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Need help with Dining Table LED Circuit

Discussion in 'LEDs and Optoelectronics' started by cuica, Jan 19, 2014.

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  1. cuica


    Jan 19, 2014
    Hello all,

    I'm trying to build the circuit shown here on a breadboard:

    It seems pretty straightforward, but I think I am doing something wrong with the supply and ground.

    I'm using a 24V supply and a voltage divider (two 4.7K resistors) to give me -12V, +12V and the ground, but I get no results from teh circuit.

    I've tried building just the transimpedance amp on its own, and i can't get this to work either.

    No matter what the intensity of light on the photodiode the voltage at the output remains a steady +11V.

    Any help much appreciated... I presume i am doing something wrong in the way I have set up the supply and ground.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    1) The voltage divider alone has a much too highh resistance to work as a virtual ground. Use a buffered circuit for this.

    2) The highly resistive ground may be a reason for the transimpedance amplifier's behavior. Any offset of the OpAmp can easily drive it into saturation. Try using a good laboratory power supply with +112V output or use 2*9V batteries to ensure a stable ground (0V). If that works, go on and build a goof virtual ground (see 1).
  3. cuica


    Jan 19, 2014

    Many thanks for your reply and the link to that excellent article!

    I'll give it a try tomorrow...
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