Emergency Light Alarm Circuit Diagram – Build Your Own!

Description

This circuit is permanently connected to a mains electricity supply and NI-CD batteries are continuously charged at a low current. During a power failure, the lamp automatically switches on. Alternatively, an audible alarm can be activated instead of illuminating the lamp. Upon the restoration of the power supply, the lamp or the alarm is automatically switched off. A switch provides a latch-up function, ensuring the continued operation of the lamp or alarm even after the power supply has been reinstated.

More about components

The operation of the circuit utilizes the characteristics of several integrated circuits. The BC547 transistor (Q1, Q3, Q4) functions as a switching transistor, enabling or disabling current flow based on the applied voltage at its base. The BC327 transistor (Q2, Q5) operates as a self-latching device, facilitating the circuit’s ability to retain its state even when the power supply is interrupted. These transistors are crucial for the circuit’s reliability and its ability to respond automatically to changes in the power supply.

Circuit Diagram:

Parts:

Circuit operation:

The mains electricity is reduced to approximately 12V DC at the terminals of C2, thanks to the reactance of C1 and the diode bridge (D1-D4). This eliminates the need for a mains transformer. A low-current trickle-charge current is provided for the battery B1 by the resistor R3, diode D5, and the green LED D6, which also monitors the presence of the mains supply and the correct battery charging. The BC327 transistors (Q2 and Q5) form a self-latching pair which activates when a power outage occurs. The base biasing of Q1 becomes positive, thereby enabling this self-latching pair. When SW3 is set as depicted in the circuit diagram, the lamp is illuminated via SW2, which is normally closed. If set the opposite way, a square wave audio frequency generator is activated by the transistors Q4 and Q5, driving the loudspeaker. If SW1 is left open, the lamp or alarm will continue to operate when the mains supply is restored. They can be turned off by opening the main on-off switch SW2. Upon the restoration of the mains supply, a positive bias is applied to the base of Q2, which terminates the operation of the lamp or alarm.

Notes: