Therevox Expression Controller

Turns an expression pedal input into a programmable Therevox modulation source. In PED mode, pick a pitch-bend interval with the rotary encoder and the pedal bends smoothly from no-bend to that interval; in LO/FM modes, the expression output can become an LFO source. The MCP4728 build also adds two independent 3.5mm LFO outputs and two assignable Therevox CV/gate inputs for patch-panel control.

Built with:

• Raspberry Pi Pico H
• MCP4728 quad I2C DAC
• 0.91 inch 128x32 I2C OLED
• Rotary encoder with push switch
• Two 1/4 inch TRS jacks: expression pedal in, Therevox expression/CV out
• Two 3.5mm TS jacks: independent LFO 1 and LFO 2 outputs
• Two 3.5mm TS/TRS jacks: assignable MOD IN 1 and MOD IN 2
• USB power

Start Here

Need	File
Arduino firmware	pico/PrecisionExpressionControllerPico/PrecisionExpressionControllerPico.ino
User/menu manual	docs/user-manual.md
Therevox sample patches	docs/sample-patches.md
Wiring and bench test	docs/pico-prototype.md
Perfboard/enclosure layout	docs/pico-physical-layout.md
BOM	docs/bom.md
Console simulator	sim/pico_console_sim.cpp
3D-print enclosure files (untested, never printed)	hardware/enclosure/

How It Works

One bend direction is active at a time for the expression output. Use the menu or serial direction up|down to change it.

UP:   heel 0 V, toe rises to the selected interval's voltage
DOWN: heel 3.3 V, toe falls to the selected interval's voltage

Tune the instrument with the pedal at heel.

Interval accuracy comes from a per-interval voltage map. Set the overall scale with response <cents> (the pitch change your instrument produces at full DAC scale; 3960 for a true 1V/oct input), then fine-tune any interval against a tuner with tune on. See docs/pico-prototype.md for the full serial command reference.

Note the DAC output tops out at 3.3 V. If your instrument needs more than that for an octave, the largest selectable interval shrinks accordingly.

LFO modes ignore the pedal interval map and output a unipolar 0-3.3V modulation signal from the DAC. LO covers slow LFO rates (0.05-20 Hz); FM covers faster modulation rates (8-160 Hz). On the expression output, the pedal sweeps LFO speed from slow to fast and the encoder adjusts depth. On LFO1/LFO2, the encoder adjusts speed directly.

LFO features:

• Eight waveforms: sine, triangle, saw up, saw down, square, pulse (adjustable width 5-95%), sample+hold random, and drift (smoothed random).
• Depth 0-100% around the midpoint, plus a -50..+50% center offset on LFO1/LFO2 (depth 0 + offset = fixed CV).
• LFO2 can lock to LFO1 at a rate ratio (1:1, 1:2, 1:4, 3:2, 2:1, 4:1) with a 0/90/180/270 degree phase offset — quadrature and polyrhythm pairs that never drift.
• Tap tempo: two short presses on a focused LFO sync it and set its rate to the tapped interval.
• sync / sync all phase resets, per-output polarity.
• The DAC's fourth channel can emit a full-swing clock square at LFO1 or LFO2's rate (clock lfo1|lfo2).

MOD IN 1 and MOD IN 2 accept Therevox 0-5V patch-panel CV or gate outputs through input dividers. Double-click to the MTRX OLED section, then use the menu to assign each input to LFO depth, LFO rate, delayed LFO fade-in, or LFO sync for EXP, LFO1, or LFO2. The same assignments are also available over Serial with the mod commands.

Build And Upload

1. In Arduino IDE, install the board package Raspberry Pi Pico/RP2040/RP2350 by Earle F. Philhower.
2. Open pico/PrecisionExpressionControllerPico/PrecisionExpressionControllerPico.ino.
3. Select Tools > Board > Raspberry Pi Pico/RP2040/RP2350 > Raspberry Pi Pico.
4. Plug in the Pico over USB, select its port, and upload.
5. Open Serial Monitor at 115200.

If the Pico does not show as a port, unplug it, hold BOOTSEL, plug USB back in, release BOOTSEL, then upload again.

Tests

# Host math test
g++ -std=c++17 test/test_pico_core.cpp -o /tmp/expctrl_test && /tmp/expctrl_test

# Console simulator (expects SIM RESULT: PASS)
g++ -std=c++17 sim/pico_console_sim.cpp -o /tmp/expctrl_sim && /tmp/expctrl_sim

# Firmware compile check
arduino-cli compile --fqbn rp2040:rp2040:rpipico pico/PrecisionExpressionControllerPico

Control Summary

Action	Result
Turn encoder with EXP focused in PED	Change interval size by one semitone
Move pedal in expression LO/FM	Sweep expression-output LFO speed within that mode's range
Turn encoder with EXP focused in LO/FM	Change expression-output LFO depth/attenuation in 5% steps
Turn encoder with LFO1 or LFO2 focused	Change that dedicated LFO output speed (or phase offset when LFO2 is linked)
Turn encoder with MTRX focused	Select MOD IN 1 or MOD IN 2 for viewing/editing
Short press in PED	Reset interval to unison
Short press in LO/FM, LFO1, or LFO2	Sync the focused LFO; two presses at tempo tap its rate
Short press in MTRX	Toggle the viewed MOD input
Double-click	Cycle edit focus: EXP -> LFO1 -> LFO2 -> MTRX
Hold ~2s	Open menu
In menu: turn / press / hold ~2s	Scroll items / edit or choose / exit
Editing a menu value	Turn to the value you want, then press to save

Settings autosave about 3s after the last change.

See docs/user-manual.md for the full OLED menu reference.

LFO serial shortcuts:

mode ped|lo|fm
focus exp|lfo1|lfo2|mtrx
wave sine|tri|sawup|sawdown|square|pulse|sh|drift
rate 1.5      set focused LFO speed; EXP LFO uses this as toe/max speed
depth 75      set focused LFO depth, 0..100
pw 60         pulse-wave width, 5..95
offset -25    shift focused LFO1/LFO2 center, -50..50
link 1:2      lock LFO2 rate to LFO1; link phase 90 for quadrature
clock lfo1    full-swing clock square on the DAC's spare channel
sync          sync focused LFO; sync all resets every phase
tap           send twice at tempo to set the focused LFO rate
mod show      show MOD IN 1/2 raw voltage, gate state, and assignment
mod 1 target lfo1-depth
mod 1 amount 50
mod 2 target lfo2-delay
mod 2 delay 750
mod 2 rise 1000

Project Layout

docs/                  wiring, layout, BOM, product notes
hardware/enclosure/    STL generator and files (untested)
pico/                  Arduino firmware
sim/                   host-side console simulator
test/                  host-side math tests