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

Tom Wiltshire tom at electricdruid.net
Sat Mar 1 14:08:49 CET 2014


No, no reason to use a transistor. You can drive an LED directly off those 74HC595 outputs you were talking about, for example. It specifies (I checked NXP, others may differ) 25mA per pin, but more significant is 750mW per package for the DIP. That would be only 6 of the 8 outputs if running at full power. Not that there's any need - you'll get plenty of light from a modern LED for a few mA, no need to go to 20mA.

Other chips sometimes specify different ratings for current sink vs current source, so be sure to check that. That's why you sometimes see LEDs hanging off the +5V rail and giving an inverted output.

T.


On 1 Mar 2014, at 12:45, Justin Owen <juzowen at gmail.com> 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
> 
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