via the normally-closed switch SW2. Alternatively, switching SW3 to the other position enables a square wave audio-frequency generator, created by the transistors Q4 and Q5, along with related components, driving the loudspeaker. When the switch SW1 remains open, the lamp or the alarm will continue to operate upon restoration of the mains power supply. These can be disabled by opening the main on-off switch SW2. If the switch SW1 is closed, the restoration of the mains supply terminates the operation of the lamp or the alarm by applying a positive bias to the base of transistor Q2.

Notes:

Ensure SW2 is switched on after plugging the circuit in.

This circuit was featured in the publication "Circuit Ideas" of ELECTRONICS WORLD, September 2001, page 708.