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)"]
    MCU -->|PWM| MOT["Motor Driver<br/>(TBD)"]

    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/>Leslie · Tabs · Effects"]

    MOT --> LSL["Leslie Motor<br/>(if DC — Scenario B)"]

    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 (if relay-switched per Scenario A), use a relay rated for the motor’s inrush current — typically 3–5× running current for an AC induction motor.

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 → motor driver	3.3V → driver input	Most motor drivers accept 3.3V logic

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.

Motor Driver

The Leslie motor interface depends on the investigation results. Three hardware options correspond to the three Leslie scenarios:

Scenario	Motor Type	Driver Hardware	ESP32 Interface
A (relay)	AC induction	Relay module (mains-rated)	2× GPIO via ULN2803
B (variable)	DC	L298N H-bridge or BTS7960	PWM output
C (VFD)	AC induction	Variable Frequency Drive	Serial or analog

Scenario A requires no dedicated motor driver — the same ULN2803 + relay chain used for tab stops handles the Leslie speed relays. The relays must be rated for motor inrush current.

Scenario B adds an H-bridge motor driver (L298N for small motors, BTS7960 for larger ones). The ESP32 generates a PWM signal; the driver converts it to motor voltage. Direction control is not needed (Leslie spins one way).

Scenario C is documented for completeness — a VFD is a separate industrial component that would be specified and installed independently of the ESP32 output stage.

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 (most dramatic, 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.