If you switch before the servo the grounds are the same.
Maybe I misunderstand but that sounds like asking the Arduino to saw through the branch it is sitting on Also, the whole point of the exercise is to switch the servo on a little later than everything else.@colubridae, if this was my project I would just put the switch on the servo V+ line. And, sorry, I am not proficient enough to offer advice on transistor or mosfet selection. I just work my limited analog electronics on the suck-it-and-see principle....R
I mean switching high side rather than low side,
Keep in mind you cannot use a NPN bjt or N-FET if you switch high side. Must be P-Channel unless you find some way of keeping your gate voltage well above threshold (charge pump) when the MOSFET is conducting. If your source voltage is the same or less as your switching voltage (and under the 5v arduino limit) you can get by without the second NPN transistor driving the gate of the P-FET.