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I have a DMX light that rotates a gobo using a 28BYJ-48 stepper motor. It has four DMX responsive settings: off, slow, medium, fast. However 'slow' is still about four times too fast for the effect I need.

To make it slower I'm considering replacing the 28BYJ-48, if possible, by just swapping it with another 5V stepper motor the same size that goes slower. Can you suggest an alternative motor?

  • I'm not able to alter the motherboard components or recode the firmware.
  • The 'standard DMX speeds' of the 28BYJ-48: slow is 1.83 rpm; medium is 2.09 rpm; fast is 2.44 rpm. I'd like anything slower, but I think .5 rpm would be ideal.
  • I've read reference to 're-gearing' stepper motors. If this would be a better alternative please advise why.
  • I haven't been able to slow the rotation with DMX software. I've sought advice on DMX forums about the software side and there doesn't seem to be a solution there. I've tried 100ms on/off iterations (the minimum time possible) through the software but this makes it jerky.

28BYJ-48 stepper motor back stepper motor front with gobo mount enter image description here

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    \$\begingroup\$ well, all else being equal, a stepper motor with more steps per revolution will go slower. \$\endgroup\$ – Hearth Oct 31 '18 at 23:40
  • \$\begingroup\$ If you want to make a stepper motor go slower. Then do it properly. Don't change the motor (asking for recommendations are offtopic on this site), change the driving signals (asking for how this can be done is very much ontopic on this site). \$\endgroup\$ – Harry Svensson Oct 31 '18 at 23:43
  • \$\begingroup\$ Thanks for the feedback. Should I amend this post title and content or re-post? I have reached out to the supplier and researched online, but I have been unable to obtain a motherboard or circuit schematic. I have the motor datasheet and photos of the motherboard. I have identified some of the micro chips used. Will it be helpful in terms of "changing the driver signals" if I link to the chips datasheets? eg. ULN2003APG: opencircuit.nl/ProductInfo/1000401/ULN2003APG-Datasheet.pdf \$\endgroup\$ – Robin Nov 4 '18 at 23:00
  • \$\begingroup\$ If anyone else is looking for info on this motor, there is a thorough analysis available here: web.archive.org/web/20150215062114/grahamwideman.wikispaces.com/… \$\endgroup\$ – Robin Nov 13 '18 at 0:43
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You are probably out of luck, unless you are really, really lucky.

Your motor is a 4-wire unipolar motor, which probably takes 64 steps per revolution (the small, cheap ones do that). It also has a gear train which reduces 64 step/rev to 4096 step/rev. In other words, it has a reduction ratio of 64:1.

What you're looking for is a similar motor with a reduction ratio of 256:1. And, sorry to say, I suspect you won't have much luck. You're getting down to mechanical gear size limits, especially since I assume you need a drop-in (mechanically identical package) replacement.

You can Google on "geared stepper motor". Who knows, you might get lucky.

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  • \$\begingroup\$ Thanks for your comment and suggestion. No luck. There doesn't appear to be a stepper motor with similar dimensions and axle position. Just the 28BYJ-48 in different specs. I may take one apart to find out exactly what I've got. (I think I have the 64:1 version) \$\endgroup\$ – Robin Nov 13 '18 at 0:42
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As an alternative solution you can try to control the 28BYJ-48 motor with a Arduino board running your own software. Maybe even connect DMX to your Arduino so that you still can control the speed.

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  • \$\begingroup\$ That won't give him smaller step sizes. \$\endgroup\$ – WhatRoughBeast Nov 7 '18 at 19:55
  • \$\begingroup\$ I think the problem isn't the step size, but the longest time between steps controlled from the current electronics and its DMX implementation. Swapping the electronics with a Arduino board would give you a 100% control over the time between each step. \$\endgroup\$ – remco_k Nov 8 '18 at 14:21
  • \$\begingroup\$ Well, sort of. I was responding particularly to the requirement that " just swapping it with another 5V stepper motor the same size that goes slower.". The only way to do this is via smaller step size. Now, in practice (particularly since the desired motion is uniform rotation), slowing the step rate is the obvious way to go, but that's not what was requested. \$\endgroup\$ – WhatRoughBeast Nov 8 '18 at 14:27
  • \$\begingroup\$ Thanks both. I still need a smooth rotation, so I'm not sure if I slow down the steps further that I can achieve this. I think it's worth pursuing and have ordered an Arduino DMX shield but it's going to be a steep learning curve so I'm not sure how far I'll get. \$\endgroup\$ – Robin Nov 13 '18 at 0:46

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