I2C does not work with external power source

Hey

I have an Arduino UNO and a 1602 LCD soldered to RCF8574 for I2C. The typical beginner LCD setup so to speak.

I just wasted 3 hours because the I2C connection didn't work. Scanning the address didn't work either. I solved it now and the problem was:

  • The Arduino connected to USB power (for sketch upload)
  • The I2C LCD connected the a external 5V power supply on the breadboard.

As soon as I connect the LCD I2C to the Arduino's 5V output everything works just fine!
Exactly the same behavior with a NodeMCU Lolin v3.

My question is:
Why does the I2C controller/device has to be on the same power source as the Arduino?

I feel I missed something critically in electric engineering class.

the two power sources must have the same ground

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