Conventional electric motors use continuous (although possibly alternating) flows of current through windings, so as to cause continuous rotation. The rate of rotation depends on a number of factors including supply voltage, winding resistance, friction, and applied load.

Stepper motors are operated by switching current flow within an array of windings. Rotation is essentially stepwise, although at fast enough rates, this will appear to be continuous.

Rotation of stepper motors is thus an accurate reflection of the total number of switching cycles which have been applied, and within the capabilities of a stepper motor, is independant of applied voltage, winding resistance, friction, or applied load. Thus, stepper motors can be driven so as to provide reliably accurate rates of rotation from very low (practically zero), up to the maximum possible for a given motor.

Note: Rotation of conventional motors can be accurately controlled by the use of 'indexing' systems, but for one-off DIY projects this is an unneccessary complication. In any case, it is difficult to control rotation of conventional motors at low speeds.

Doug Jones has provided a fairly comprehensive introduction to stepper motors and their use. For our purposes, we need condsider only uni-polar and bi-polar types.

Stepper motors are driven digitally, so they can either be driven from a fixed frequency 'clock', or from a computer. In the latter case, the 'clock' frequency can be varied so as to account for errors inherent in the geometry of barn doors, thus allowing for extended tracking periods.

A number of published stepper motor driven designs use uni-polar stepper motors and the SAA11027 chip. This chip has been obsolete for some time. Alternative solutions can be found on Rob Paisley's website along with ready made printed circuit boards. Rob provides designs for uni-polar and bi-polar stepper motors. Timer circuitry is based on an NE555 chip, but there's no reason why this shouldn't be replaced by a crystal controlled divider/timer, or by a PC. This is a well thought out site and provides a good understanding of how to use stepper motors. I have used one of Rob's circuit boards to motorise the RA drive of an equatorial mount and can recommend them.

Some barn door designs show the use of ordinary motors in combination with electronic timers, but greater stability, accuracy and simplicity is likely to be achieved by using stepper motors.

Old printers can be a source of stepper motors. Modern printers tend to use ordinary motors (ie not steppers) with indexing systems. Very old floppy disc drives (ie 5 1/4" drives) contain suitable stepper motors.

Other sources of stepper motors can be found here.