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  • 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

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    7 Responses to "Automatic Emergency Light Circuit"

    1. Dominic T Peelipose says: on November 13, 2010 at 3:20 am

      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


    2. Pentaconto says: on November 16, 2010 at 8:20 am

      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.

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


    4. This circuit DOES NOT WORK

    5. shorting the switch might be work

    6. 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 ?

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

      The battery has dried out.

      Replace the battery

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