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logic pull up through LED

Discussion in 'Electronic Design' started by panfilero, Mar 10, 2013.

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  1. panfilero

    panfilero Guest

    I was wondering if it's ok to pull up an logic gate, 74HCxx family, lets say an AND gate, through a series LED and resistor?

    I think it is, I see the input of the logic gate as a high impedance that is still referenced to ground, so you are forward biasing the LED but because of the high impedance only the leakage current will flow into the gate, which is not enough to turn the LED on, but you will see a voltage (minus the LED drop) on the input of the gate...

    but, doesn't a diode need a certain amount of current to become forward biased? is the leakage current into the gate really enough to forward bias the LED? or am I thinking about it wrong... maybe it's not really forward biased...

    anybody know the right way to think about this? maybe it's more about electric fields than current flow.... i dunno
     
  2. panfilero

    panfilero Guest

  3. Though I agree it will probably work most all the time in most
    situations, I think this sort of thing is an accident waiting to
    happen.

    74HC inputs (ST and regular) have Vih(min) 3.15V at 4.5V supply, so if
    the 5V supply is allowed to go -10%, the LED must drop less than 1.35V
    with whatever leakage current it is feeding (say 1uA, since that's
    what the spec says). A green LED will drop about 2V at 1uA.
    Photovoltaic effects don't help*, and if it's near the threshold it
    will pull up relatively slowly and have little noise immunity.


    Sometimes the clever thing to do is to just do it the obvious way..
    either a resistor in parallel with the LED+resistor, or another
    resistor across the LED.

    * With a lower voltage supply and maybe a bit of tweaking you could
    make a sunlight detector with that same circuit. Try a few LEDs in
    series for a 5V supply.


    Best regards,
    Spehro Pefhany
     
  4. Jasen Betts

    Jasen Betts Guest

    yes, but it's a very small amount.
    give it a go, if it doesn't work parallel the LED with a 100K resistor.
     
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