drill control

There are two basic types of speed control for electric drills. The most common type simply reduces the power to the drill to obtain a lower speed; this has the disadvantage that the speed of the drill depends on the load. A more sophisticated type uses some form of feedback to hold the speed more or less constant at the required number of revs. The control described here is of the latter type. It is suitable for most electric drills, no matter what their power rating is, although some minor modifi- cations will be required for really high-power drills.