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Make this Useful Emergency Light using 'Cool' White LEDs

written by: Swagatam • edited by: KennethSleight • updated: 10/9/2012

Using high efficiency white LEDs and a unique voltage controller circuit, an almost ideal LED emergency light circuit can be constructed. Let's look at the procedure involved.

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    We are all aware of the ill effects of global warming gripping the world strongly with each passing day. Although the major cause of this phenomenon is almost certainly the use of fossil fuels as the main source of energy by many countries, the contribution of lighting in different forms to the problem in cannot be entirely ignored. Common, but age-old incandescent bulbs have become almost obsolete, but even modern CFLs emit quite a bit of heat. White LED lamps today are becoming a popular (and smart) alternative due to their rather "cool" nature and high-efficiency.

    In this article we learn how simple it is to construct a versatile LED emergency lighting circuit using a handful of white LEDs and a unique circuit design that helps to increase the overall efficiency of the unit.

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    Principle Functioning of the Circuit

    Automatic LED Emergency Lights, Circuit, Image 

    To illuminate a white LED, we need a minimum of 3.6 volts and a maximum of 20 mA of current, which is the forward voltage drop of a white LED.

    In the present circuit we use a 6 volt battery as the power source. So what's the big deal in illuminating a group of white LEDs with this battery? It's true, at first glance that the circuit is pretty ordinary, just connecting a group of LEDs in parallel to a battery. But something is special about this circuit. It's the inclusion of four diodes in series with the LEDs and, of course, the addition of S1.

    We know that according to Ohms law, voltage is directly proportional to current, which implies that if the 6 volt supply is directly applied to the LEDs, they will start drawing excessive current, resulting in unnecessary dissipation of heat through the resistors and the LEDs and fast discharge of the battery, decreasing the efficiency of the circuit. This will happen because a potential difference of 6 volts is simply too high than the forward voltage of the LEDs (which is 3.6 volts as discussed above).

    By adding 4 diodes we are able to drop the voltage to exactly the forward voltage of the LEDs (since each diode drops a voltage of approx. 0.6 volts).

    Thus, when the battery is fully charged at about 6 volts, the LEDs will be receiving 3.6 volts, just enough to make them glow brightly without dissipating extra power.

    Now suppose the battery voltage drops to an extent when you see the LEDs become weaker in intensity. You can simply adjust the switch S1 a step ahead by-passing one of the diodes. This will immediately restore the brightness of the LEDs, maybe for another couple of hours. After that the procedure may be repeated by by-passing the next diode.

    Thus you are able to get multiple back-ups using the same battery- which wouldn’t be possible if the LEDs were connected directly.

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    Parts List

    You will need the following parts for this plug & play emergency lights circuit:

    • R1 = 1M, ¼ Watt, 5%, CFR
    • R2 = 10K, ¼ Watt, 5%, CFR
    • R3 = 10 Ohms 1Watt
    • ALL LED RESISTORS ARE = 22 Ohms, ¼ Watt
    • C1 = 205/400V, PPC
    • C2 = 100uF, 25V
    • Z1 = 9 V, 1 WATT
    • T1 = BD 140
    • ALL DIODES = 1N4007
    • S1 = SINGLE POLE 3-WAY SWITCH
    • BATTERY = 6 VOLTS 4 AH
    • LEDS = WHITE, 5mm, HIGH EFFICIENCY
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    Plug & Play Emergency Lights, Image 

    Connect a group of white LED s with a few series resistors to a 12 volt battery, and here we go... an LED emergency light circuit is ready! You will find many such trivial circuits on many other websites.

    The present LED emergency light circuit is different from them because it consumes very little battery power (6 Volts) and thus remains illuminated for at least 40% longer than its ordinary counterparts.

    It is also interesting that it is the voltage and not the current that matters in keeping the white LEDs glowing. This property of white LEDs has been exploited here.

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    Construction Clues

    CAUTION: THE ENTIRE CIRCUIT MAY CONTAIN DANGEROUS FLOATING AC CURRENT WHEN SWITCHED ON, SO BE EXTREMELY CAREFUL WHILE HANDLING AND TESTING THE DEVICE.

    The construction procedure is as follows:

    • Finish the emergency light board assembly by soldering the white LEDs and the other parts properly following the diagram.
    • Placement and spacing of the white LEDs will depend on the cabinet size and structure, which may be home-built or may be obtained ready-made.
    • Next, insert and solder the remaining passive components and also the transistors and the diodes. Be careful with the transistor pin-outs and the diode orientations.
    • Inter-link correctly each component lead by bending and soldering the component leads together with the help of the circuit schematic.
    • Clamp the battery and the transformer tightly in the cabinet with only the AC power cord coming out of it.
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    Automatic Switch-Over and Battery Charging

    This plug and play emergency light circuit is completely automatic and may be kept connected to the AC mains permanently.

    As long as mains power is alive, T1 receives a direct positive rectified potential through D1 and thus is unable to conduct. This keeps the LEDs shut off and they are not able to light up.

    In the meantime, the battery gets slowly trickle charged through D1 and D2.

    The moment AC supply fails, the positive potential from T1's base is removed, so it gets biased through R2 and starts conducting, instantly illuminating the LED array.


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