Midi trs switcher

badascule-bridge board

A small, USB-powered MIDI-to-dual-TRS switcher that receives MIDI over a 5-pin DIN input and switches the tip and ring contacts of two TRS jacks. It is intended for equipment that accepts external contact-closure controls on TRS connectors. The switching logic is implemented in firmware on a Seeed Studio XIAO ESP32-S3; the schematic describes the hardware, not a fixed MIDI message map.

The included firmware supports both DIN and USB MIDI Program Change messages, with configurable mappings and manual control through a Wi-Fi web UI.

Electrical warning: The TRS outputs are intended only for low-energy footswitch or contact-closure control inputs. Never connect an amplifier speaker output, mains voltage, a power-supply rail, or another powered load. Doing so can damage the PhotoMOS relays, the PCB, or the connected equipment. A compatible control input may place a small sensing voltage on its contacts; check the target device's documentation and confirm its voltage and current are within the relay and board ratings before connecting it. The G3VM-61VY2's absolute maximum load ratings are 60 V AC peak/DC and 500 mA at 25 °C; these are limits, not recommended operating targets. See the manufacturer's specifications.

Architecture

5-pin DIN MIDI IN → H11L1 optocoupler → XIAO ESP32-S3
                                         ├─ output 1 tip  → solid-state relay → TRS jack 1
                                         ├─ output 1 ring → solid-state relay → TRS jack 1
                                         ├─ output 2 tip  → solid-state relay → TRS jack 2
                                         └─ output 2 ring → solid-state relay → TRS jack 2

• The DIN input uses a 220 Ω resistor, 1N4148 protection diode, and H11L1 logic-output optocoupler. Its output goes to the XIAO's D7 input with a 470 Ω pull-up to 3.3 V; a 100 nF capacitor decouples the optocoupler supply.
• Four GPIO outputs drive the input LEDs of four G3VM-61VY2 MOSFET relays through 390 Ω resistors. Each channel has an indicator LED with a 1.5 kΩ resistor and a 100 kΩ pull-down at the drive node.
• D0 switches jack 1 tip; D1 switches jack 1 ring; D3 switches jack 2 tip; D4 switches jack 2 ring. Each relay closes its contact between the named tip or ring and the corresponding jack sleeve. The two TRS jacks are independent.
• The XIAO supplies the 3.3 V logic rail. Power the assembled board through the XIAO's USB-C connector.

Main components

Check the KiCad schematic, footprints, and BOM before ordering: package and mounting choices are defined by the project files. The described schematic also includes four PCB mounting holes.

Resistor and capacitor size

Use 1/4 W through-hole axial resistors in the DIN0207 size. The PCB footprints are designed for a body approximately 6.3 mm long × 2.5 mm diameter, with 7.62 mm lead spacing.

For C1, use a 100 nF non-polarized through-hole capacitor rated for at least 50 V. The prototype uses a film capacitor, but another suitable capacitor type may be used. Its PCB footprint has 5 mm lead spacing and is sized for a body approximately 5 mm wide × 2.5 mm thick, so verify that the chosen part fits.

Enclosure fit and recommended connectors

Mount the Seeed Studio XIAO ESP32-S3 on pin headers (often sold as goldpin headers) rather than soldering it directly to the PCB. The header height is part of the provided enclosure design and positions the XIAO correctly relative to the enclosure openings. If the XIAO is mounted at a different height, the enclosure design will need to be adjusted.

Use Neutrik NMJ6HFD2 TRS jacks when building the board for the provided enclosure. This model sits flush against the PCB and matches the dimensions used for the sample enclosure. Other jacks may require changes to the PCB footprint or enclosure.

The recommended 5-pin DIN connector is the REAN NYS325. It offers better build quality than many cheaper equivalents. If substituting another connector, verify its footprint, and enclosure fit before ordering the PCB or making the enclosure.

LED brightness and custom enclosure wiring

The indicator LEDs use 1.5 kΩ series resistors to keep them from being overly bright. If more brightness is needed, a lower resistance can be used, provided the resulting current stays within the LED and GPIO ratings. These are the indicator LED resistors, separate from the 390 Ω relay input resistors.

The board also includes footprints for Molex KK254 series (2.54 mm pitch) connectors for the indicator LEDs and DIN MIDI input connector. Other 2.54 mm pitch connectors can be used if their pin layout and mechanical dimensions match the footprints. These are optional: builders can use the board-mounted components or fit the connectors and wire the LEDs and DIN connector to positions of their choice in a custom enclosure. Check the footprint and pinout when choosing mating connectors, and preserve LED polarity and DIN pin assignments when wiring them.

Known board issue

The current MIDI input wiring connector uses a 3-pin footprint, although the MIDI input only needs two wires. The middle pin is not connected; use the two outer pins for the MIDI input wiring, following the schematic's pin assignments. Changing this to a 2-pin footprint is planned for the next board revision.

Files and building

The board files include the KiCad project, schematic, PCB layout, and Gerber files ready for fabrication. Use the editable KiCad files as the source of truth for the circuit, footprints, and component values. The Gerber files can be sent directly to a PCB manufacturer.

• Bill of materials (CSV)
• Fabrication-ready Gerber archive (ZIP)

Additional manufacturing exports, including component designators, positions, and the IPC netlist, are available in the production folder.

A direct link to the PCBWay shared project will be added here when it is available, allowing the board to be ordered without uploading the Gerbers manually. Ordering through that link also supports this project: I receive a small commission from qualifying orders placed through it.

Keep the .kicad_pro, .kicad_sch, and .kicad_pcb files here, together with any project-specific symbol or footprint libraries. Use project-relative library and 3D model paths so the project opens on other machines. Keep fabrication outputs and the bill of materials in the production/ folder.

The enclosure is a prototype, so check physical fit against your chosen jacks, board, headers, and other hardware before making a finished case.

Populate the PCB and follow the firmware build and upload instructions to program the XIAO. The included firmware drives D0, D1, D3, and D4 and lets you configure Program Change mappings in the web UI. Check each output with a multimeter before connecting the target device. Confirm the target's TRS control pinout and electrical requirements: these outputs are contact closures to sleeve, not audio outputs or MIDI-over-TRS ports. The schematic itself does not define the MIDI mapping; that behavior belongs to the firmware.

Status

Prototype design. Verify the PCB, enclosure fit, and behavior with your target equipment before relying on it in a live setup.

License

See the project license and attribution terms for the license applicable to these materials.