Small bi-color LED circuit

Discussion in 'General Electronics Chat' started by Teq, May 19, 2007.

  1. Teq

    Thread Starter New Member

    May 19, 2007
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    I'm not an electronics expert by any means (about 6 months experience 14 years ago), so would like a little help with a project I am working on.

    I am trying to power a bi-color LED from a 5 volt supply, in the 0 state this should glow red, when 1 blue. (red having 2.1 and blue having 3.3 forward voltages), I've thought of a not gate circuit but am not sure if this will work properly for my means. (if you do come up with a solution a resister ohms value, any diode and transister models that would fit into the project would also be helpful).
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Is this a two lead or a three lead bicolor?
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    A part number and manufacturer's name for the LED you are using would also help us in providing you guidance.

    hgmjr
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
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    It seems to me that it is a bicolor led that glows green when biased in one direction and red on the other. If that is the case, nothing better than a H-bridge with transistors, like the ones used to drive a bidirectional motor.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    Some bicolor LEDs are RED/GREEN, but not all. He might have a RED/BLUE bicolor and the problem is more complex than you might think. Because the BLUE LED has a much higher forward voltage, the resistor you would compute for the RED half would not allow sufficient current through the BLUE half. So an H-bridge with two resistors might work
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    The mystery should disappear once we get a datasheet in hand.

    hgmjr
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    I might turn blue and red and have to take a breath before that happens.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    LOL....

    Just as long as it is not black and blue....

    hgmjr
     
  9. Teq

    Thread Starter New Member

    May 19, 2007
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    Sorry I didn't have the data sheet at hand. I've actually now solved my problem using a 4069 CMOS logic driver.

    Just FYI the LED did have two forward voltages on a 3 pin package. Red being 2.1v, Blue 3.3v.
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    That's great. Good work.

    hgmjr
     
  11. hgmjr

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    I don't suppose you would be willing to post your final solution for the benefit of others who might have a similar project in mind.

    hgmjr
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    Danger Will Robinson. You're going to be quite disappointed in the 4069's ability to light a LED especially the BLUE one from a 5V supply. The spec sheet says you can expect less than 1mA. I guess this is OK if they are very bright and super efficient. If the 4069 does not work, may I suggest a 74AC04 which should be more than enough drive at 24 mA.
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    I agree with papabravo that you will get far better results by directly driving your LEDs using a 74AC04 than you can directly driving them using a 4069B.

    If you find the brightness of the LEDs is not what you were looking for, then at least you have another option.

    We would still be interested in seeing your circuit diagram.

    hgmjr
     
  14. cumesoftware

    Senior Member

    Apr 27, 2007
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    As far as I can tell, neither the red LED should lit.
     
  15. Papabravo

    Expert

    Feb 24, 2006
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    Oh..ahdonno. There are super efficient RED's that will blind you @ 2 mA, so 0.88 mA should be acceptable.
     
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