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Thread: Indicator LED has a faint glow when off

  1. #1
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    Default Indicator LED has a faint glow when off

    Hi,

    I have just built my first 8 port 595 on strip board with an indicator LED on each channel. When the channel is turned off, the LED still has a faint glow. I have not got around to adding the TRIACS and looking at what the lights are doing but should I be worried about the LEDS or is this normal?

    The actual circuit is being driven/sunk by a ULN2803 through a MOC3021 opto, to the LED, a 240ohm resistor and then the 5V supply.

    I am getting my power from the USB port.

    Any opinions?

    thanks, Jon

  2. #2
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    Default Re: Indicator LED has a faint glow when off

    Quote Originally Posted by jpb
    The actual circuit is being driven/sunk by a ULN2803 through a MOC3021 opto, to the LED, a 240ohm resistor and then the 5V supply.

    Jon
    Jon, where exactly is the LED?? Sounds like it is after the opto...I am probably misreading it. But the LED should be either before, or after the 2803 (between the 595 and 2803, or between the 2803 and the SSR output).

    The LEDs should NOT be glowing dim when the 595 is off.

    If the LEDs are in the right location in the circuit, do you have the 595 hooked to a computer, and have you given it an "all off" command?? Just thinking...that depending on the LEDs, they might actually be on (the channels on), and that is as bright as they get with your setup....just a thought.

    Sean

  3. #3
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    Sean, the LEDs are glowing when off and are hard on when on.
    I have attached a drawing of the circuit somehow ??
    [/img]

    I have had a closer look on how the LEDs have been done on the Co-op PCB and will try to replicate that. With the Co-op boards is the common lead of the SIP Gnd or 5V? I am guessing 5V but can not find a clear image to study.
    I am wanting the LED on the far side of the 2803 if possible.

    thanks, Jon
    Attached Images Attached Images

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    Maybe a ground problem on the ULN2803?

    --

    Phil

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    The first thing I see is your trying to run the opto with +5 through the LED. I don't think that is a good idea.
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    That should be OK, he reduced the value of the resistor to compensate for the drop across the optical LED.

    --

    Phil

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    Jon, the COOP board has the the LED after the ULN2803. The output of the 2803 is a ground. So the signal coming through the SIP should be whatever is left of the 5V after it goes through the SIP.

    As to your layout that you posted, I would have no idea why the LED would be on. The 5V is always there...so I would guess it is picking up a ground somehow. If you pull the 2803 If it is socketed), does the LED go out??

    Make sure your grounds are all tied together if you are getting power from more than one place...

    Sean
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    Thanks to all for the assistance.
    Phil put me on the right track with his idea of a ground problem. I found that I had put my ground link for the 595 on the wrong strip on the strip board, as Maxwell Smart would say "missed by that much".
    Jon

  9. #9
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    Jon,

    All part of the learning opportunity. Great to hear u have it solved.

    John.
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