I recently purchased a DIY music box kit from a website which I will not link to here. If you watch this video you can see that it has a serious design flaw: it has a tiny hand-crank, and it's thus next to impossible to play your custom tunes at a constant rate. The thing sounds like crap for this reason alone.

I'd like to automate the crank with an electric motor, since I am under the impression that it's pretty hard to make your own wind-up music box (difficult to get the wind-up mechanism to turn at a constant rate, involves fans and other intricate timing mechanisms, etc).

So it doesn't take much torque to turn the crank, but it needs to be turned approximately 100-150 times per minute in order for the music to sound decent. What kind of motor am I looking for? It should be relatively small, so I can fit it in the wooden music box I'm making along with the musical mechanism itself and whatever other contraptions I need to put it all together. I'm also pretty noob with electronics, so I'd prefer not having to put together my own circuit board or anything; at most I'd like to just make a few solders and hook up the battery, then snap on a few widgets & gears and pump out those tunes.

What kind of motor should I use?

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    \$\begingroup\$ Since you need to have constant rotational velocity of the motor, you'll have to make a circuit board of some kind. Since you need precise speed control, you'd probably need a stepper motor. If you have experience with microcontrollers, they are the way to go, but otherwise, a 555 timer would be useful, since it has much lower setup costs. \$\endgroup\$ – AndrejaKo Feb 5 '12 at 10:22
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    \$\begingroup\$ EGM has some DC-driven music boxes, you might be able to use the mechanics. goldmine-elec-products.com/prodinfo.asp?number=G18177 \$\endgroup\$ – Wouter van Ooijen Feb 5 '12 at 19:45
  • \$\begingroup\$ Any luck with this project? I am actually attempting to do the same thing: physicsforums.com/showthread.php?t=610663 \$\endgroup\$ – user10094 Jun 1 '12 at 22:41

You will need an electric motor fitted with a simple gearbox: An example would be this one: sparkfun gear motor

Then your job is to tinker an mechanical connection to the crank of the music box and provide 6V to the motor. A 9V battery and a linear regulator like the LM317 will do the job; your circuit will look very similar to this:

typical lm317 circuit

When you mount the motor into a case, you might want do decouple it from the case so the sound of the motor doesn't get amplified by the case bottom. So don't just bolt it down but use some kind of rubber feet.

Update: I thought about other sources for electric motors and came up with the idea of gutting an old cassette player: The bigger ones (read: not walkmans) used to have a flywheel which would provide a very smooth rotation of the gears (obviously to avoid variation in tape speed which would alter the pitch of the sound). Mounting such a thing an connecting could be a little bit more tricky, and I don't know how much current these motors need (especially when the heavy flywheel is standing still).

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    \$\begingroup\$ Would the motor speed be constant enough with this setup? Also wouldn't the standard 9 V batteries be very expensive? I think that it would be much better to use a wall wart instead of the battery, since 9 V batteries have very small capacity. \$\endgroup\$ – AndrejaKo Feb 5 '12 at 11:34
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    \$\begingroup\$ If the gearbox turns smoothly there will not be much jitter I guess (the music box needs only a minimal torque to be turned, so it doesn't interfere much). As for the power source: machine yearning mentioned a battery, so I guessed he/she did not want to use a wall wart (which would be my personal preference). \$\endgroup\$ – 0x6d64 Feb 5 '12 at 11:53
  • \$\begingroup\$ Thank you very much for the thoughtful and comprehensive answer. And with pictures to boot! \$\endgroup\$ – machine yearning Feb 5 '12 at 13:18
  • \$\begingroup\$ So what's the advantage of using a wall wart besides the fact that it won't run out of juice on me? \$\endgroup\$ – machine yearning Feb 5 '12 at 13:20
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    \$\begingroup\$ @machine yearning Compared to non-rechargeable cells, pretty much none (well except the price, since the 9 V batteries have really really bad capacity, but you could make a 9 V battery by connecting say AA cells in series to avoid the problem and get say 4 times the capacity). Compared to rechargeable batteries, you don't need a low-voltage cut-off circuit. \$\endgroup\$ – AndrejaKo Feb 5 '12 at 14:31

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