Leslie Control

For Players

The Leslie speaker is probably the most visceral effect in any organ. Instead of just amplifying the sound, the Leslie physically spins a horn (treble) and drum (bass) around the speaker drivers. The result is a Doppler pitch shift, amplitude modulation, and room interaction that no digital effect truly replicates. The 555 has a built-in Leslie with a console switch for slow (chorale), fast (tremolo), and off. Controlling that switch from MIDI means a foot controller, mod wheel, or sequencer can spin the Leslie up and down in time with the music — the way Hammond players have done with half-moon switches for decades.

How the 555 Controls the Leslie Today

The console has a three-position Leslie switch: Off / Slow / Fast. The slow (chorale) and fast (tremolo) speeds produce the characteristic Leslie sound — the spin-up and spin-down transitions between speeds are where most of the musical drama lives.

What we know:

• The console switch is functional — the Leslie spins at both speeds and stops cleanly
• The motor(s) appear healthy — no grinding, hesitation, or speed instability
• The physical speaker assembly is mounted in the lower cabinet behind a baffle

What we don’t yet know:

• Motor type — AC induction (most common in Leslie speakers), DC, or shaded-pole
• Control mechanism — relay-switched speed taps? Variable voltage/frequency? Direct switch to motor windings?
• Wiring — how many conductors run from the console switch to the motor assembly?
• Whether there are separate fast and slow motors (common in Leslie 122/147) or a single motor with speed taps

Caution

The Leslie motor control circuit handles mains voltage (120VAC) or significant current. All investigation work must be done with the organ unplugged, and any MIDI-controlled switching must use properly rated relays with appropriate isolation from the ESP32’s 3.3V logic.

Investigation Plan

Before designing the MIDI interface, the Leslie control circuit must be traced from the console switch to the motor:

1. Trace the console switch — with the organ unplugged, follow the wires from the Leslie Off/Slow/Fast switch to the first junction point. Are they going directly to motor windings, to a relay board, or to a motor controller?

2. Identify the motor type — inspect the motor nameplate or housing. AC induction motors have speed determined by winding configuration (not voltage). DC motors can be speed-controlled by voltage. Shaded-pole motors are single-speed.

3. Measure control signals — with the organ powered, carefully measure the voltages at the console switch terminals in each position (Off/Slow/Fast). This reveals whether the switch selects between motor windings, relay coils, or voltage levels.

4. Count motor windings — classic Leslie speakers (122, 147) use dual-speed AC induction motors with separate slow and fast windings. If the 555’s Leslie uses the same approach, control is simply a matter of energizing the correct winding.

5. Document the wiring — photograph and diagram the complete path from console switch to motor. This becomes the reference for the relay interface design.

MIDI-to-Leslie Interface Options

The interface design depends entirely on what the investigation reveals. Three scenarios cover the likely possibilities:

Scenario A: Relay-Switched Speed (Most Likely)

If the console switch selects between motor windings or relay coils — the most common arrangement in organ Leslie speakers:

Interface: ESP32 GPIO → ULN2803 Darlington driver → relay module (SPDT or dual SPST)

The ESP32 drives two relays that replicate the three-position console switch:

• Both relays off = Leslie off
• Relay 1 on = slow (chorale)
• Relay 2 on = fast (tremolo)

MIDI mapping: CC#80 (General Purpose 5) with three zones:

CC#80 Value	Leslie State	Behavior
0–42	Off	Motor stops
43–95	Slow (chorale)	~40 RPM
96–127	Fast (tremolo)	~340 RPM

This mirrors the CC#80 convention used by Hammond-Suzuki and several Leslie MIDI controllers.

Note

The spin-up and spin-down transitions are entirely mechanical — the motor accelerates or decelerates through its own inertia. No firmware ramping is needed. The musical character of the Leslie comes precisely from these physical transitions, which are different in each direction (spin-up is faster than spin-down due to motor torque vs. air drag).

Scenario B: Variable Voltage (DC Motor)

If the Leslie uses a DC motor with voltage-controlled speed:

Interface: ESP32 DAC → op-amp gain stage → PWM motor driver (L298N or equivalent)

Continuous speed control becomes possible — the mod wheel could sweep the Leslie from stopped through chorale to tremolo speed in a continuous arc.

MIDI mapping: CC#1 (Mod Wheel) for continuous speed, 0–127 mapping to full RPM range.

This is less likely in a 1974 organ but would be the most musically expressive option.

Scenario C: AC Induction with VFD (Future Path)

If precise continuous speed control of an AC motor is desired:

Interface: ESP32 → serial/analog → Variable Frequency Drive (VFD)

A VFD controls motor speed by varying the AC frequency. This is the approach used in high-end Leslie clones and digital organs. It’s expensive ($100–200 for a small VFD), mechanically complex, and almost certainly overkill for this project — but documented here as a known option.

Status: Future consideration only. Relay switching (Scenario A) covers 95% of musical use cases.

MIDI Mapping Recommendation

Regardless of motor type, the recommended MIDI mapping provides two complementary controls:

MIDI Message	Function	Use Case
CC#80	Leslie speed preset (Off/Slow/Fast)	Foot switch, sequencer automation
CC#1 (Mod Wheel)	Continuous speed (if Scenario B)	Expressive real-time control

CC#80 is the primary control — it works with the simplest relay interface and matches existing Leslie MIDI conventions. CC#1 is reserved for continuous control if the motor type supports it.

Prior Art

The Leslie has been MIDI-controlled since the late 1980s. These existing solutions inform both the MIDI mapping and the electrical interface:

• Hammond-Suzuki Leslie 2101mk2 — purpose-built MIDI Leslie with built-in MIDI input. Uses CC#80 for speed selection, exactly as proposed above. The industry standard reference.
• Neo Instruments Ventilator — Leslie simulator pedal with MIDI input. Demonstrates the CC#80 slow/fast convention and adds CC#1 for continuous rotation speed in its “advanced” mode.
• Voce V5+ Organ Module — MIDI-to-Leslie interface that drives real Leslie speakers from MIDI. Relay-based speed switching, confirming Scenario A is the standard approach for real motors.
• Motion Sound Pro-3T — rotary speaker with MIDI speed control, using a DC motor and continuous speed adjustment via MIDI CC.

The consensus across these products: CC#80 for three-state switching (off/slow/fast) is the de facto standard. Our implementation follows this convention for maximum compatibility with existing MIDI controllers and DAW setups.