Transistor

Controls a large current with a small one.

Schematic letter: Q
Unit / key rating: Gain (hFE), current (A)

Pinout

C, Collector: Where the load connects.
B, Base: The control pin; always drive it through a resistor.
E, Emitter: Usually connected to ground on an NPN.

Real photos

Small-signal transistors in TO-92 packages

Transistor and MOSFET packages: TO-220, TO-247, D²PAK, SOT and others

What is it for?

A transistor uses a small base current to control a large current between collector and emitter. It works as a switch or an amplifier.

The BC547 (NPN) and BC557 (PNP) are the most common transistors to start with.

Things to watch out for

⚠ The pin order varies by model. Always check the datasheet before fitting it.

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