The CEREBEL USB MIDI HOST has several design errors

1. MCU runs with 8MHz only, where 16MHz should be used
2. MIDI Pin 4 = +5V/220 Ohm where MIDI Pin 5 = 3.3V/220 Ohm, both is definately wrong.
a. you can not use a 5V pullup voltage at MIDI OUT PIN 4 and 3.3V signal voltage at MIDI OUT PIN 5
b. both voltages has to be absolutely equal, otherwise the receiving MIDI DEVICE can't work properly.
The optocoupler LED then will allways light because its cathode has 3.3V where the annode has 5V.

CEREBEL USB MIDI HOST bug fixes by r.meurer

1. Use a 16MHz 3.3V MCU or use a 16MHz 5V MCU and change the power regulator to 3.3V
2. cut MIDI PIN 4 and solder a 33 Ohm resistor between MIDI PIN 4 and 3.3V
3. change the resistor at pin 5 from 220 Ohm to 10 Ohm
4. and flash the latest firmware version (USB MIDI HOST by Yuuichi Akagawato)