What they look like:

How they are shown on circuit diagrams

There are two main types of transistor the NPN type and the PNP type.  Note the direction of the arrows on the diagrams.

An easy way to remember is, on the NPN type transistor the arrow is Not Pointing iN.

Leg Label

What it stands for







Their purpose

Transistors act like small and very fast switches that can turn electricity on and off thousands and even millions of times a second. 

Take the NPN transistor shown above.  If a positive electricity source is connected to the Collector, the negative is connected to the Emitter and no electricity is connected to the Base - no current would flow.  If a small positive voltage (with respect to the Emitter) is applied to the base, a small current flow would result from the Base to the Emitter, but a much larger current would flow from the Collector to the Emitter.


The arrow in the transistor diagram shows the flow of electricity both from the Base to the Emitter and from the Collector to the Emitter.

The PNP transistor is similar:

With the PNP Transistor, a low voltage on the Base (with respect to the Collector) causes the bulb to illuminate.  Again follow the arrows to trace the current flow.

A Summary - A NPN Transistor allows current to flow when the base is taken positive. A PNP transistor allows current to flow when the Base is taken negative.

How to identify them

Transistors are printed with their description on the main body of the transistor.

As shown in What they look like, there are many shapes and sizes for transistors.  Generally the larger the transistor, the more current they can switch.  So for a small circuit, say to light a LED, a small transistor will suffice say type TO-92.  For a large application, say to illuminate an automotive bulb, a TO-3 type will suffice (a TO-92 type would soon be destroyed by the current flow).

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