We talked about transistors in the previous chapter. We used them as digital switches. They can also be used as amplifiers, voltage stabilizers, and many other related applications.

You can find transistors almost everywhere and they are quite cheap. You can find the complete datasheet at http://www.fairchildsemi.com/ds/BC/BC547.pdf.

The following is a basic diagram explaining how transistors work:

The transistor used as a digital switch in logical circuits

A transistor has the following legs:

If we saturate the base by applying a 5 V power supply to it, all the current coming from the collector will be transmitted through the emitter.

When used like this, the NPN transistor is a nice way to switch on/off high current that Arduino wouldn't have been able to drive by itself. By the way, this switch is controllable with Arduino because it only requires a very small amount of current to be provided to the base of the transistor.

In any case, where you need to have an external power supply to drive motors, we use this kind of design pattern.

Let's now learn about small current servos and then move further using transistors.