I decided after my first couple of attempts using a webcam that I needed an electric focuser, but after scouring the internet,
I was simply unable to source one with the correct curve on it's base for my telescope (Orion Optics (UK) 12" F4), so
decided to try and make one myself.

The whole project has cost £29.99 (2004) but the same could be made with new parts for as little as £12 if you use my parts list below.

Being from the ‘what parts do I have’ design school, I decided to buy a kit of cogs, worm gears, rack and pinions, motors etc. from Maplin and see what I could come up with.

If you know exactly what gears you need you can get these direct from Maplin's – at a much lower cost than it has cost me buying a full kit. However with a 'keep-trying-until-it-works' approach buying a kit with lots of bits was the best strategy.

I settled (experimentally) on a gear ratio of 256:1 plus a worm drive giving 40:1 a total gearing of 10240:1, quite steep gearing, but since I only wanted the electric focus to do fine focus, using the manufacturers (MFA) data this gives me a full revolution of the focus knob in 42 seconds.

I might try reducing the gearing a little, this would allow a single turn in 10 seconds, but might be too much for the little 3V motor to handle - a modification might be to use a 12V motor

The kit came with perforated steel sheet and strips, using these I fashioned two mounting brackets, these are mounted to the focuser using the original focuser mounting holes – no drilling of the tube :-)

The focus knob is fixed to the shaft of the focuser by a single screw threaded into the focuser’s shaft. I screwed a thread into the shaft in place of the original screw, slid on the focuser knob, then used a nut to secure the knob, the gear for the worm drive is slid over the thread. A nut has been glued to a wheel from the kit using a hot glue gun, and is threaded onto the shaft tightening this wheel secures the gear engaging the electric drive.

A modification I intend to make here is to stick thin rubber sheets to the surface of the gear and focus knob so that the locking wheel doesn't have to be tightened as much

Note: these are the parts I used and not necessarily the best design. The MFA-comodrills web-site has some great stuff, had I been aware of this manufacture, before purchasing the kit I might have chosen a different design...

Geared Motor Kit MFA/como drills Part No 917D14
Actual Parts used in kit - prices are those quoted on literature I received with kit (2004)
Gearbox Multi-ratio (Assembled) Part No 917D/A (£6.99)
Worm Drive Set Part No 917D/4 (£1.69)
Perforated metal strip Part No 917D2531 - (£3.53) could use leftovers from metal sheet below.
Perforated metal sheet Part No 917D2540 (£1.96)
Robotic Wheel - 36mm Part no 917D2502 (£0.98)

Threaded shaft
2x nuts for threaded shaft

Here is the Cct diagram to control the focuser, nothing elaborate, note pushing both push buttons at the same time preferences Sw1, this is to ensure there are no short ccts!

Electrical Parts List:

Note: All these parts I had lying around...

A 3V power source
This is a bench power supply I've had knocking around for years, it will suffice until a 3V AC adapter can be sourced for next to nothing...
Wire
DPDT non locking push buttons
- commercial units seem to favour push buttons
See Photo of Julian T's electric focus using the same design :-)