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logic pull up through LED

I was wondering if it's ok to pull up an logic gate, 74HCxx family, lets sa y 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 i s still referenced to ground, so you are forward biasing the LED but becaus e of the high impedance only the leakage current will flow into the gate, w hich is not enough to turn the LED on, but you will see a voltage (minus t he LED drop) on the input of the gate...

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

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

Reply to
panfilero
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here's a link to what I'm thinking....

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Reply to
panfilero

an AND gate, through a series LED and resistor?

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...

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...

electric fields than current flow.... i dunno

No. The voltage on the input won't be well-defined because the LED has such a high impedance for forward voltages below a volt or so. If you want to have an indication that the input is being driven low, you can put your LED+resistor in parallel with the normal pullup.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

OK, you show this is a 5 V supply, the LED usually has a forward drop of about 1.7 V, so the gate will be held at 3.3 V minimum, assuming nothing else pulls it lower. The LED will not light up, and the gate will be held at the positive logic level, for just about any logic family I can think of.

Jon

Reply to
Jon Elson

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

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Reply to
Spehro Pefhany

an AND gate, through a series LED and resistor?

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...

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...

yes, but it's a very small amount.

electric fields than current flow.... i dunno

give it a go, if it doesn't work parallel the LED with a 100K resistor.

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Reply to
Jasen Betts

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