Hello,

The TIP120 is NOT a FET.
It is a NPN Darlington Transistorhttp://search.datasheetcatalog.net/key/DARLINGTON+TRANSISTOR as stated on this page:
http://www.datasheetcatalog.com/datasheets_pdf/T/I/P/1/TIP120.shtml

Bertus

 

I stand corrected

Thanks Bertus

 

I just left the uC and the rest of the hardware sharing a common ground as assumed, I probably should have connected it though, so yes it was assumed.

Well, if the downstream device can have a different ground isolated from your µC and step-up, you could use my approach first suggested. Connect the drain pin to "ground" of the downstream signal, and Vcc to high. Turning on the MOSFET creates a ∆V across these, and you don't need to invert it.

That's a questionable move to avoid just one more component, though. Depends how "simple" you need it to be.

 

I meant "connected it" in the drawing. It cannot have a different ground in the physical world.