Some time ago, I constructed a digital driver for a stepper motor driven barn door tracker. It was a cheep and effective way of driving the tracker. I also built the gearbox and drive screw system for the tracker. It works well, constructed from a few components (555-timer, decade counter, power transistors, a few diodes etc) can be driven via a resistance substitution box to achieve a multitude of different drive speeds.
If anyone is interested, I could post a copy of the plans and circuit board used with a few hints and suggestions for construction.
