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

[rburnett at richieburnett.co.uk]

> This is also a good reason to soft-switch LEDs to remove the sharp
> edges, and if possible run the LEDs between the power rails and not go
> dumping current transients into the ground.
> 
> Neil

I've seen VU meter designs in pro-audio kit where all of the LEDs are 
connected together in series and driven from the supply rails via a 
constant current source.  Current is allowed to pass through the LEDs 
that we want illuminated, and "diverted around" the LEDs that are meant 
to not be illuminated.  The total current drawn from the power supply is 
therefore 20mA regardless of whether all the LEDs are lit, all are 
extinguished, or any combination in between.  That way it doesn't trash 
the power supply rails with switching transients.

-Richie,