This year for the Scavenger Hunt I mostly worked on this item:

49. Bring us an Animusic-style instrument that plays itself. Once activated, it should perform, unaided, a composition of no less than 90 seconds. While your device may have electronic components, it must not produce electronic music. [175 points. 50 bonus points if the music is played by multiple distinct mechanisms that join in gradually as it proceeds]

I saw this item at around 3 in the morning on Thursday, a few hours after the craziness of list release. It grabbed my attention, both because I knew I was expected to do it and because instruments have fascinated me for some time, though I'd never gathered the gumption to make anything more than a simple didgeridoo. So I cast around in my mind for ideas on how to make something like this in the next few days. First I had to know what the judges wanted, so I looked up the source material on YouTube:

Clearly, I needed some sort of mechanical actuator. Luckily, I had kept some solenoids from my failed attempt at a water curtain in 2013. Another quick YouTube search confirmed that it was possible for solenoids to hit things hard enough to make sounds. Unfortunately the solenoids I had were pulling solenoids, which are not optimal for striking things. So I modeled a simple lever that the solenoid could pull on, and a mount to keep everything in line.

I cut them out the next morning on the lasercutter at the Hack Arts Lab. Unfortunately, I had forgotten to get materials, so I had to use the scrap in the shop. I ended up cutting the main levers out of 3/4" thick plywood, which required 3 passes at high power and low speed to get through. Even then I had to use a Dremel to fully separate the pieces from the stock.

Some judicious application of superglue to skateboard bearings ensued. I like using skateboard bearings for moving parts because they're easy to get and relatively cheap. They also fit great on things that are 5/16" in diameter, such as Bic pens. I like superglue because I like taking risks and do not know fear.

Once the superglue dried, I hooked up the whole apparatus to an Arduino. The solenoids ran on 12V and 1.2A, which is far more than the Arduino can output, so I needed to use some power transistors and an external power source. Fortunately, I had planned ahead and converted an old computer power supply into a serviceable benchtop power supply, with binding posts for +12V, +5V, +3.3V, -5V, -12V, and GND. It even had fancy things like short protection and everything! I got the power transistors from RadioShack. Fat little suckers the size of your fingernail, rated for 90W.

After all the wiring was sorted it was relatively simple to get the lever to move. All I had to do was send a 5V, 20ms pulse to the base voltage on the transistor. Even with my limited knowledge of C I was able to manage that.

It remained to wire up several more solenoid-lever modules so we could play multiple instruments, and have some sort of method for writing music. I ran into some issues which were solved by adding a '\0' to the end of a string and felt like a greenhorn, but it all came together pretty quickly.

My friend Zach, an incredibly talented drummer (and the captain of our team), brought over his drumset on Saturday night. We spied some large pieces of MDF that nobody was using, and made a nice big base for everything. By 6:30 in the morning we had everything wired up and ready. Zach took a quick nap and about half an hour later we had this:

We sat there watching it loop over and over, sleep-deprived, triumphant, shocked and awed by our own magnificence. A teammate snored loudly on the couch, somehow sleeping through everything.

Then Zach scrapped all the music and re-wrote it. Here is the final result.

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