Expression Pedal

Two-Axis Controller

The expression pedal on the 555 is more than a simple volume swell — it’s a two-axis controller:

Vertical Axis (Toe-Heel)

Standard volume swell. A potentiometer controls the main amplifier gain. The full range goes from silence (heel down) to maximum volume (toe down).

MIDI mapping: CC#11 (Expression) or CC#7 (Volume)

Lateral Axis (Push Right Foot Left)

Pitch slide/bend. Physically pushing the pedal sideways engages a second potentiometer or switch that routes to the Orbit III’s Slide circuit, bending the VCO pitch.

MIDI mapping: Pitch Bend

Tip

This two-axis design is significant for the MIDI conversion — the expression pedal contributes at least 2 analog inputs, not just 1. Both axes need their own ADC channel on the ESP32.

Physical Design

The pedal mechanism uses a rocker assembly with the vertical pivot at the heel and a lateral slide mechanism at the toe end. The lateral motion is spring-loaded to return to center when released. The volume pot likely uses a logarithmic taper for natural-feeling volume response, while the pitch bend pot uses a linear taper for predictable pitch movement.