MIDI Conversion Approach

For Players

Every key, pedal note, and Orbit III mini-key in the 555 already sends an electrical signal when you press it — that is how the organ makes sound. This project taps into those same signals with a small computer board and translates each key press into a MIDI message, sent over USB, Bluetooth, or a standard MIDI cable. The organ keeps working exactly as it always has; the MIDI encoder just listens in. Think of it like a pickup on an acoustic guitar — it does not change the instrument, it just lets you plug in.

Goal

Scan all keyboard contacts and transmit MIDI, optionally coexisting with the original analog sound generation. The organ keeps working as-is — the MIDI encoder taps the key contact bus wires in parallel.

Bidirectional MIDI

The ESP32-S3 is inherently bidirectional — the same USB, BLE, and DIN-5 interfaces that send MIDI-Out also receive MIDI-In. The reverse path takes incoming MIDI messages and converts them into voltages and switch closures that drive the organ’s analog circuits: spinning the Leslie up to tremolo speed, sweeping the Orbit III’s Wah-Wah filter, selecting voice presets, or injecting pitch CV into the VCO. The Output Inventory catalogs every controllable target, and the Output Stage Hardware describes the DACs, relay drivers, and op-amp gain stages that bridge the ESP32’s 3.3V logic to the organ’s 12V analog world.

Hardware Design

Microcontroller: ESP32-S3

The ESP32-S3 handles all three MIDI output formats simultaneously:

• USB-MIDI — wired, lowest latency
• BLE-MIDI — wireless, convenient for mobile/tablet DAWs
• DIN-5 MIDI — classic, universal compatibility

Digital Input Scanning: 74HC165 Shift Registers

151 digital inputs (keys + pedals + Orbit III dual contacts) are scanned through 19 daisy-chained 74HC165 parallel-in/serial-out shift registers (8 inputs each, 19 chips = 152 inputs, one spare).

The ESP32 reads the entire chain via SPI at high speed — a full scan of all 151 inputs completes in microseconds.

Analog Inputs: Expression Pedal

Two analog channels on the ESP32’s built-in ADC:

• Expression pedal vertical axis (volume swell)
• Expression pedal lateral axis (pitch bend)

Scanning Approach

Tip

The 555 uses bussed keyboards (common in pre-1983 organs). The shared bus connects to ground; when a key is pressed, it pulls the shift register input LOW. We scan the key contact INPUT side (bus wires going to divider-keyer ICs), not the audio output side.

The approach reads the electrical state of each key contact directly — the same 0-12V square wave signals that drive the organ’s voicing circuits. The shift registers convert these parallel signals into a serial data stream the ESP32 can read as a single SPI transaction.

graph LR
    KEYS["🎹 Key Contacts<br/>151 digital"] --> SR["74HC165 ×19<br/>Shift Registers"]
    SR --> SPI_IN["SPI In"]
    PEDAL["🦶 Expression Pedal<br/>2 analog"] --> ADC["ADC"]

    SPI_IN --> MCU{ESP32-S3}
    ADC --> MCU

    MCU --> USB["USB-MIDI"]
    MCU --> BLE["BLE-MIDI"]
    MCU --> DIN["DIN-5 MIDI"]

    USB --> MCU
    BLE --> MCU
    DIN --> MCU

    MCU --> SPI_OUT["SPI Out"]
    MCU --> GPIO["GPIO"]

    SPI_OUT --> DAC["DAC ICs<br/>(MCP4728 ×2)"]
    DAC --> OPAMP["Op-Amp<br/>Gain Stages"]
    OPAMP --> CV["Organ CV Nodes<br/>VCO · Filter · LFO"]

    GPIO --> DRV["ULN2803<br/>Relay Drivers"]
    DRV --> RLY["Relays"]
    RLY --> SW["Leslie · Tabs<br/>Effects"]

    style KEYS fill:#92400e,color:#fef3c7,stroke:#d97706
    style PEDAL fill:#92400e,color:#fef3c7,stroke:#d97706
    style MCU fill:#78350f,color:#fef3c7,stroke:#d97706
    style USB fill:#065f46,color:#d1fae5,stroke:#10b981
    style BLE fill:#065f46,color:#d1fae5,stroke:#10b981
    style DIN fill:#065f46,color:#d1fae5,stroke:#10b981
    style CV fill:#92400e,color:#fef3c7,stroke:#d97706
    style SW fill:#92400e,color:#fef3c7,stroke:#d97706

MIDI Channel Assignment

Input	MIDI Channel	Messages
Upper Manual (Swell)	Ch 1	Note On/Off
Lower Manual (Great)	Ch 2	Note On/Off
Pedalboard	Ch 3	Note On/Off
Orbit III Contact 1 (note)	Ch 4	Note On/Off
Orbit III Contact 2 (aftertouch)	Ch 4	Channel Pressure or CC
Expression Pedal (volume)	Global	CC#11 (Expression)
Expression Pedal (slide)	Ch 4	Pitch Bend

MIDI-In Channel Assignment

Target	MIDI Channel	Messages
Leslie speed	Global	CC#80 (Off/Slow/Fast)
Orbit III CV (pitch, filter, LFO)	Ch 4	Note On, CC#74, CC#76, CC#77
Orbit III voice select	Ch 4	Program Change 1–10
Expression volume	Global	CC#11 (Expression)
Tremolo / Vibrato	Global	CC#92, CC#77

See the Output Inventory for the complete mapping and the Leslie Control and Orbit III CV Interface pages for detailed rationale behind each assignment.

Coexistence with Analog Sound

The MIDI encoder is designed to be non-invasive. The shift registers present high-impedance inputs to the key contact bus — they read the state without loading or altering the signal. The original analog sound generation continues to work exactly as designed.

This means you can play the organ acoustically through its Leslie speaker while simultaneously sending MIDI to a DAW, hardware synth, or any other MIDI device.