When using a transistor to turn on a device, it’s important to chose the right transistor, and to trigger it properly. NPN transistors get triggered with a high, PNP transistors get triggered with a low, and get connected differently in a circuit. You need to know how to size the base resistor properly, to make sure you drive the transistor hard enough, but not draw too much power from the Arduino. The typical hookup in the Arduino world is that a Arduino digital pin is driving the base of the transistor, through a resistor, and that the voltage is typically (but not always) 5v.
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Electrical Engineering 101: Everything You Should Have Learned in School…but Probably Didn’t (w/ CD)