[sdiy] LED current source needed for 'digital' outputs?

Gordon JC Pearce gordonjcp at gjcp.net
Sat Mar 1 14:01:18 CET 2014

On Sat, Mar 01, 2014 at 04:45:24AM -0800, Justin Owen wrote:
> Hello, just a little double-check here...
> For an LED displaying a changing or AC signal I would always use some sort of transistor current source.
> For an LED displaying a simple on/off digital signal - e.g. a logic output - is there any reason to not just use: Source > LED > R > GND?
> This is a circuit being used in a modular environment so it will need to play nice with its neighbours and optimal/'reasonably' low current draw is a normal priority.
> Thanks,
> - J

Most digital outputs pull down rather better than they pull up, so you'd be better to wire it between +5 and the pin.  Put a 150 ohm resistor in series (or really anything you have within about 100-470 ohm, it's not at all critical) and you'll be fine.

Gordonjcp MM0YEQ

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