I wanted to make a music box as a gift. I'll extend this post with more details as I photograph components.
First, what are the design goals? It certainly has to be a box that plays music. That means an MP3 player, a speaker, some buttons for control, a rechargeable battery, and a way to charge the battery. These will have to fit into the box, which should be nice.
I also wanted it to have motion -- like the common 'dancing ballerina' you see in music boxes. However, I wanted it to have more than just one, it should have 3! That means it needs at least one motor. I also want the dancing figures to be removable -- so the box can close properly, and so that a variety of dancing figures can be used.
Finally, it needs to include elements of Vietnamese culture wherever possible. To this end, I chose the porcelain figures of dân tộc Việt Nam (Vietnamese ethnic groups) produced by Minh Long, a local porcelain company:
I also wanted to include the pattern from the Dong Son drum, and the music should of course be Vietnamese instrumentals.
Then we get to the specifics. The electronics were pretty easy. I chose a DY-SV5W MP3 player, as the 5W output means I don't need a separate amplifier (and space is limited). The charge controller is a TP4056 based board, with an adjustable output (which I set to 5V). For the battery, I chose a Panasonic 21700 lithium cell, as this allows quite a lot of playback time, and is safer than a pouch cell. For the control switches, I chose small chromed steel industrial switches as they are high-quality and look good.
The we get to the mechanical parts -- this is where things get harder. First off, it is a waste of power and (very limited) space to use 3 motors to move 3 figures. So we need clockwork, specifically a set of gears that transmits the (somewhat too fast) motor rotation to a slower turn rate for the 3 figures. It also has to be quiet, so as not to interfere with the motion. So for this I chose a small gear that connects to 3 large gears of equal size. For the teeth I chose a very low pressure angle to make them run quieter at the cost of some efficiency.
Then the gears need to somehow rotate the figures. Initially, I though a bar shaped socket that they plug in to would work:
However, testing proved that this was fragile, difficult to use, and risked bits of broken plastic falling into the gears. So instead, to the tops of the gears I attached disc-shaped magnets to act as a magnetic friction drive. The figures have a small steel bolt attached to the bottom (invisible once inserted) that causes them to rotate along with the magnet:
To hold everything in place, I used brass sheets cut to size, separated by brass hex spacers. I chose brass because it has a good aesthetic, and is easy to drill holes in. Though trial and error, I found that the gear spacing needed to have a tolerance of about 0.1mm for each part. Annoying, but not impossible with hand tools.
The brass sheet presented an opportunity -- I can etch brass using the same techniques as etching circuit boards. This meant I could include the Dong Son drum pattern on the brass:
This came out well, but left a lot of unused space around the pattern, as the brass place was much longer than it is wide. So I added some traditional Buddhist patterns that are common here. Western audiences should note that the symbol that looks superficially similar to the symbol of a certain terrible regime has a very different meaning here.
