> One question on the base resistor... without it, how do you ensure that
the
> transistor won't take more current than the AVR can safely supply?

You don't - but you wanted the simplest possible circuit regardless :-)
In practice it is a good idea BUT in practice it also probably doesn't
matter. As long as the emitter follower is able to drive its load the
emitter will track below its base by aboput 0.6v and the transistor is
"happy". Even if you do short the emitter to ground the AVR output is
shorted to ground via  adiode and will quite probably survive (don't try
this at home. YMMV).
Note that this is NOT the case when you are driving the base with DC, when a
resistor will be needed.

> understood that it was a good (safe) design practice to always use a base
> resistor to limit current.

Yes

> What am I missing here?

Not much - except that for simplest solution it *probably* wont matter.



        RM

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