A teacher I know is stuck, with only npn transistors in the supply cupboard and the need to create a reversible motor ... these circuits without extra diodes (see below). And all but the last circuit ...

The Arduino has been used for many purposes, and “shields” are available to make many common tasks easier. However, [Nick] wanted a stackable motor driver shield, so he build one himself!.

Maker, developers and hobbyists that enjoy building Arduino platform based projects might be interested in a new Arduino Motor Shield called the Arc controller which has been created by Arc Robotics.