automatic emergency light circuit schematic

Automatic Emergency Light Circuit

This low cost automatic emergency lighting circuit turns on a lamp during power failures. It is powered by a NiCad battery that is being charged by the main power line when there is no blackout. The circuit design is very simple and can be made by almost anyone.

How does the automatic emergency light works

The voltage from the step down transformer is rectified by diode D1 and filtered by C1. The NiCad battery gets charged with about 100 mA via the diode D2 and resistor R1. The battery must have a capacity of at least 2 Ah to tolerate the charging rate.

When the main power supply fails during a blackout, the charging current is interrupted and a current flows from the base of T1 via resistor R2 which triggers the transistor to conduct and the two emergency lamps light up. When the main power returns, the charging current flows again through D2 and the transistor turns the lamps off.

The “TEST” button S1 is used to check the function of the circuit. If the secondary coil of the transformer delivers a higher voltage level, replace R1 with a higher value resistor to avoid exceeding the maximum charging current allowed for the NiCad battery being used.

Emergency light parts

  • 220V to 4.5V / 2A Transformer
  • D1 = 1N4004
  • D2 = 1N4001
  • R1 = 33Ω/1W
  • R2 = 470Ω
  • C1 = 470µF/16V
  • T1 = 2SB242
  • La1, La2 = 2.5V bulbs
  • Battery = 2.5V / 2Ah Nicad

Automatic Emergency Lighting Circuit Diagram

automatic emergency light circuit schematic


Join the conversation!

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  • Dan

    Does anybody can help me? I have problem with emergency lighting, how I can replace LEDs in it without any replacement of it? I have this model and I afraid to do something with it because of overvoltage!

  • sabari

    what wonderful circuit!

  • Colin Mitchell

    “We plugged it and used it 2X then after that it would not function anymore. ”

    The battery has dried out.

    Replace the battery

  • paz

    someone gave me an emergency lght that turns on automatically during brownouts. We plugged it and used it 2X then after that it would not function anymore. I would like to buy something like that ….. but apprehensive about length of lifespan. Help. What did I do wrong ?

  • gonio

    shorting the switch might be work

  • Colin Mitchell

    This circuit DOES NOT WORK

  • Erick Nava Aldana

    Could adap this to use 12v Sealed Battery, from 1 to 7 A, and use 110v AC?


  • Pentaconto

    Dear sir:

    This circuit is not correct: The T1 PNP transistor is always in ‘on state’, because always a bias current flows from the base of T1 via resistor R2 to ground (negative of battery). The two emergency lamps light up ALWAYS.

  • Dominic T Peelipose

    Dear Sir,

    Thank you for your simple and practical circuits with low cost components. Some circuits are not ligible in the screen hence components specifications could not be recorded. Please arrange to attach a ligible diagram so that everybody can get the correct components.

    With regards


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