I'm trying to power my servo from an external power source and control it with my Pi and I've found following circuit: http://razzpisampler.oreilly.com/images/rpck_1001.png
Why is there a resistance of 1k Ohm? Is it to limit the Pi's amperage to 3mA?
Ohm's law: V = I * R 3.3v = I * 1000ohm | / 1000ohm 3.3v / 1000ohm = 3mA
If yes, why 3mA?