I wanted a Barn Door/Scotch/Haig type camera platform to photograph Comets, Meteors etc. (This was in 1999 when Comet Linear 1999 S4 was looking likely to be impressive).
I don't have much in the way of workshop facilities, so it had to be simple mechanically. I read had Stephen Tonkins book on Amateur Telesope making, and learned that the bog-standard Scotch mount will only track with reasonable accuracy for about 10 minutes. The issue is that a simple-to-build tangential drive, driven at a constant rate soon diverges from the rotation of the sky.
Ten minutes may well be long enough given how light-polluted our skies are, but ever the optimist, I wanted the option of longer exposures. Mr. Tonkins' book details improvements by Dave Trott, but in the end I decided to go for a simple mechanism that can run at a variable rate.
The second thing driving this is the wish to make a simple device that others could also build.
Stephen Tonkins book also had a section on Mel Bartel's computer-driven stepper motor Dobsonian drive - and that was the seed for my plan.
That drive depends on a full-size PC or laptop, but I did not fancy the idea of carrying a tripod, camera platform and laptop into a muddy field in the dark, so casting around for a solution I eventually discovered you can get a complete computer on a chip - a Microcontroller.
The platform is a basic scotch mount with an ATMEL AVR microcontroller driving a stepper motor to give a continuously varying rate, with the aim of tracking accurately for up to one hour.
Well, Comet Linear 1999 S4 came and went behind a familiar veil of cloud, not to mention disinegrating, so since then I have been in a continuing cycle of redesigning and rebuilding. So far I have not gone beyond 10-minute exposures, but the addition of a pause button has been an advantage, in that I don't have to reset the drive back to the start position for each new exposure - it just carries on where it left off, running at the rate appropriate for the current angle of the arms.