"How to choose the right transistor for controlling object A (which uses more current) through a GPIO (which can give less current).
As you have likely read, the in general most folks find an MosFet a more desirable choice than a BJT (transistor). Why....
- A Mosfet can mimic a switch to ground with lower voltage across it when on than a transistor.
- A Mosfet takes nearly no current to control, where as a transistor used as a switch required ~ 1/10 of the current being controlled to be provided from the µP output pin.
So to answer your question, you start with the requirements:
- how much current do you wish to control. This will help chose a specific part.
- are you trying to switch to ground or to +V
- If trying to switch to +V, is the voltage you are trying to control higher than the arduino voltage?