Midi2BLE

midi2BLE

Overview

This device is a portable USB-to-BLE MIDI bridge designed to interface with USB MIDI controllers such as the Akai MPK Mini III.

It supports two user-selectable modes:

• Wireless BLE MIDI operation
• Wired USB passthrough operation

The device is battery powered with USB-C charging and includes an onboard OLED display for status and battery information.

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Operating Modes

1. Wireless Mode

In this mode, the device acts as a BLE MIDI bridge.

Behavior

• ESP32-S3 operates as a USB host

• MIDI data is read from the keyboard and transmitted via BLE

• Device is powered from internal LiPo battery

• OLED displays:

  • battery percentage
  • estimated runtime
  • BLE connection status
  • keyboard connection status

Power

• Battery supplies system power
• Optional charging via USB-C

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2. Passthrough Mode

In this mode, the device behaves as a direct USB cable between the keyboard and a computer.

Behavior

• Keyboard is directly connected to upstream USB-C port via hardware switching

• ESP32 is not part of the USB data path

• BLE functionality is disabled

• OLED remains active and displays:

  • "WIRED MODE"
  • charging status
  • battery percentage

Power

• System is powered from upstream USB-C
• Battery does NOT power the system
• Battery may still be charged

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Mode Selection

• A physical switch selects between:

  • WIRELESS
  • WIRED

• The switch is authoritative

• Mode switching is NOT live

• Device must be restarted after changing modes

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Ports

Upstream USB-C Port

• Used for:

  • charging the device
  • connecting to a computer in passthrough mode

• Supplies system power when connected

Keyboard USB-C Port

• Used to connect the MIDI controller

• Behavior depends on mode:

  • Wireless: connected to ESP32 USB host
  • Passthrough: connected directly to upstream USB-C

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Power Architecture

• 1-cell LiPo battery

• USB-C charging input

• Power-path design allows:

  • operation while charging
  • seamless transition between power sources

Power Priorities

Wireless Mode

1. Battery powers system
2. USB-C supplements or charges battery

Passthrough Mode

1. Upstream USB-C powers system
2. Battery is isolated from load
3. Battery may still charge

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USB Routing

Wireless Mode

• Keyboard D+/D- connected to ESP32 USB host

Passthrough Mode

• Keyboard D+/D- connected directly to upstream USB-C port

This is implemented using a USB 2.0 switch/multiplexer.

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Keyboard Power (VBUS)

Wireless Mode

• Supplied by internal 5V boost converter

Passthrough Mode

• Supplied directly from upstream USB-C VBUS

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Display (OLED)

• SPI-connected monochrome OLED display

• Displays:

  • battery percentage
  • estimated runtime
  • current mode
  • connection status

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Firmware Behavior

Wireless Mode

• USB host enabled
• BLE MIDI enabled
• MIDI data forwarded USB → BLE

Passthrough Mode

• USB host disabled
• BLE disabled
• ESP32 used only for UI/status

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V1 Scope

Included

• BLE MIDI output
• USB passthrough
• battery operation
• USB-C charging
• OLED display
• manual mode switch

Excluded

• custom RF protocols
• MIDI remapping
• display UI complexity
• automatic mode switching
• USB hub functionality

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Hardware Design

The schematic and PCB layout are both complete. The design is implemented in KiCad 10.0.

The schematic is split across seven sheets:

Key ICs

PCB

The PCB is laid out in KiCad. All components are placed and fully routed.

Board dimensions: 75 × 40mm
Thickness: 1.6mm
Layers: 4

Stackup

Inner layers are solid copper fills: In1 = GND, In2 = SYS_3V3.

Test points: TP1–TP6
Via size: 0.45mm diameter, 0.2mm drill

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Summary

This V1 design prioritizes:

• simplicity
• reliability
• clear user control

The system uses a manual mode switch and hardware-level USB routing to ensure predictable behavior and robust operation.

Current status: Schematic and PCB layout complete. Ready for fabrication review.