A pc psu provides power for:
arduino uno (via de standby 5v power + ground so it always has power)
raspberry pi (via a 5v + ground connection)
usb hub (via a second 5v + ground connection)
Arduino is connected from an analog port to the psu POWER-ON connector to shutdown / start the psu after a button press has been detected. (how to detect pi powered off state is for another day).
Can there be any problems here?
I would eventually like to use the psu 12v to power 3 to 9 rgb leds. Can i do that?
i know all electronic things connected together should have a common ground. Does a psu have a common ground for all black wires? or should i use power from the same pair of wires for all things?
ethernet/hdmi cables to the pi not shown but they will be connected.
i've read that it would be better with 1ohm resistance on the green wire (psu power on)