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.

  • 2
    If using separate power supplies a connection of the both GNDs is mandatory!
    – Ghanima
    Jan 10, 2016 at 18:57
  • It seems to work now
    – Moritz
    Jan 10, 2016 at 19:02
  • Ditto -- they must have a common ground.
    – goldilocks
    Jan 10, 2016 at 19:12
  • 2
    Feel free to write a short answer yourself and accept it.
    – Ghanima
    Jan 10, 2016 at 19:12
  • ... and on a side note: please notice that I reverted your title edit as we do not mark questions as answered that way.
    – Ghanima
    Jan 10, 2016 at 19:53

1 Answer 1


Try i2cdetect -y 0, when I used 1 I could NOT detect my HAT, but it worked with 0.

  • Well spotted! To talk over the I2C bus on pins 27&28 that is reserved for HAT ROM you need to use the other bus. You might need to enable it in the config.txt file IIRC. This applies to the later (HAT compatible) RPis - and that was when they moved the normal I2C bus from the first(0) to the second (1) built-in ones on the Broadcom processors - they reserved the first two extra pins on the longer 40 pin GPIO header for a HAT ID ROM channel (a bit like DDC2B on PC Monitors) but wired the previously used I2C bus to there instead of using the previously unused one.
    – SlySven
    Mar 3, 2016 at 15:05

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