WonderTiger:
Is it possible for somebody to explain me why I need to use a resistor on the base from the transistor? I did alot of reading about why a resistor is needed on the base from the transistor (as NPN switch), but I keep seeing different answer on different forums this makes me understand it less and less. So if someone briefly can explain why a resistor is needed on the base (and how you calculate it) I would be very happy with it :)!
sorry for the newbie questions, still in the learning process
It's because once the voltage applied across the emitter/base junction of a transistor is higher then the forward voltage drop rating for that base/emitter junction it will fully turn on and act like a short circuit drawing current well in excess of it's maximum base current limit (and the maximum output current being drawn from say an arduino output pin when set high that one might wire to the base terminal), so something in the base circuit needs to limit the base current to at or below it's maximum rated current value, and a resistor wired in series with the base terminal just happens to be the simplest way to do that.
Lefty