Introduction
Ever since I was a child, I wanted to make a music box. I always found them charming, and all the gears fascinating!
Unfortunately, I am pretty terrible at mechanical engineering. So I cheated a little and used electricity -- although it still needed custom gears. This project took a long time, about 8 months. Mostly, it was hard to find a box that was right for the task.
The final result was pretty good, featuring etched brass, tropical hardwood, hand-painted porcelain, and traditional Vietnamese music. There's a custom friction drive mechanism using magnets that's mildy clever.
Electronics
Unlike most of my projects, the electronics here are very simple. Everything is a standard component or module, and there is no custom firmware. I am also (purposefully) light on the schematics and build details.
This was meant to be an artifact: something unique. Produced once, and never again. Then given away.
Nonetheless, the build details I include here should be sufficient for you to help you build something similar, but not exactly identical.
Power
Power is supplied by a rechargeable 21700 lithium cell. It's recharged using a standard USB lithium cell charger module. There is a boost converter to provide a 5V line, and a buck converter to provide a ~ 3.3V line.
Sound
I used an MP3 music player module for the sound -- the DY-SV5W. I chose this one because it can properly drive an 8Ω speaker loudly enough. A lot of the really cheap MP3 player modules are more suitable for headphones. Due to space constraints, an external amplifier module was undesirable.
This module accepts a standard microSD card, which I loaded with 16 tracks of traditional Vietnamese music. I provided chromed buttons to control volume, and rotate through the tracks.
Electromechanical
A standard N20 geared motor controls the rotation of the 3 porcelain dancers. I used one with a 90° gearbox with a high gearing ratio -- the dancers need to rotate slowly and gracefully, not spin like a top.
Mechanics
This motor was still too fast, and the motion not smooth enough, so I 3D printed gears to provide mechanical advantage. I used a very low pressure angle for the teeth to keep operation silent at the expense of some efficiency.
I used 4 gears -- one small one and 3 large. The large gears have a strong magnet on top. The porcelain dancers have a magnetic metal piece adhered to the base -- they snap into place onto these magnets, through holes in the brass plate. The friction between the magnets and the metal piece is enough to turn the dancers.
If the dancers get stuck on something the friction is not enough to stall the gears. In fact, you can easily swap the dancers while the device is in operation (this is by design). There are 10 dancers available, any 3 can go in at once.
The Top Brass Plate
The mechanical components are encased in a brass frame -- the top half of which needed some kind of design applied to it. I am not an excellent designer, so this took me a while to figure out.
Etching the Top Brass Plate
I decided to etch in a Đông Sơn drum pattern that I modified a little to fit. This was done with the standard toner-transfer method used to etch PCBs at home. It took a long time to etch, paint, and coat. Manufacturing the top plate took a solid 7 hours of labor.
Painting
I used standard water-based acrylic paints, and then applied a matte clear-coat to protect it. Then I noticed that the plat was 4mm too big, and had to completely restart.
The Box
Finding the right box took ages. I looked through many sources. Finally, I found something perfect in the Hàn market in Đà Nẵng. Sadly, this market is aimed at tourists so the prices are quite high for me. The vendors have to pay more to rent stalls around there, so I can't negotiate too hard.
Most markets I frequent (especially wholesale and industrial markets) are Vietnamese-language only, they are very busy and don't have time to deal with tourists. Although this was a market for tourists, speaking Vietnamese is a shibboleth and the vendor was nice enough to cut me a little slack on the price.
The Porcelain Dancers
The porcelain dancers were comparatively easy to find! A locally famous high-end porcelain manufacturer, Minh Long, makes a line of small hand-painted porcelain figures wearing the traditional costumes of different ethnic groups in Viet Nam (dân tộc Việt Nam).
Concluding Remarks
This turned out super well, although it took most of a year of slowly finding parts, figuring out gear formulas, and so on. It was not the only project I worked on during this time, but it was always on my mind.
The mechanical accuracy required for getting the gears to not jam was about 0.1mm. If a single gear was off by more than that, they would jam. This was definitely a bit of a pain, but quite achievable on a small desktop drill press with some care.
