I cannot solder the hat to the raspberry like suggested in the manual because the pins are alreade blocked by a proto-shield. However, I connected the SCL and SDA pins to the board and used an external power supply. Unfortunately, the PWM led on the Raspberry does not light up and
i2cdetect -y 1 shows no detected device (i2c is activated).
Do I have to connect an additional pin ? e.g GND ?
It is working if I do not use an external power supply and use the raspberry directly. Probably there is something wrong with the external power supply.