Automatic Emergency Light Circuit Design: 555, LM317 & X1

Description

This straightforward emergency illumination device possesses several advantages compared to traditional emergency lights.

The circuit architecture is comprised of an inverter and a charger section. The inverter section is constructed around a timer NE555, while the charger section utilizes a 3-terminal adjustable regulator LM317. Within the inverter section, NE555 is configured as an astable multivibrator, producing a 15kHz square wave. The output pin 3 of IC 555 is connected to a Darlington pair composed of transistors SL100 (T1) and 2N3055 (T2) via resistor R4.

The Darlington pair drives a ferrite transformer X1 to illuminate the tubelight. For fabricating the inverter transformer X1, utilize two EE ferrite cores (with dimensions of 25×13×8mm each) along with a plastic former. Wind 10 turns of 22 SWG on the primary and 500 turns of 34 SWG wire on the secondary, incorporating insulation between the primary and secondary. To connect the tube-light to ferrite transformer X1, initially short both terminals of each side of the tube-light and then connect to the secondary of X1. (Alternatively, a Darlington pair of transistors BC547 and 2N6292 can be used for a 6W tube-light with the same transformer.)

Circuit diagram:

When mains power is available, reset pin 4 of IC 555 is grounded through transistor T4. Therefore, IC1 (NE555) does not generate a square wave, and the emergency light turns off when mains electricity is present. When mains fails, transistor T4 ceases to conduct, and reset pin 4 receives a positive supply via resistor R3. IC1(NE555) begins producing a square wave, and the tube-light is activated via ferrite transformer X1. In the charger section, input AC mains is reduced in voltage by transformer X2 to deliver 9V-0-9V AC at 500mA. Diodes D1 and D2 rectify the output of the transformer. Capacitors C3 and C4 function as filters to minimize ripples.

The unregulated DC voltage is supplied to IC LM317 (IC2). By adjusting the potentiometer VR1, the output voltage can be adjusted to provide the charging voltage. When the battery reaches a voltage above 6.8V, a zener diode ZD1 conducts, and regulator IC2 ceases to deliver the charging voltage. Assemble the circuit on a general-purpose printed circuit board and enclose it within a cabinet offering sufficient space for the battery and switches. Connect a 230V AC power plug to supply the charging voltage to the battery and create a 20W tube outlet within the cabinet for switching on the tube-light.