Arduino Mega 3.3V and 5V Pins

[awneil]

@gilshultz

ALL Grounds get connected together

And that connection should be a single "star" point - not a "daisy chain".

Recent thread on this:

I2C protocol - Common power supply mandatory?

You don't need a common power supply, but you certainly do need to have a common GND. The GND is what provides the reference point for each board to determine whether signals are at zero or one (or any other level) - without a common reference, that won't work. But the power supplies can be different - just beware of leakage if one is turned off while another is still on ... Addenda Sometimes you might appear to get away without a specific GND link - but it needs to be provided somehow;…