ExpressionController

expad - Expression Adapter

A four-jack USB MIDI adapter for expression and sustain pedals, built on a Raspberry Pi Pico 2 W. It works out what is plugged into each jack - whichever contact a pedal's wiper is on, and whether it is a potentiometer, a switch or a two-wire rheostat - and sends every pedal's position as a MIDI control change, ~190 times per second. A WiFi access point serves a web interface for watching the jacks and changing the settings live.

Hardware

The PCB is the KiCad project in hardware/. Each jack's tip, ring and sleeve (plus the tip's switch contact, for plug detection) can be switched to a shared 1 kΩ pull-up to the 2.5 V reference or a 1 kΩ pull-down, and is read back by one of two AD7718 24-bit ADCs. The four WS2812B LEDs below the jacks glow a dim white while a plug is in but no known pedal is behind it. A recognised pedal is announced for a second in its own color - purple for an expression pedal, lime for a footswitch, yellow for a rheostat - after which the LED shows the value it sends, from dark red at 0 through red, orange and yellow to white at 127. They run off a linear regulator, so the firmware caps them at ~4% brightness.

Supported pedals

Pedals are recognised within ~0.1 s of being plugged in, and unplugging is noticed just as fast.

• A normally-closed sustain pedal, or an expression pedal wired the other way round, needs Invert.
• A rheostat pedal on a mono cable first reads as a potentiometer at rest, until its resistance has changed by a fifth.
• A potentiometer pedal needs a stereo cable: with a mono one, most pedals short their ring to the sleeve, which leaves a resistance that rises and falls again over the travel.
• A mono cable with nothing at the other end reads as a released switch.

Using it

1. Plug the Pico into a computer over USB. It shows up as a USB MIDI device; by default the first two jacks send CC 11 (Expression) and the last two CC 1 (Modulation), all on MIDI channel 1.
2. Join the WiFi network Expression Adapter (password: the one it was built with, see below). Phones, tablets and computers open the interface by themselves as the network's sign-in page; otherwise open http://192.168.4.1.
3. Per jack, set the MIDI channel and controller, Invert, and the Range: move the pedal to one end and press Min, to the other and press Max, so its travel covers the full MIDI range. Drive bends the response against a pedal's nonlinear track: centered is linear, turning it right makes the value rise early in the travel, left late (double-click resets it). Wiper only matters for pedals whose wiper is on the ring or sleeve and that rest on an end stop when plugged in; Auto handles the common wirings. The LED slider sets the brightness.

The topology panel on the right draws the selected jack's measured network.

Setup

Needs Rust, Node.js (see engines in web/package.json) for the web interface, and a debug probe (e.g. a Raspberry Pi Debug Probe) on the Pico's SWD pins.

rustup target add thumbv8m.main-none-eabihf
cargo install probe-rs-tools --locked
cp .env.example .env   # then set EXPAD_WIFI_PASSWORD (8-63 characters)

Building and flashing

cargo run --bin expression_controller   # build, flash and show the log
cargo run --bin expression_controller --features no-wifi   # the same with the radio off
cargo build --no-default-features       # without the web interface (no Node.js needed)

The other programs in src/bin/ test parts of the board:

Development

cargo test -p expad-topology --target x86_64-pc-windows-msvc   # solver tests, on your host triple
cargo fmt
cargo clippy --all-features
cd web && npm run dev:mock   # web interface against a simulated device, no hardware needed

AGENTS.md describes the repository in detail, and docs/fast-tracking.md the measurements and design behind the pedal detection.

General TODOs

• missing standard trait implementations for all types (Debug, Default, Clone, Copy, PartialEq, Eq, etc.)
• missing with_field builder methods for all config structs
• thiserror (nostd) or similar better error handling