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I want to make an analog subway countdown clock. The idea is to have an ordinary clock with three hands. These hands will point to the number of minutes before the next, second next, and third next train arrives at my local subway station (one of the L stations in Brooklyn, NYC).

I was thinking to implement this using a Raspberry Pi. The software part is easy; however I have no idea how to get started on the hardware part. Is it possible to get a motor which can control three clock hands completely separately but on the same shaft? Or is there another mechanism that could address this problem?

I did some research, and you can buy hardware to control an hours-minute-second clock, but the problem is that you can't decouple the hands and get separate control on each of them.

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    \$\begingroup\$ Pull a centre-seconds movement (most quartz ones) apart, remove the train and the minute wheel. You now have 3 independent concentric hands. Drive the outer one via the hour wheel, the middle one by the cannon pinion, and the inner (second) hand directly. I'd use the hour wheel and cannon pinion from a second movement to give each a 1:1 drive (via idlers if necessary to reach the motors) \$\endgroup\$
    – user16324
    Commented Apr 22, 2016 at 23:31
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    \$\begingroup\$ You will need custom gears and motors. A 3D printer or service may be necessary. \$\endgroup\$
    – Passerby
    Commented Apr 23, 2016 at 0:04
  • \$\begingroup\$ Did you ever get this working? I'm interested in doing something similar and I'm wondering which suggestion you followed. \$\endgroup\$
    – isedwards
    Commented Aug 2, 2017 at 14:53
  • \$\begingroup\$ @isedwards I never did... \$\endgroup\$ Commented Aug 4, 2017 at 3:51

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You can use 3 successive hollow shaft stepper motors like this one: http://www.ebay.com/itm/CNC-Hollow-Shaft-28-2-phase-28HB3302K-Dual-Shaft-1-8degree-33mm-Stepper-Motor-/271809907038?hash=item3f4922095e:g:hwoAAOSwJkJWjO1P

Then use successively smaller shafts through the center of the one in front like this: enter image description here

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    \$\begingroup\$ Keep in mind that stepper motors are incremental devices which will need something to establish an initial position. For example, an optical sensor at "0" and you independently rotate each through that while keeping track. Or more crudely, weak motors and a mechanical stop at 0 which you stall each of them against. You'll have to do this after every power loss, or any time you think your software's idea of where the hands are could have gotten out of sync with their actual position. \$\endgroup\$ Commented Apr 23, 2016 at 0:43

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