Smart MIDI box

11 months ago 464
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(still work in progress, not fully tested)

A smart MIDI box based on a STM32 "blue pill" microcontroller (STM32F103C8T6).

Software is hosted on Github:

Uses a HD44780 16x2 LCD screen with the usual I2C converter.

The interface is made of 4 push buttons arranged in a directional pad (up, down, left, right).

A SD card can be attached for more features.

Optional parts

Some parts on the PCB are optional, depending on what you wish to do with the board.

Extender address

J1 and J2 are only useful for port extenders so it's useless to put them on the main board.

J3 places the firmware in extender mode so it must be omitted on the main board.

Optional inputs and outputs.

  • To remove MIDI IN 1, omit R1, R2, D1 and U1.
  • To remove MIDI OUT 1, omit R3, R4, R5, R6, Q1 and Q2.
  • To remove MIDI IN 2, omit R7, R8, D2 and U3.
  • To remove MIDI OUT 2, omit R9, R10, R11, R12, Q3 and Q4.
  • To remove analog gates, omit U4 and R13-R20.

Optional SD, LCD and buttons.

If you remove the SD card, the LCD screen and buttons, all MIDI messages will be routed to the first MIDI output. It acts like a buffered MIDI merger.

MIDI port extenders can be used in that configuration.

If you omit the SD card, you can omit C2 (decoupling capacitor).

Building a multiplexed extender

  • Build the same PCB as the main board
  • Omit SD, LCD and buttons as they are unused
  • You can put analog gates on an extender, they will share MIDI OUT 2. Analog gates are mapped to C-4 and up on extenders.
  • Place a jumper on J3 (or link LT and RT pins together).
  • Set J1 and J2 to either open or closed to select the 2 MIDI ports it will route (use 0 ohm 0805 resistors or just solder a piece of wire between the pads). Each extender must have a different J1/J2 combination.
  • Connect "MUX TX" of the main board to "MUX RX" of the extender
  • Connect "MUX TX" of the extender to "MUX RX" of the next extender, and so on and so forth (up to 4 extenders, order is not important)
  • Connect "MUX TX" of the last extender to "MUX RX" of the main board



PCB 1.0

PCB 1.1


ID Name Designator Footprint Quantity
1 POWER P1 HDR-TH_2P-P2.54-V-M 1
2 MUX P2 HDR-TH_2P-P2.54-V-M 1
3 1N4448 D1,D2 DO-35 2
4 BC547B Q1,Q2,Q3,Q4 TO-92_3PIN-L4.95-W3.94-PITCH2.54-L 4
5 MIDI1 IN H2 HDR-TH_3P-P2.54-V 1
6 MIDI1 OUT H4 HDR-TH_3P-P2.54-V 1
7 MIDI2 IN H5 HDR-TH_3P-P2.54-V 1
8 MIDI2 OUT H6 HDR-TH_3P-P2.54-V 1
9 100u C1 1206 1
10 BTN H1 HDR-5X1/2.54 1
11 6N137EE U1,U3 DIP8 2
12 STM32F103 U2 STM32F103C8 1
13 LCD H3 DIP-1X4P2.54 1
14 100n C2 0805 1
15 GATES P3 HDR-8X1/2.54 1
16 SD P4 HDR-8X1/2.54 1
17 ULN2803AN U4 DIP18 1
18 10k R13,R14,R15,R16,R17,R18,R19,R20,R3,R1,R6,R9,R7,R12 0805 14
19 220 R2,R4,R5,R8,R10,R11 0805 6
20 ADDR_H J1 0805 1
21 ADDR_M J2 0805 1
22 EXT_EN J3 0805 1


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