Transistors: Electronic Switches and Amplifiers
A tiny current controlling a big one. This single idea powers every computer, phone, motor driver and robot on Earth. In this lesson you will learn to use a transistor as a switch a small microcontroller can flick on and off to run something powerful.
The big idea
Some parts (LEDs, resistors) just sit there and do what electricity does. A transistor is different: it is a control part. You feed it a small, weak signal on one leg, and it uses that signal to open or close the path for a much larger current on its other two legs. Nothing you have met before can do this. It is why the transistor is often called the most important invention of the 20th century.
Think of a water tap. Your fingers push with almost no effort, yet you control a powerful jet of water. The transistor is an electrical tap: a light "finger" of current at the base controls a strong "flow" of current through the load.
- Explain in plain words what a transistor does: a small current controls a big current.
- Name the three legs of an NPN transistor and say what each one is for.
- Use the current-gain rule (beta) to work out how much collector current a base current allows.
- Build the standard low-side switch circuit and understand every component in it.
- Calculate the base resistor you need so a microcontroller pin can drive a transistor safely.
- Understand why you cannot just connect a motor straight to a microcontroller pin.
- Recognise a transistor used as an amplifier, and know when to reach for a MOSFET instead.
- Add the safety parts every motor switch needs: a flyback diode and a heatsink for big loads.