Transistor basics

What is a transistor?

The transistor is an electronic component generally used to amplify current/voltage or act as a switch in a circuit. There are generally two types of transistors which are called NPN and PNP. The differance between these types of transistors will be explaind later in the article.

Transistors also come in many shapes and sizes (usually called packages), but all of them will have three pins: collector(C), base(B) and emitter(E). The pin-layout may vary between transistors depending on transistor type and manufacturer. To verify the pin configuration, refer to the datasheet of your specific transistor.


So, what is the differance between NPN- and PNP transistors? Bluntly it is that NPN transistors are “on” when the base (B) is pulled HIGH (i.e. when the base-pin receives an electrical current) and PNP transistors are “on” when the base (B) is pulled LOW (i.e. when the base pin is connected to ground).

Symbol of a NPN transistor.
Symbol of a PNP transistor.

Breadboard example

Here we’ve set up an example of how a NPN transistor can be used to switch a light (in this case an LED). The transistor will be triggered using 5VDC to let a higher voltage (10VDC) through to the LED. The components used in this example set up are:

Components used
Component Quantity Name in diagram
500Ω resistor 1 R1
100Ω resistor 2 R2, R3
LED (red) 1 D1
BC547B transistor 1 Q1
The circuit diagram of the set up.
The set up on a breadboard.