Randofo writes –
An H-bridge is a type of circuit that you can use to get a reversible DC motor to spin both clockwise and counterclockwise.
In other words, this circuit allows you to quickly reverse the direction a motor is spinning by using a switch or controller chip to change its direction.
I’m going to show you how to make the simplest and most reliable form of H-bridge that I know how to make. I must warn you that this is by no means the best H-bridge design and, in fact, it has a couple shortcoming which I will explain later.
Although, should you never have made an H-bridge before or simply need a circuit that can reverse the direction of a motor’s spin, then this circuit is a quick and easy solution.
How to make an H-bridge – Instructables – [via] Link.