# Simplest solution for a fixed speed stepper motor?

#### McPit

Joined Aug 17, 2014
11
Hi all,
Newbie here..
I'm looking at replacing an old and broken DC motor+gearbox which drive a rotating planet in a Judge Dredd pinball machine. (Very expensive to repair.)

I'm thinking what if I could find a stepper motor with enough torque and control it with the most basic standalone driver (does that even exist?) which I would program once and for all for a specific direction and speed (unknown at this stage, would need to be tuned) and which would simply be activated by the machine's original 12V signal!?

Is that any realistic? What kind of motor and driver should I be looking at? Any view/direction most appreciated!

Thanks a lot!
Anthony

P.S. For those who might know the machine, the planet does actually rotate correctly. Only the disk, which is supposed to rotate the opposite direction, doesn't, due to heavily worn out gears. So the new motor would actually come on top of the existing one. I'm thinking MOSFET if the original motor's driving transistor isn't able to handle two motors.)

#### MrSoftware

Joined Oct 29, 2013
2,027
What are your requirements for space, and do you have the ability to code in C, or alternatively to program micro controllers? And how smoothly does it need to rotate?

What I'm thinking is if you want smooth rotation, you'll need a stepper controller that can micro step. In my limited stepper experience, you can use something like an arduino uno, or the $3 generic version, to control a stepper based on the L6470 controller IC, which will micro step. The stepper controller has a lot of configuration options and you talk to it via SPI, which the arduino will support. There is a pretty good arduino library for that stepper controller (Google for dSPIN library) so your coding will be pretty high level. So using C code you can customize the stepper controller parameters for whatever stepper motor you end up with. When the arduino boots it will initialize the stepper controller and start it stepping. I'm sure there are multiple other stepper controllers that will work, I just happen to be familiar with this one. #### mcgyvr Joined Oct 15, 2009 5,394 Just find another suitable dc motor/gear Post specs or pics of the original motor for suggestions.. #### cmartinez Joined Jan 17, 2007 7,686 Just find another suitable dc motor/gear Post specs or pics of the original motor for suggestions.. Yeap, some pics and the original motor's general dimensions would come in handy #### MaxHeadRoom Joined Jul 18, 2013 25,183 You can buy stepper controllers on ebay for a low as$2.65 p paid!
As per @cmartinez the original motor spec would help.
Max.

#### McPit

Joined Aug 17, 2014
11
My initial naive thought was that I would have to go with a stepper motor because of torque constraints, and because I didn't want to start fiddling with gearboxes. But Dodgydave's initial reply made me realize there ARE affordable plain DC motors out there, sometimes called "high torque", which are bundled with a small gearbox. I estimated the RPMs I need to about 7, so I just ordered this little guy:
http://www.ebay.com.au/itm/Replacem...ox-Speed-Control-Electric-Motor-/381196549506
..and I'll just give it a resistor or potentiometer to slow it down to what I need.
(Now I'm going to have to play with some shaft couplers and spur gears but that's another story. Let's see how it goes.)

Again thank you all heaps! Much appreciated!

P.S. For those interested here's the actual forum thread: https://pinside.com/pinball/forum/topic/judge-dredd-planet-gearbox-revealed-2/page/2#post-2882749
I looked everywhere and those exact original gears are impossible to find. Just not the kind of pitch diameter/number of teeth/module/bore diameter that people manufacture nowadays.

#### cmartinez

Joined Jan 17, 2007
7,686
..and I'll just give it a resistor or potentiometer to slow it down to what I need.
You could use a resistor to slow it down, but you're going to have some heat generated. On the other hand, you'll probably run into trouble if you try to use a common pot, since most are rated for 1/2 watt tops.

Using pwm would be much wiser, but a little more complicated. If that's the case, come back here, and we'll try to help you with that too.

Good luck!