Midiface 4×4 Thru/Merge button hack for merge mode

The Miditech MIDIface 4×4 Thru/Merge returns to its default mode whenever it is powered on. On the unit used for this build, the number of MODE button presses needed to select MERGE depends on how its USB port is connected.

The QT Py and relay automate the 5 or 6 presses required for the selected connection. The relay contact is wired in parallel with the existing button. Once the sequence is complete, the relay remains released and the NeoPixel stays off until the microcontroller starts again.

The code defaults to 5 presses for an external USB power supply. Before programming the QT Py, change N_PUSH_BUTTON to 6 only if the Midiface will be connected to a computer. The build does not detect the connection automatically.

The Midiface USB port and the QT Py USB-C port serve different purposes: the QT Py USB-C port is connected to a PC temporarily for programming.

For modes, indicators and MIDI connections, refer to the Midiface 4x4 Thru / Merge DEU-ENGmanual, also available on the official Miditech manuals page.

MC Hobby links for components and wire, provided for reference.

MC Hobby does not sponsor this project. Nomadly simply bought the parts there, without any special benefit or compensation. Nomadly recommends this Belgian shop for its service quality and prices.

Disconnect USB power and MIDI cables before opening the Midiface 4×4 Thru/Merge.

1. Remove the two screws at the ends of the enclosure faceplate.
2. Gently release and remove the black plate. Keep the plate and both screws for reassembly.
3. Slide the circuit board out of the enclosure. It is held in rails: slide it gently along them to remove it.

The firmware runs only once in setup(). The loop() function is empty. The N_PUSH_BUTTON constant defines the number of presses: 5 with an external USB power supply, 6 if the Midiface USB port is connected to a computer.

Timing according to the Midiface connection

Total programmed delays: 8 969 + N_PUSH_BUTTON × (264 + 744) + 7 546. * Excluding QT Py startup and instruction execution time. The durations come from the supplied MidifaceMerger.ino file.

The two COM / NO wires simulate closing the SW1 button contact. They are not connected to the relay logic control circuit. The colours are used as reference markers in this document.

Leave NC unconnected. The relay is released when EN = LOW ; EN = HIGH causes a button press.

The photos retain their original orientation. The MIDIface pads and relay terminal order were confirmed by the owner of the build on 18 October 2026.

Before soldering anything to the QT Py, program it, then disconnect its USB-C cable.

QT Py ↔ relay

Relay ↔ MODE / SW1 button

COM and NO can be swapped on this pair: the dry contact has no polarity.

Power from the MIDIface

Do not use the point measured at approximately 3 V. The relay is powered from the QT Py 5V and GND pins, never from the A2 output.

#include <Adafruit_NeoPixel.h>

/*
QT Py           Relais
5V   ------->   VDD
GND  ------->   GND
A2   ------->   EN

Relais          MidifaceMerger
COM  ------->   A1
NO   ------->   A2

MidifaceMerger  QT Py
5V   ------->   5V
GND  ------->   GND
*/

const int RELAY = A2;
// Adafruit indique que sur ce modèle la broche 12 coupe l’alimentation du NeoPixel
const int PIN_NEOPIXEL_POWER = 12;

// Si boîtier connecté à l'ordinateur : 6 appuis du bouton (il n'y a aucune raison de relier le boîtier au PC en mode Merge)
// Si boîtier non connecté à l'ordinateur, mais uniquement à alimentation usb externe : 5 appuis du bouton
const int N_PUSH_BUTTON = 5;

Adafruit_NeoPixel pixel(1, PIN_NEOPIXEL);

void setLed(int r, int g, int b)
{
  pixel.setPixelColor(0, pixel.Color(r, g, b));
  pixel.show();
}

void setup()
{
  // Init A2/Relais
  pinMode(RELAY, OUTPUT);
  digitalWrite(RELAY, LOW);

  // Init LED
  pixel.begin();
  pixel.setBrightness(30);

  // Démarrage / attente que le MIDIface soit prêt
  setLed(255, 0, 0); // rouge
  delay(8969);

  // 6 appuis sur le bouton MODE
  for (int i = 0; i < N_PUSH_BUTTON; i++)
  {
    // Bouton pressé
    setLed(0, 0, 255); // bleu
    digitalWrite(RELAY, HIGH);
    delay(264);

    // Bouton relâché
    digitalWrite(RELAY, LOW);
    setLed(0, 255, 0); // vert
    delay(744);
  }

  // Terminé
  setLed(255, 255, 255); // blanc
  delay(7546);

  // Sécurité : relais au repos
  digitalWrite(RELAY, LOW);

  // Eteindre complètement la LED NeoPixel
  pixel.clear();
  pixel.show();

  pinMode(PIN_NEOPIXEL_POWER, OUTPUT);
  digitalWrite(PIN_NEOPIXEL_POWER, LOW);
}

void loop()
{
}

The loop uses N_PUSH_BUTTON to count relay activations. The choice between 5 and 6 is made before uploading; the Midiface USB connection is not detected automatically.

RELAY = A2 ; PIN_NEOPIXEL_POWER = 12 is explicitly declared in this firmware. Adafruit documents setting pin 12 LOW to switch off NeoPixel power on the SAMD21/M0.

Compilation and upload must be checked in the Arduino environment used. Keep the core and library versions that work with this file.

The QT Py USB-C connection to a PC is used to program the microcontroller. It does not determine the number of presses: that setting depends on the Midiface USB connection during use.

1. Connect the QT Py on its own to a PC with a USB-C data cable.
2. Set up Arduino IDE using the official Adafruit guide : Adafruit SAMD support and the Adafruit NeoPixel library.
3. Open MidifaceMerger/MidifaceMerger.ino. Keep const int N_PUSH_BUTTON = 5; as the default for an external USB power supply for the Midiface. Change it to 6 only if its USB port will be connected to a computer. Select the Adafruit QT Py M0 (SAMD21) board and the corresponding port; verify, then upload.
4. Disconnect the QT Py USB-C cable. Perform the power supply checks, then wire and solder the build with all devices powered off.
5. Connect the MIDI devices, then power the Midiface through its USB port using the chosen source: external USB power for the default 5 presses, or a computer for 6 presses.
6. Check the complete sequence using the test procedure.

Provide an easily disconnectable +5 V connection inside the enclosure for future reprogramming.

Check the power supply before connecting the build, then test the actual assembly before closing the enclosure.

Measure 5 V before connecting the QT Py

1. With the Midiface powered off, identify the 5V and GND points shown in the photo. Keep the QT Py disconnected.
2. Power the Midiface through its USB port and use a multimeter in DC voltage mode to measure the voltage and polarity between these two points.
3. Disconnect Midiface power and wait for it to switch off before soldering and connecting the QT Py.

Wiring checks with power off

• Check the solder joints, absence of solder bridges and board insulation.
• Check that COM/NO is open at rest, NC is unconnected and only the two selected SW1 contacts are wired.
• Check that no USB-C cable connects the QT Py to a PC.
• Check that the Midiface USB connection matches the programmed number of presses: 5 with an external USB power supply, 6 with a computer.

For each test from a complete startup

1. Switch off MIDIface power; wait for it to turn off completely before powering it on again.
2. Observe red for approximately 9 s, then count 5 activations with an external USB power supply, or 6 if the Midiface is connected to a computer, with alternating blue/green indications. The observed number must match N_PUSH_BUTTON.
3. Check that the mode indicator shows MERGE by referring to the Midiface manual.
4. Observe white for approximately 7.5 s, then the NeoPixel switching off. The relay must remain released.
5. Leave the device powered on for at least one minute: no further button presses should occur.
6. Test MIDI data transmission using the connections described in the Midiface manual, then check that the manual button still works.
7. Repeat several complete cycles; record any missed press and the resulting mode.

If the result is different

• Secure the QT Py and relay so they cannot touch the MIDIface PCB or any conductive parts of the enclosure.
• Insulate exposed solder joints and terminals; secure the wires without putting strain on the pads.
• Keep the MODE button and disconnectable +5 V connection accessible.
• Label the wires at both ends; clearly distinguish QT Py A2 and SW1 A2.
• Check the voltage and correct operation during relay activations before closing the enclosure.
• Work on and change the wiring with power off. Do not accidentally leave NC connected.

Keep this page, the SVG diagram, annotated photos and .ino file with the device. After testing, record the Arduino/core/library versions used and any hardware differences.

The support helps fit the QT Py and relay inside the Midiface 4×4 Thru/Merge enclosure. Its STL file is available below for 3D printing.

Support view

360° rotation

Assembly on the Midiface board

This tutorial is shared free of charge by Frédéric Delchambre/Nomadly SRL as an account of their own build, with no guarantee of results. Anyone choosing to reproduce it must check the compatibility of their hardware and remains responsible for their own actions. These provisions apply within the limits set by law.

It presents an independent, experimental modification; it is not an official manufacturer procedure.

Modifications and risks

Each person chooses to carry out this modification at their own risk and remains responsible for their checks, choice of hardware and actions. They must have the necessary skills and check their device's compatibility before starting any work.

Opening the enclosure, soldering and electrical modifications can damage the MIDIface or connected equipment, cause malfunctions, data loss or injury. They may also affect the manufacturer's warranty or eligibility for support or repair.

To the extent permitted by applicable law, Frédéric Delchambre and Nomadly SRL guarantee neither the outcome of this modification nor its suitability for another device or a particular use. They disclaim liability for property damage, data loss and repair or replacement costs attributable to wiring errors, inappropriate handling, incompatible components or failure by the person carrying out the modification to follow the safety instructions.

Trademarks and third-party rights

The trademarks, product names and logos mentioned or shown, including Miditech, MIDIface, Adafruit, QT Py, Pololu and Arduino, belong to their respective owners. They are mentioned solely to identify the hardware and tools used. No affiliation, endorsement, certification or warranty by these owners is claimed in relation to this tutorial or build.

This tutorial transfers no intellectual property rights belonging to third parties. The rights of trademark owners and other rights holders remain applicable.

Nomadly SRL legal information and contact details on the home page.