Building an electric focuser for an LXD75

Oh one other question, are you using ful steps or half steps? The way I wrote the PIC software was to use full steps for a fast slew and halfsteps for fine focussing. I considered microstepping but the thought of all the PWM etc doesn't appeal to me much. I think half steps are more than enough anyway.

The stamp uses a PIC16C57 and has a built-in RS232 port so it can be programmed from your PC. You can also use this port for your programs to communicate. The motor I've scavenged from the printer has only two windings (=bipolar), so I have to use two H bridges to power it. From your component list I assume you're using a four winding motor. I intend to use a ULN2064B and four TIP150 for the bridges as I already have those parts. (The TIP150s are even more overkill than your TIP120s). I hope to build the driver circuit sometime this week.

As for full or half steps, I've not decided yet. I'm not even sure if a bipolar motor can run in half steps. The motor has a step size of 7.5 degrees, so I think even with half stepping I'd need to use gears. What level of resolution are you aiming for at the focuser? I reckon that without gearing I'd get about 0.5mm per full step. Luckily, I also retrieved the gear train from the printer.

Now that I'm planning to use the Stamp, I'm thinking of using a variable speed in manual mode, the longer the button is pressed, the faster the motor is pulsed.

I see from this article www.cs.uiowa.edu/~jones/step/an907a.pdf that I can half step (or even micro-step) a bipolar motor.
If I half step and use a pair of gears, I can get a resolution of 1/16 mm which I think should be good enough.

Dave,

I've built a stepper microfocuser which I'm using with a WO crayford on a C9.25. Uses a microswitch to find a reference point on power up & as a focuser which will let you return to a specific point it is great. At the moment I'm using it as a PC/Manual controlled focuser as I've made the unit ASCOM compatible.
However I'm using the motor in half step mode & the vibrations are causing me problems when I try to use it with focus max. I was going to use microstepping, but using PWM drive also seems to vibrate the setup enough to smear things a bit. I'm thinking of re-designing to use a DC motor with an encoder. I'd say much of the problems I'm having with the stepper are down to mechanical issues, but I find the electronics easier to deal with than the mech stuff. Anyone else had these issues using a stepper for focusing?

Joe