Fracture

Fracture - The Infinitely Chainable MIDI Keyboard

A modular, glowing wall of keys with its own brain, synth, and buzzer per board.

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Renders

Visuals because everyone loves eye candy.

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What is this?

It's an infinitely expandable MIDI keyboard system built around an ESP32-S3, designed so each board is a fully-featured synth module. When you connect a bunch together, they behave like one giant, continuous instrument. Each board gives you 2 octaves of hall-effect keys (24 main keys), plus 8 “function” keys for extra tricks. It also has onboard I2S audio output, a buzzer synth per board, and 45 addressable RGB LEDs for underglow + status. Boards talk to each other over RS-485 chaining so note events get shared, and it also does USB MIDI, meaning any board plugged into your computer speaks for the whole keyboard.

One board is fun.
Four boards gives you a full velocity sensing piano.

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Features

Infinitely Chainable Boards

Each board is its own little 2-octave chunk: 24 main keys and 8 extra keys for function/mode stuff. The boards discover each other physically using an RS-485 bus (SP3485) and left/right neighbor sense pins, so you can keep chaining them together.

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Hall-Effect Velocity Sensing

No mushy rubber domes here. Keys are read using dual 74HC4067 analog multiplexers, and velocity is computed using the time between “start of motion” and “fully pressed”.

It’s all done in firmware - no external velocity hardware, just some math and a lot of analog readings.

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Onboard Polyphonic Synth (I2S DAC)

Each board is a tiny synth. It uses an ES9023P I2S DAC driven at 44.1 kHz, stereo, and supports up to 8 simultaneous voices.

You don’t need a computer to hear anything, the keyboard can sing on its own.

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Polyphonic Buzzer Per Board

Because one audio path wasn’t enough. Each board also has a timer-driven buzzer synth that runs at a 20 kHz sample rate. It’s used for a startup jingle, any feedback sounds, and mild harassment of anyone nearby.

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RGB Overlay (Key Underglow + Debug UI)

Each board has 45 WS2812 LEDs and they’re not just there to look pretty (but they do). Every note triggers a “splash in the pond” ripple, and the ripple spreads based on MIDI note distances, not physical LED position. Since note events are shared over RS-485, the ripple effect works across all boards.

Basically: the keyboard tells you how it feels in RGB.

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USB MIDI for the Entire Chain

Any board plugged into USB exposes MIDI for all boards. So you can plug in literally any board in the chain, open your DAW / synth, and play the entire keyboard from that one cable.

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Rotary Encoder

Because knobs are scientifically proven to make things better.

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BOM

All the silicon, magnetics, and glowsticks live here.

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CAD

The CAD is open source here.

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Why?

Because regular MIDI controllers are boring. Off-the-shelf keyboards don’t re-map themselves and you usually cant chain them together.

Fracture exists to be modular, mildly smart, and just slightly over-engineered.

I honestly thought this would be an easier project, but feature creep got the best of me...

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Usage

After you have your board, follow this guide for flashing settings and flash Fracture_Keyboard.ino onto your board.

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Contribute

Ideas, improvements, and constructive roasting welcome.

• Open an issue
• Submit a PR
• Or suggest cursed new expansion ideas

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Disclaimer

This project is open-source and powered by vibes and a sprinkle of C++.

If you:

• Somehow mess up the RS-485 bus
• Drive speakers directly off something that shouldn’t
• Misconfigure power and let the smoke out

That’s on you.

Be careful, be smart, and enjoy building a keyboard that expands with your musical ability.