162,742
Well, yeah, you need to know how to straighten out crooked volts and untangle ohm's knots first, in order to not blow up your microcontrollers, but that's not all that hard.
Ohms law is the only thing that you need for 90% of the time, and it's quite easy to use.
Just take R = U / I, U = I x R, R=V/I
Let's say you wanna run a led from a 9 volt source, but the led is rater for 30 mA @ 2 volts, all you have to do is take the 2 volts off the 9 volts, wich makes 7, and then divide that 7000mV by the 30mA, wich makes 233,3 ohms. And, that right there is the resistance value you need to use to make the led burn without letting the smoke out!