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Hardware

Components

Part Quantity Notes
ESP32 WROOM-32 DevKit 1 Any common dev board works
Thin silicone wire (AWG28-30) ~30cm For button pad taps

No optocouplers, no level shifters, no resistors, no separate power supply. The ESP32 is powered directly from the ROL desk controller's 5V rail.

Wiring

Button connections

Panel button pad ESP32 GPIO Mode
UP GPIO26 Output, open-drain
DOWN GPIO27 Output, open-drain
M1 GPIO33 Output, open-drain
M2 GPIO32 Output, open-drain
GND (shared) GND Common ground

LIN bus tap

Source Destination Notes
RJ45 pin 3 (LIN bus) GPIO16 (UART2 RX) Direct connection
RJ45 pin 8 (GND) ESP32 GND Shared ground

Power

Source Destination Notes
ROL desk controller 5V ESP32 VIN Powers the ESP32 directly
ROL desk controller GND ESP32 GND Shared with panel GND

Full wiring diagram

[ROL Group Controller]
       |
       | RJ45 cable (LIN bus + power)
       |
[Mittzon Handset Panel]
   |   |   |   |   |
  UP  DOWN M1  M2  GND
   |   |   |   |   |
   |   |   |   |   +-----+
   |   |   |   |         |
   |   |   |   +---+     |
   |   |   |       |     |
   |   |   +---+   |     |
   |   |       |   |     |
   |   +---+   |   |     |
   |       |   |   |     |
[GPIO26] [27] [33] [32] [GND]   <-- ESP32 WROOM-32
                                    (open-drain mode)

LIN bus tap (separate connection):
RJ45 pin 3 ----------- GPIO16 (UART2 RX)
RJ45 pin 8 ----------- ESP GND (shared with panel GND)

Power (from ROL desk controller):
5V  ------------------- ESP32 VIN
GND ------------------- ESP32 GND

Panel teardown

  1. The Mittzon handset panel clips off the desk frame. Remove it and undo the screws on the back.
  2. Inside is a small PCB with 4 momentary tactile buttons.
  3. Each button has a signal pad and shares a common ground.

Button pad electrical characteristics

  • Signal pads idle at 3.3V (pulled up from the panel's MCU)
  • Pressing a button shorts the signal pad to GND
  • This is exactly what our ESP32 GPIO replicates in open-drain mode

Soldering the taps

Solder one thin wire to each of the 4 button signal pads (UP, DOWN, M1, M2) and one to any GND point on the PCB. Route the wires out through the cable channel.

Why direct GPIO works (no optocoupler needed)

  1. Voltage match — Panel buttons operate at 3.3V, same as ESP32 GPIO
  2. Common ground — ESP32 and panel share GND, so voltage references are aligned
  3. Open-drain mode — GPIO configured as open-drain can only pull LOW (assert) or float (release). It never drives HIGH, so there's no risk of back-current into the panel's pull-up resistors
  4. Galvanic isolation unnecessary — Both devices are grounded together intentionally. The desk controller's power rail is separate from the button signals

This saves 4 optocouplers and simplifies the build significantly.

LIN bus tap

The LIN bus runs on RJ45 pin 3 between the controller and handset panel. We tap it receive-only directly to ESP32 GPIO16 (hardware UART2 RX). Since we're only receiving (not transmitting), there's no risk of bus disruption from this tap.

RJ45 pinout (observed)

Pin Function
3 LIN bus (data)
8 GND
Others Power rails (not tapped)

Caveats

  • Parallel wiring — Physical buttons remain fully functional. The ESP32 GPIO taps are wired in parallel with the panel's own buttons.
  • Assert-only — ESP can drive a button LOW (pressed) but cannot force it HIGH if the user is physically holding the button down.
  • Safety via "two buttons = STOP" — The ROL Group controller has built-in behavior: if two buttons are active simultaneously, it stops all motion. The firmware exploits this for physical button safety (see JOURNEY.md).