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    Content by Mika Satomi and Hannah Perner-Wilson


    Hannah is currently a graduate student in the MIT Media Lab's
    High-Low Tech research group


    Supported by Distance Lab
    Circuits

    LED with Light Sensor

    By using light sensor with transistor switch, you can make a “LED that turns on when dark” circuit without bothering with microcontrollers.

    You will need
    -1 transistor (BC546)
    -1 photocell
    - 100k ohm potentiometer
    -LED
    and pliers and soldering iron.

    Here is the schematic

    and here is how the components are physically connected..

    You should be careful for these points when connecting the circuit. Some of them has directions and it will not work if you connect it the other way around…

    This circuit is based on information from following sites. You can also just follow the schematics from these sites, and should work.
    http://www.kpsec.freeuk.com/trancirc.htm
    or this project is also nice
    http://www.evilmadscientist.com/article.php/nightlight

    I made a “Granny Throwie” with this circuit, so that the LED will turn on only in dark. I am hoping to send this piece to Stitches on the Bridge project, a massive guerilla knitting over the Skye bridge in Scotland. Perhaps “Granny Throwie” will light up the Skye bridge some day..

    3 Comments so far

    1. DIY – Simple Dark-Detecting LED Circuit - Instructables, Evil Mad Scientist Laboratories, Mika Satomi, Hannah Perner-Wilson, eTextile, Scientist, Sensors, Perner-Wilson, Laboratories - talk2myShirt on December 7th, 2009

      [...] Satomi and Hannah Perner-Wilson have posed on ‘How To Get What You Want‘ another variant of this simple sensor using a 100k ohm potentiometer instead of the 1k [...]

    2. Stan Axe on May 14th, 2010

      Suggest you add a current limiting resistor to protect your circuit. There are 2 possible places:
      1. Between the top end of the pot and the 3V.
      2. Between the base of the transistor and the wiper of the pot.

      Otherwise, with the circuit as shown, if you turn the wiper all the way to the 3V, you will either cook your transistor, drain your battery, overheat the battery, or all three at once.

      Also, depending on how hard the transistor gets driven, you may want to consider a resistor in series with the LED to avoid cooking that part as well.

    3. Mika on May 17th, 2010

      Thanks for the suggestion!
      The schematics on this post is working (or at least it did not burn or overheat my circuit, yet:), but I am sure there can be some improvement.
      This granny throwie lasted several weeks (emitting light only at dark) but maybe it can last even longer.
      I will try your suggestion when I get time and post it here.

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