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.