> >The current source has PIC output to transistor base, transistor
emitter to
> >ground via suitable resistor (see below), + supply to LED anode, LED
> >cathode to transistor collector.  It only works if the PIC supply
voltage +
> >the voltage required by the load is less than the power supply, so you
> >would need a >10V supply for a 5V PIC driving a 5V LED.
>
> An additional series base resistor of perhaps 0.3~1K is not a bad idea.

Actually, it is.  The emitter resistor will limit the current drawn from
the PIC nicely.  Additionaly base resistance only decreases the apparent
impedence of the current sink driving the LED.  The impedence of an ideal
current sink is infinite.


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