Need a little help bidirectional LED supply to drive 3 pin LED

Discussion in 'Electronics & Electrics' started by MEX, Feb 11, 2018.

  1. MEX

    MEX Member

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    I've got a small electronics problem that's beyond my knowledge.


    I'm swapping in a replacement AMP on a sub, the problem is the led supply on the board if for a 2 pin led that's bidirectional.

    The supply to the led is GND and then the other leg is either -1.7v or +1.9v

    This creates two problems the LED on the front of the sub needs to be brighter than these voltages supplied and the led itself is a 3 pin style led.

    I'm sure there is a fairly simple way with a few components to convert this signal into something that will drive the main LED. I can easily get + or - 12v from the internal PSU


    I'm sure someone with more advanced circuit knowledge would be able to solve this very quickly.



    Regards
    Luke
     
  2. _zak

    _zak Member

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    Have you considered just buying a replacement bidirectional LED? There are plenty on Digikey if you search for bidirectional LEDs (example here).
     
  3. OP
    OP
    MEX

    MEX Member

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    I had thought about that but their is not enough power voltage coming from the current LED to run the front LED at sufficient brightness or even turn it on.


    Cheers
    Luke
     
  4. Technics

    Technics Member

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    You should be able to do it with a pair of external diodes. Essentially you make the LED part of a bridge rectifier with a shorted output (which is not an issue as current will be limited by the source). Exactly how to wire them depends on if the three pin LED is common anode or common cathode. I suspect the 1.7V and 1.9V measurements are down to the forward voltage of the red and green diodes. This voltage is not what is determining the brightness, that is determined by the current. If the brightness is insufficient you will need to decrease the value of the resistors that are limiting current.
     
  5. Technics

    Technics Member

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    You will probably need to remove the exiting LED. It may have a lower forward voltage than the one on the front. If that is the case it will prevent any current from flowing in the new LED. A photo of everything you're trying to wire up would make helping much easier.
     
  6. OP
    OP
    MEX

    MEX Member

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    Thanks for your input Technics,

    I don't have a photo unfortunately and I'm also unable to remove the existing LED due to it being behind a glued PCB.

    I was playing with some circuit software and I think its possible to create a simple circuit with a couple of transistors and resistors to sense the voltages on the existing LED and in turn switch the second LED on appropriately.


    Regards
    Luke
     
  7. Technics

    Technics Member

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    In that case, yes you certainly can do it with transistors and resistors. You would still need to know if the 3 pin LED is common anode or common cathode to design the circuit though.
     
  8. OP
    OP
    MEX

    MEX Member

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    Dam I forgot to check that on the LED, I suppose both circuits can be designed. They shouldn't be overly complex.

    Luke
     
  9. Technics

    Technics Member

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    Something like this for common anode.

    [​IMG]

    For common cathode your swap the supply rails and the NPN for PNP and vice versa.
     
  10. Technics

    Technics Member

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    This should work from a single +12V supply for common anode.

    [​IMG]

    Edited: Put the wrong one in the first time.
     
    Last edited: Feb 15, 2018
  11. rainwulf

    rainwulf Member

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    Thats a pretty damn nifty circuit!
     
  12. OP
    OP
    MEX

    MEX Member

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    This is great!

    Thank you for the input.

    Once I find some spare time I'll collect the components and see if I can build this!


    Regards
    Luke
     

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