Transistor as a Switch: NPN, PNP Bipolar Junction Transistor BJT

Here are the calculations for how to use a Bipolar Junction Transistor (BJT) as a switch for NPN and PNP transistors. Examples are shown for NPN and PNP and the circuits are simulated using Tinkercad. Transistors can be used similar to relays to electrically control circuits that are too powerful to be directly controlled, for example if a micro-controller operates at 5 volts and can only deliver 25 mA of control current, a transistor can be used to control higher voltages or currents. The theory and equations shown here allow you to calculate the base resistor required to turn on a transistor for a specified maximum collector (load) current. Remember to use a flyback diode to protect the transistor from voltage spikes if switching inductive loads like relay coils.