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light sensing LEDs

Discussion in 'LEDs and Optoelectronics' started by vadfamous, Sep 5, 2012.

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  1. vadfamous


    Sep 5, 2012
    I am building a clock and would like to add about ten LEDs. I am sorry ahead of time if there is already something on this. I am not very good at electronics although i can solder. I would like the leds to come on automatically when light is dim. I would like this all to be powered by 2 or 4 AA batteries if possible. any idea if this is an easy thing to do? any help would be greatly appreciated.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010

    You're looking for something that's NOT mains powered. 4 AA batteries will probably be best.

    You may find circuits turning on relays, this may be replaced with a LED (or LEDs) with current limiting resistors.
  3. vadfamous


    Sep 5, 2012
    So I would have to use a relay on a timer? There is no inline light sensor for LEDs that turns the LEDs on when light is low?
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    No, Steve said:
    A relay would consume way too much power. Your batteries would be drained fast.
  5. wingnut


    Aug 9, 2012
    Here is a simple circuit which I have tried and it works.

    It is in the thumbnail at the end of this post.

    D2 is actually the phototransistor which looks like a diode and, like a diode, works in only one direction.

    The phototransistor I used was MT-P500M4D but I think almost any phototransistor will work, as will almost any npn transistor.
    You can also add more LED's in series.
    How the circuit works is that in the day when there is IR light around, the transistor conducts, and it is as if the - side of the battery were connected to the base of the npn transistor. Negative switches the transistor off.

    In the dark, the phototransistor becomes an infinite resistance, so now the base is connected to the + side of the battery through the 1k resistor. Positive switches the npn on. Thus the transistor conducts, and the LED goes on.

    Attached Files:

    Last edited: Sep 5, 2012
  6. iknowvator


    Sep 5, 2012
    Better use 555 which have enough current capacity to drive all the 10 LEDs. No other transistors needed. Use CMOS version to keep current low.
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