RobertPN:
Sorry if this is a stupid question, but I notice that the wiring of my first proper project means that the arduino uno r3 and my 120 neopixel WS2812B LED strip share a common ground but not the +ve.
Cool, that is the correct arrangement.
My power supply unit (psu) is 5v 10A for the LEDs and the board is USB powered.
My question is, why can't they both share the external psu? It has ample power and 5v. Would it make any difference if the psu was, say, 7v since I read somewhere that the board prefers higher than 5v when not USB powered? Or is it just a no-no to share a psu for higher ampage projects?
The barrel connector input to the Arduino goes through a 5V regulator, and then to the board. A regulator requires a slightly higher voltage than it's intended to output. That'd be ~6.5 minimum, so 7V is typical.
I don't know what the specs are for your LED strip but unless it says it can operate at 7V then it 'shouldn't.'