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28BYJ-48 Stepper motor questions

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I've completed fitting my DIY Onstep goto system to my 200p, EQ3-2. Works great. I'm delighted.

Onstep can also control electric focusers so that's my next project.

I want to use a 28BYJ-48 stepper motor because it's small , light & cheap & I've seen it used to drive focusers on the forum, (but indirect drive with pulleys or gears)

My focuser is the stock Skywatcher 2"/1.25" rack & pinion 1:1 . I want to use direct drive for simplicity & minimum cost.

      Has anyone direct driven this focuser with this motor? (Has it got enough torque?)

Since the motor is unipolar (and 5v) & Onstep only supports bipolar steppers I will be rewiring it to bipolar & running  it at 12volts.

     Does using the motor like this give the same torque , or double or half?

     Does it give the same step angle or double or half ?

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4 minutes ago, Gina said:

IMO a NEMA17 is overkill for focussing of small to medium telescopes.

Yes I would prefer the small motor if possible so that if something goes wrong it will probably just stall rather than destroy the focuser.


4 minutes ago, Gina said:

Depends on what weight you hang on the focuser.  I used a step down pulley or gearing system.

 I'm just visual so the max payload is a 32mm Plossl.

 I want to avoid pulleys if I can, to maintain my reputation as a cheapskate.?

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My 200p is F/5. It is very difficult to nail the focus with the stock 1:1 manual focuser.

The critical focus distance is about 60 microns which equals a movement of about 0.3mm at the perimeter of the focus knob.

I should be able to get a resolution of about 6 microns with the BYJ if it works. 

Also since the mount is a bit shaky I should get less vibration  focusing with the phone app rather than manual.

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I use a 28BYJ48 / ULN2003 combo via rubber-band (Meccano motor belt drive pulley drilled out to fit the BYJ shaft, driven with the stock meccano rubber band) direct to the 10:1 fine focus knob of my 200PDS. Works brilliantly. Tie wrap round the end of the knob to stop the band slipping off. Not enough torque to drive the coarse focus knob via rubber-band without slip, but would probably do so via belt or direct drive. As it’s rubber-band drive, it just slips when it hits the stops and has no chance of causing any damage, but has plenty of grip to shunt my DSLR about / hold in position.

Gives me around 110,000 steps from full in to full out, so rather fine control, but this gets translated down 4:1 step range for use with TekkyDave’s Ascom driver & APT (one of which doesn’t support the native step range - can’t recall offhand which). Motor is mounted via Meccano to a spare scope ring, onto the camera thread. Entire thing is driven by a Raspberry Pi 1 model B running node-red to translate the serial protocol of TekkyDave’s driver to the simple bash commands to drive the motors.

As it’s such a cheap n’ cheerful (sub £30 in total) chuck-on-and-go solution, I never bothered thinking about step angles and torque etc... just threw it together and tried it for giggles, and it worked, so I stuck with it *shrug*. 



Edited by Marci
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Love it!  I've used ordinary elastic bands as supplied by the postman wrapped round post items.  For the 1:1 (only) focuser on my MN190 I used a timing belt and pulleys.  The 28BYJ-48 little stepper motor is quite adequate and oh so cheap.

Edited by Gina
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Brilliant design Marci. I love the use of Meccano. Wish I hadn't thrown mine away decades ago.

For the sake of science I've just measured the torque required by the (newly greased) SW 1:1 r&p focuser.

I wrapped some string around the focus knob and added 10gm oxo cubes till it turned smoothly.

It took 200gm which equates to a torque of 400gm.cm (5.6 oz.in) with a typical eyepiece.

Since the 28BYJ-48 should be good for about 10 oz.in in bipolar mode I'll go with that, direct drive.

Since Onstep includes focus control software I will only need the motor, bracket, cable & a drv8825 driver so the cost should be less than £10!

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