Automatic xylophone with homemade solenoids 🎶🧲

I used a hair straightener to melt the glue. Its broad heated surface made it easier to spread the glue evenly, which was faster and more convenient than using a glue gun.

• Counting every turn by hand was tedious, so I used a timing estimate:
  About 400 turns gave me 2 Ω. I counted the turns made in five seconds, then used that winding rate to estimate how long 400 turns would take.
• I also got lucky: filling the space between the washers gave almost exactly the coil thickness needed for 2 Ω. That became a useful visual guide, without relying entirely on counting turns or timing each winding.

I drilled holes in a wooden support to hold the solenoids. It held them firmly and was simpler to build than the 3D-printed design, so I used it for the final mount.

Quick note

Adding a permanent magnet on top can dramatically increase the force for the same electrical input, but I chose not to use one because:

• My washers are metal, so > the magnet and nail would stick to them.
• Striking a key with the nail’s tip worked better than using the magnet’s wider face. The larger contact area could damp the key’s vibration, although the effect was small.
• Most importantly, it already worked well without the magnet using the 5 A power supply.

BJTs vs MOSFETs

• I chose TIP120 NPN transistors. Several Reddit users suggested MOSFETs instead to reduce voltage loss & heat, but the MOSFETs I found cost about three times as much, and I did not want to wait for shipping.

If it works, don’t touch it?

• I was used to the approach “if it works, don’t touch it” . I initially ignored the solenoids’ back EMF and left out the protective diodes, since the circuit appeared to work without them.
• The early tests used relatively little current. Once I connected the 5A, 12V supply and tried the full load, some transistors burned out. So much for “don’t touch it” :)

Digital audio software

LMMS is the digital audio software I used to:

• Read MIDI files.
• Mute the unwanted tracks, such as bass and drums, to isolate the instrument I wanted to play.
• Send the MIDI output to a virtual port (see below).

Virtual MIDI port

• LMMS needs a MIDI output device, so I created a virtual port to pass its output to the next program. The setup may differ on macOS; on Windows, I used loopMIDI successfully.

Bridge

• I used Hairless MIDI to forward the MIDI output to the Arduino over serial. The “Note: XX” values are note numbers defined by MIDI; for example, middle C (C4) is 60.

Arduino and MIDI library

• Map each solenoid to an Arduino pin and assign it the corresponding MIDI note number.
• Use a function to read incoming MIDI messages.
• The xylophone has 24 notes across two octaves. Notes outside that range are shifted to an equivalent note within it, for example, C6 becomes C5. It is not a faithful reproduction of every piece, but it is better than dropping the note entirely. The mapping accepts all 88 piano notes while keeping playback within the instrument’s physical range.