Halfmoon Chimes: A Raspberry Pi-Powered Instrument for a Music Festival

Ben Holmen, a developer from Eau Claire, Wisconsin, built a unique interactive instrument called Halfmoon Chimes for the 2026 Eaux Claires music festival. The instrument, inspired by the festival's ethos of collaboration and audience participation, allows attendees to play a 37-note chime, record their performance, and receive a physical receipt that can be scanned by others to replay the performance.

Concept and Design

The core concept was to create an instrument that was playable, recordable, and replayable. Holmen chose a MIDI keyboard interface, with solenoids acting as hammers to strike aluminum pipes tuned to notes from C4 to C7. The design was constrained by material properties: larger pipes are needed for lower notes, and achieving a good tone below middle C is difficult. He settled on a half-moon arrangement of the chimes, a nod to Halfmoon Lake near the festival grounds, which also solved several engineering problems.

Engineering and Implementation

The instrument is powered by a Raspberry Pi 4 running a multi-threaded Python script that handles MIDI input, barcode scanning, recording, and solenoid control. To drive 37 solenoids, Holmen used chained solenoid breakout boards, requiring only a few GPIO pins. He mapped MIDI velocity to solenoid pulse duration, ranging from 10ms for soft hits to 40ms for the hardest.

Each chime was carefully tuned by cutting and refining aluminum pipe lengths. The pipes were polished using a custom lathe setup with a drill. Custom brackets, called 'canoes,' were fabricated to support each pipe at the nodal points (22.4% from each end) to allow free vibration.

The Receipt System

A distinctive feature is the receipt system. When a participant records a performance, the instrument prints a receipt with a barcode and a visual representation of the notes played (circles sized by velocity). Other attendees can scan the barcode to replay the performance. The receipts become part of the installation, surrounding the instrument with the collective contributions of attendees.

Festival Success

At the festival, the instrument was a hit, attracting thousands of visitors over two days. It recorded 447 performances, printed 716 feet of receipt paper, and saw over 400,000 notes played. The project demonstrates a creative blend of hardware, software, and interactive design, and serves as an inspiring example of what's possible with accessible technology like Raspberry Pi and Python.