Keep in mind that our motor is an inductive load. Every motor will be different, so be sure to check the specs of the motor. Overall, the motor will pull around 81mA of current when running full speed at 5v. For this reason, I’ll always limit the current flow to 20mA per pin. It’s not a good idea to run 40mA for long though because of heat buildup. Microprocessors such as the Atmega328 have a maximum current limit of 40mA per pin. For the purpose of simplicity, we are controlling the load with a button. Always check the data sheets and wiring diagrams of the transistor you wish to use. On the other hand, if you wish to interrupt the positive side of the load, use a PNP transistor. For an NPN transistor we interrupt the negative side of the load. At this point current flows from the emitter of the NPN transistor to the negative side of the power supply. Current flows through the motor, then through the collector of the transistor. Whenever you apply a positive charge with a certain amount of current flow to the base, the transistor will start to conduct. Personally, I like to use the 2n2222 for such applications.
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