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Driving DIO input from a LED

Discussion in 'Electronic Design' started by pdq, Oct 21, 2003.

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  1. pdq

    pdq Guest

    I have an existing device with an LED on it that I'd like to be able to
    "sense" at the digital input of a PIC chip. I can't just jook it up
    directly because its ground is different from that of the analog signal that
    I'm already sending to the PIC. I need to isolate the LED with a transistor
    or something.??

    The existing LED circuit has a 470ohm resistor connecting it to a +16VDC
    supply. There are a couple of 2N3904 transistors "stacked together" on the
    negative lead of the LED: A Flip-Flop goes to the Base or Q1, Emitter of
    Q1going to the Base of Q2. Both Collectors go to NEG lead of LED.

    I'm guessing I could use a transistor between the LED and the digital in...
    but I'm not sure which one or the best way to hook it up.
  2. Ian Stirling

    Ian Stirling Guest

    Are the grounds of the PIC and LED circuit the same?
    If not, one way may be to use an optoisoloator in series with the LED, to
    give you plenty of isolation.
  3. pdq

    pdq Guest

    The grounds are different. I have to go parallel because I can't mod the
    existing circuit.
    Is there a way that draws less current than an Optotransistor?

  4. Ian Stirling

    Ian Stirling Guest

    If the LED is indeed driven from a 16V rail, then the effect of putting the
    optoisolator LED in series with it is going to be almost none.
  5. James Meyer

    James Meyer Guest

    Try this:
  6. pdq

    pdq Guest

    James, very helpful. I think I've got it now.
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