Ok, I'd probably have saved time by buying an RC controlled relay from pololu,

Ok, I’d probably have saved time by buying an RC controlled relay from pololu, but I like the SmartFPV RCCCv2 board (switch 3 video inputs, plus extra functions), so I managed to control a dual relay to switch 5 and 12V with it. That way I can turn off/turn on my VTX and cameras (before and after flights). I can even do an in flight reboot of my mobius cams if needed.

http://marc.merlins.org/perso/rc/post_2015-10-29_Controlling-5-and-12V-swtiching-with-SmartFPV-RCCCv2-Video-Switching-Board.html