How to connect arduino and modules from different power supplies?

Question

Arduino

Hello, I am assembling a smart home. There are a lot of sensors and various modules, I decided to power it all not from arduino, but from a separate PSU. I heard that you need to make a common ground. How to connect everything correctly? On arduino mega PSU 9v 1A On modules and sensors 12v 1A (I will put a 5v step-down converter in front of the consumer)

Answer 1

As an option - distribute a change of 36 Volts over a separate "network", this will reduce losses in the wires. True, you will have to put an additional "diode bridge" in front of each device, but the increased efficiency of the converters will be a plus.