CIRCUIT

IDEAS

1 0 0 • F E B R U A R Y 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

� DR C.H. VITHALANI

FULLY AUTOMATIC EMERGENCY LIGHT

S.C. DWIVE

DI

T his simple automatic emer-gency light has the followingadvantages over conventional

emergency lights:1. The charging circuit stops auto-

matically when the battery is fullycharged. So you can leave the emer-gency light connected to AC mainsovernight without any fear.

2. Emergency light automaticallyturns on when mains fails. So youdon’t need a torch to locate it.

3. When mains power is available,emergency light automatically turnsoff.

The circuit can be divided into in-verter and charger sections. The in-verter section is built around timerNE555, while the charger section is

built around 3-terminal adjustableregulator LM317.

In the inverter section, NE555 iswired as an astable multivibrator thatproduces a 15kHz squarewave. Out-put pin 3 of IC 555 is connected to theDarlington pair formed by transistorsSL100 (T1) and 2N3055 (T2) via resis-tor R4. The Darlington pair drives fer-rite transformer X1 to light up thetubelight.

For fabricating inverter transformerX1, use two EE ferrite cores (of25×13×8mm size each) along with plas-tic former. Wind 10 turns of 22 SWGon primary and 500 turns of 34 SWGwire on secondary using some insula-tion between the primary and second-ary.

To connect the tubelight to ferritetransformer X1, first short both ter-

minals of each side of the tubelightand then connect to the secondary ofX1. (You can also use a Darlington pairof transistors BC547 and 2N6292 for a6W tubelight with the same trans-former.)

When mains power is available, re-set pin 4 of IC 555 is grounded viatransistor T4. Thus, IC1 (NE555) doesnot produce squarewave and emer-gency light turns off in the presenceof mains supply.

When mains fails, transistor T4does not conduct and reset pin 4 getspositive supply though resistor R3. IC1

(NE555) starts producing squarewave and tubelight turns on viaferrite transformer X1.

In the charger section, inputAC mains is stepped down bytransformer X2 to deliver 9V-0-9V AC at 500 mA. Diodes D1and D2 rectify the output of thetransformer. Capacitors C3 andC4 act as filters to eliminateripples. The unregulated DCvoltage is fed to IC LM317 (IC2).By adjusting preset VR1, theoutput voltage can be adjustedto deliver the charging voltage.

When the battery getscharged above 6.8V, zener diodeZD1 conducts and regulator IC2stops delivering the chargingvoltage.

Assemble the circuit on ageneral-purpose PCB and en-close in a cabinet with enoughspace for the battery andswitches. Connect a 230V ACpower plug to feed chargingvoltage to the battery andmake a 20W tube outlet in thecabinet to switch on thetubelight. �