Output Stage Hardware

For Players

The Input Inventory page described shift registers — chips that read the organ’s key contacts and send that data to the ESP32. This page covers the reverse hardware: chips that take MIDI commands from the ESP32 and translate them into the voltages and switch closures that the organ’s circuits understand. Digital-to-Analog Converters (DACs) generate smooth voltages for things like filter sweeps and pitch bends. Relay drivers flip switches for things like Leslie speed and voice selection. Together, they form the translator between the digital MIDI world and the organ’s analog world.

Architecture

The output stage sits between the ESP32-S3 microcontroller and the organ’s analog circuits. It converts digital commands into the voltages and switch closures the organ expects.

graph LR
    MIDI["MIDI-In<br/>(USB · BLE · DIN-5)"] --> MCU{ESP32-S3}

    MCU -->|SPI| DAC["DAC ICs<br/>(MCP4728 ×2)"]
    MCU -->|GPIO| DRV["Relay Drivers<br/>(ULN2803)"]

    DAC --> AMP["Op-Amp<br/>Gain Stages"]
    AMP --> CV["Organ CV Nodes<br/>VCO · Filter · LFO"]

    DRV --> RLY["Relay Modules"]
    RLY --> SW["Organ Switches<br/>Tabs · Effects"]
    RLY --> LSL["Leslie Reed Coils<br/>(Off / Slow / Fast)"]

    style MIDI fill:#065f46,color:#d1fae5,stroke:#10b981
    style MCU fill:#78350f,color:#fef3c7,stroke:#d97706
    style CV fill:#92400e,color:#fef3c7,stroke:#d97706
    style SW fill:#92400e,color:#fef3c7,stroke:#d97706
    style LSL fill:#92400e,color:#fef3c7,stroke:#d97706

DAC Subsystem

The DAC subsystem generates analog voltages for the organ’s continuous-control circuits (VCO pitch, filter cutoff, LFO parameters, expression). Channel count is derived from the Output Inventory continuous targets table.

DAC IC Selection

Primary choice: MCP4728 (4-channel, 12-bit, I2C)

Feature	Specification
Channels	4 per chip (2 chips = 8 channels)
Resolution	12-bit (4096 steps)
Interface	I2C (2-wire, addressable)
Output range	0–2.048V (internal VREF) or 0–VDD
Update rate	400 kHz I2C → ~50 kHz per channel
EEPROM	Stores power-on default values

Two MCP4728s on the I2C bus provide 8 DAC channels — enough for all identified continuous targets with room for expansion. Each chip has a unique I2C address (factory-set, changeable via special command).

Note

The MCP4728’s EEPROM feature means the DAC powers up with known safe voltages even before the ESP32 boots. This prevents voltage spikes from reaching the organ’s circuits during startup — important when driving a VCO that would otherwise emit an uncontrolled pitch.

Alternative: MCP4922 (2-channel, 12-bit, SPI)

Faster update rate than I2C (SPI clock up to 20 MHz), but requires more chips (3–4 for 6–8 channels) and more GPIO for chip-select lines. Better suited if update rate proves critical for pitch tracking.

Op-Amp Gain Stage

Each DAC output feeds a non-inverting op-amp gain stage that scales 0–3.3V to the organ’s operating voltage:

Target Voltage	Gain	R1 (kΩ)	R2 (kΩ)	Op-Amp Supply
0–5V	1.52	10	5.1	+12V
0–10V	3.03	10	20	+12V or +15V
0–12V	3.64	10	27	+15V

Op-amp selection: TL074 (quad JFET op-amp) — one chip handles four channels. Low offset voltage, rail-to-rail not required since the supply rails exceed the output range. Two TL074s cover all 8 DAC channels.

The exact gain for each channel will be set after bench measurement of the target CV node voltage ranges (see Output Inventory investigation queue).

Relay Driver Subsystem

Binary targets (Leslie speed, tab stops, tremolo/vibrato, rhythm) require switch closures. The ESP32’s 3.3V GPIO drives relay coils through Darlington transistor arrays.

ULN2803 Darlington Array

Feature	Specification
Channels	8 per chip
Input	3.3V–5V logic compatible
Output	Open-collector, 500 mA per channel
Protection	Internal flyback diodes for relay coils

The ULN2803 is the standard interface between microcontrollers and relay coils. Each input pin connects directly to an ESP32 GPIO; each output pin drives a relay coil. The internal flyback diodes suppress the inductive voltage spike when a relay de-energizes — no external protection diodes needed.

Relay Count

From the Output Inventory binary targets:

Group	Relays	Notes
Leslie (Off/Slow/Fast)	2	Two relays encode three states
Orbit III voice tabs	10	One relay per tab stop
Tremolo	1	On/off
Vibrato	1	On/off
Rhythm select	2–4	Depends on switch matrix investigation
Total	16–18

Two ULN2803 chips (16 channels) cover the minimum count. A third chip provides headroom for rhythm selection and future expansion.

Relay Selection

Preferred: SRD-05VDC-SL-C (Songle 5V SPDT relay) — ubiquitous, cheap, available on pre-built relay modules with screw terminals for prototyping. Switching capacity: 10A @ 250VAC, far exceeding the organ’s tab stop and signal switching requirements.

For the Leslie motor control (confirmed reed switch coil drive), use a relay rated for the reed switch coil current — see remaining investigation for the bench measurement.

Caution

Relay modules that switch mains voltage (Leslie motor) must be physically separated from the low-voltage signal relays (tab stops, effects). Use separate relay boards and route mains wiring away from signal wiring. The ESP32 and DAC circuits should never share a ground path with mains-voltage relay contacts.

Level Shifting

The ESP32-S3 operates at 3.3V logic. The organ’s circuits use 5V and 12V signals.

Direction	Voltage	Method
ESP32 → ULN2803	3.3V → relay coil	ULN2803 handles directly (3.3V input threshold met)
ESP32 → DAC (I2C)	3.3V	No shifting needed (MCP4728 operates at 3.3V)
DAC → organ CV	0–3.3V → 0–12V	Op-amp gain stage (see above)
ESP32 GPIO → Leslie coils	3.3V → relay coil	ULN2803 handles directly (same relay subsystem)

The ULN2803’s internal Darlington pairs have a low enough input threshold (~2.5V) to trigger reliably from 3.3V GPIO. No separate level shifter is needed for the relay subsystem.

Leslie Motor Driver

The Leslie investigation identified the 555’s Leslie as a CBS/Electro Music unit (Part No. 660890) with a TRIAC-based speed controller and magnetically actuated reed switches. The console’s Off/Slow/Fast switch energizes reed switch coils that configure the TRIAC for the corresponding motor speed.

Scenario A: Reed Switch Coil Drive Confirmed

No dedicated motor driver is needed. The ESP32 drives two GPIO pins through the existing ULN2803 Darlington array, each energizing a small relay across a reed switch coil. Two relays encode three states:

GPIO State	Leslie State	MIDI CC#80 Value
Both off	Off	0–42
Channel 1 on	Slow (chorale, ~40 RPM)	43–95
Channel 2 on	Fast (tremolo, ~340 RPM)	96–127

The CC#80 mapping follows the Hammond-Suzuki convention used by the Leslie 2101mk2 and other MIDI Leslie controllers. The relays must be rated for the reed switch coil drive current — one bench measurement remains to confirm the coil voltage and current spec.

The spin-up and spin-down transitions are entirely mechanical. The MIDI interface just flips the relay; the motor accelerates or decelerates through its own inertia. The musical character of the Leslie transition is preserved exactly because the same TRIAC circuit drives the same motor.

Scenario C: ESP32 Micro-VFD Future Upgrade

The ESP32-S3’s MCPWM peripheral can generate variable-frequency SPWM to drive the motor at any speed from ~10 Hz (60 RPM) to 60 Hz (360 RPM) through a full H-bridge, with about $10 of additional components. This unlocks continuous speed control via MIDI CC#1 (mod wheel) and programmable ramp profiles shaped in firmware. See Leslie Motor Simulations for SPICE waveform data comparing VFD and TRIAC drive. Build after Scenario A is proven.

Power Supply

The output stage needs to power DACs, op-amps, relays, and potentially a motor driver. Rather than adding an external supply, the design taps the organ’s existing power rails:

Rail	Source	Used By
+5V	Organ’s 5V regulator or ESP32 USB	Relay coils, ULN2803 VCC
+12V (or +15V)	Organ’s Orbit III power supply	Op-amp supply rails
+3.3V	ESP32 onboard regulator	DAC ICs, ESP32 GPIO
GND	Common ground	All subsystems

Caution

The organ’s existing power supply must have enough headroom to support the added load. The relay coils draw ~70 mA each; with 16 relays the worst case is ~1.1A at 5V. The op-amps draw < 10 mA total. Measure the organ’s 5V and 12V rail capacity before committing to this approach — if headroom is tight, a small external 5V supply for the relays is a simple fallback.

Prototyping Approach

Phase 1 uses off-the-shelf breakout boards and modules wired to an ESP32-S3 dev board. No custom PCB is needed until the circuit is proven.

Component	Module	Interface	Approximate Cost
ESP32-S3	ESP32-S3-DevKitC-1	—	$10
DAC (8 ch)	MCP4728 breakout × 2	I2C	$8 × 2
Relay (16 ch)	16-channel relay module	GPIO via ULN2803	$12
Op-amps	TL074 on breadboard	DAC outputs	$2
Resistors	Assorted 1% metal film	Gain setting	$5
Total			~$45

The prototyping phase validates each output channel independently:

1. DAC → op-amp → voltmeter

   Confirm voltage scaling for each target range

2. GPIO → relay → continuity test

   Confirm relay switching for each tab stop

3. Connect one channel to the organ

   Start with the Leslie — two relay channels driving the confirmed reed switch coils, most dramatic result, easiest to verify by ear

4. Add channels incrementally

   Expression volume, then Orbit III CV, then remaining targets

This incremental approach minimizes risk to the organ’s circuits — each new connection is validated in isolation before moving to the next.