Building an interactive instrument for a one-of-a-kind festival

Source: https://benholmen.com/blog/halfmoon-chimes/


Photos by Andrea Paulseth, Evelyn Nelson, Luong Huynh, Nick Meyer, and Kip Holmen.

Halfmoon Chimes at Eaux Claires 2026

My hometown - Eau Claire, Wisconsin - is home to a singular music, literary, and art festival that is the brainchild of Justin Vernon (Bon Iver). Called Eaux Claires, it's unlike any other major music festival, and has a heavy emphasis on unique collaboration, audience participation, and unexpected delights. Besides world class musicians performing unique sets and collaborating in new ways, the festival has hosted a wide range of art installations to be discovered if you're willing to dig for them. I've experienced every version of this festival and even helped with an art installation in the 2016 edition. After running from 2015-2018, the festival took an indefinite hiatus.

!Image 2: Eaux Claires - July 24-25 - Carson Park - More Something, For More Everyone

In the 2018-2026 interim, I'd been pondering an idea for a custom instrument / art installation. It was very specific to the ethos of the Eaux Claires festival and the community around it, and I doubted that I'd have the chance to actually build it and share it with others. In the meantime, I focused on my kilopixel, and had just finished a successful installation when I heard Eaux Claires was returning in 2026. I immediately started prototyping my idea, figured out who the organizers were, and pitched it. My success with the kilopixel gave me confidence that I could pull this off.

Video 3

The core concept

The instrument would have three essential functions:

- playable live - walk up to it and start playing immediately

- recordable - push a button and record what you're playing to share with others

- replayable - select a recording from another festival attendee and replay it

Recording would produce a physical artifact - a receipt of your work - that would become part of the art installation and slowly surround the instrument with the contributions of attendees.

I built a prototype of the core functionality to prove that I could do it, sketched the concept, and sent a video to the organizers. They said yes! I would actually build this dream instrument!

Material selection + ideation


I knew that I wanted to base it on a MIDI keyboard of 2-4 octaves, and I knew I'd use a solenoid as a hammer to strike the note. I went full percussion section mode, learning as much as I could about different types of vibraphones, marimbas, glockenspiels, xylophones, and more. I eventually settled on an improvised chime made from metal pipe - a bright, pure, sustaining tone. I found the most incredible old internet site, where I learned everything I could possibly need to know about chimes.

I learned so much about chime material and constraints! For one, the diameter and wall thickness is critical. Thin steel pipes sound clanky and thin; thick aluminum pipes sound pure and sweet. Large pipes are required for low notes, and it's really hard to push below middle C with good tone. I shifted the entire instrument up one octave due to material constraints, settling on C4 (middle C) to C7. 37 notes to build and play.

!Image 4: Aluminum and other metal stock on a rack at a metal supplier.

My early prototyping with solenoids was successful - a 12V, 5N solenoid can hit hard enough and consumes a reasonable amount of power. I experimented with hammer materials - wood, plastic, felt, and cork. I found the right cork pad to dampen the blow enough but still ring out.

So I had a solution: aluminum pipe, a solenoid, cork pad, 12 volts, and a computer.

Tuning chimes

You intuitively know that the length of a chime has something to do with the note it makes. This is called the fundamental frequency, and is determined by the material, the wall thickness, the diameter, and the length. I had a chart of approximate lengths for a given note, and I cut them long and refined them millimeter by millimeter. Some notes tuned easily - others gave me trouble. But I stuck with it, carefully shaving down each note until it rang at just the right frequency.

!Image 5: A pile of aluminum pipe is sitting on a work table. They're arranged by length and cut roughly.

!Image 6: Aluminum pipe pieces and chips are seen on a bench.

Polishing chimes

I sourced the chime material from a local metal supplier. It's in good condition but inconsistent, and not really up to the standards of an art installation. I found the best strategy was to make a mini-lathe and polish the pipe with abrasive pads. It's possible to get to a mirror finish this way, but I just needed consistency - all chimes should look the same.

I fabricated some chocks and rigged a lathe setup with my drill. It was slow going, but the results looked great.


!Image 8: Three aluminum pipes are shiny and clean, alongside five rough unpolished pipes.

!Image 9: A dirty hand is covered in aluminum slurry

!Image 10: Clean, soapy, and shiny aluminum pipes are seen on a wooden deck. They're being washed

Supporting chimes

A chime tube can be held at two places without interfering with the vibration - 22.4% of the length of the tube from each end. Wind chimes are always supported at one of these points, but I decided to use both points to hold the chime but let it ring freely. I created custom brackets to hold the pipe, suspended it on a copper wire, and attached the bracket to a piece of walnut. I called these my canoes, as they were reminiscent of the shape and fit a water theme I was using.

These canoes were each unique - 37 distinct parts that I shaped by hand. I made four different bracket sizes, one for each pipe diameter. And to attach them to the instrument, another custom bracket that could hold the canoe, a support arm, and the solenoid in just the right orientation.

!Image 11: Screenshot of 3d models for chime brackets

!Image 12: Screenshot of 3d model for support bracket

!Image 13: Wooden supports hanging from a rope. They're made of walnut and aligned in a pleasing array


This completed my chime and solenoid assembly and represented many hours of careful work.

Under the hood

The instrument is built around a Raspberry Pi 4 running a multi-threaded python script. The script is doing a few things as fast as it can:

- listen for any new MIDI messages

- listen for any new barcodes scanned

- track the state of the recording button

- save MIDI messages (if recording)

- if a new barcode is scanned, add MIDI messages to a queue to be played at the right time

- turn on any solenoids for any new MIDI messages or playback MIDI messages

- turn off any solenoids if they've been held on long enough


Controlling 37 outputs with one Raspberry Pi isn't possible unless you use an addressable bus of some kind. I used solenoid breakout boards that allowed me to chain up to 6 breakout boards with just a few Raspberry Pi outputs. This was more than enough - 8 outputs per breakout board meant I could control my solenoids with room to spare. My code could turn on and off the solenoids at will.

I experimented with converting a MIDI velocity to a solenoid hit, and found that I needed to power the solenoid for about 10 milliseconds to barely tap the chime, and 40 milliseconds was the maximum duration. I mapped this to a MIDI velocity - the hardest hits get a full 40ms, the softest key presses get 10ms.

!Image 16: Solenoids attached to long wires. Ready for installation

!Image 17: Solenoids breakout boards wired up to 37 wires.

Overall form

I knew that the project had to hit three notes:

- sound beautiful

- look beautiful

- play intuitively

I puzzled over the complete design of the instrument, and specifically where to place the chimes in relation to the participants. I happened into the name _Halfmoon Chimes_ in homage to Halfmoon Lake that surrounds the festival grounds. This gave me the inspiration of arranging the chimes in a half moon shape, which solved a number of design and engineering problems at once. I never really looked back.

!Image 18: A circle cut out of a sheet of plywood with a router on a simple circle cutting jig

!Image 19: Walnut pieces radiate from a central point in a pleasing manner. They contrast with a birch plywood base.


!Image 21: A circle cut out of a sheet of plywood with a router on a simple circle cutting jig


The receipts

I knew from the beginning that I wanted a physical artifact of a recording, and I settled on a receipt. There's something appealing about the immediacy and tangibility of a receipt! I had conceived a vending machine style feeding mechanism that would slowly pull the paper into the instrument, but I rejected that due to complexity and breakdown potential. However, to achieve that feeding I needed to include a barcode and it had to be able to be scanned from either direction which led to the design of stretching a barcode and hiding it in plain sight. I liked the distinctive and abstract look of this so I kept it in the final design.

This meant that each receipt was completely unique - the circles represent the note position, and the size of each circle is determined by how hard the note was played. You can kind of tell the style of the song by the receipt, but it's still very serendipitous - the song might be great, or it might be boring. You'll only know once you scan it with the barcode scanner and replay it!


!Image 24: Receipts clipped to a line, blowing in the breeze. Photo by Nick Meyer

A pre-festival open house

I wanted to open the festival with some music already recorded - without it, early participants might have a hard time understanding how to interact with the instrument. So we hauled it into our backyard and held an open house for friends, acquaintances, and strangers alike. The party was warm and encouraging - about twenty generous folks showed up and played together. It proved to me that this thing had the juice.


!Image 26: The instrument, ready to play in the back yard. Photo by Nick Meyer


Live and up close

I could not have been more pleased with the reception at the festival! A few thousand people visited my quiet corner of the festival over two days, and the instrument consistently had a crowd of people around it trying to figure it out or play with it.

!Image 28: Photo by Evelyn Nelson.

!Image 29: Photo by Evelyn Nelson.

!Image 30: Photo by Evelyn Nelson.

!Image 31: Photo by Andrea Paulseth.

!Image 32: Photo by Luong Huynh.

!Image 33: Photo by Andrea Paulseth.

!Image 34

The instrument recorded 447 distinct performances and printed 716 feet of receipt paper. Attendees replayed 336 performances, and more than 400,000 notes were played.

#### Photo credits

Thanks to Andrea Paulseth, Evelyn Nelson, Luong Huynh, and Nick Meyer for the photos. All hail Volume One!