Got it, so optoisolation isn't really necessary then. Is it that hard to set it up so a HIGH output on the pin correlates to the solenoid being activated? Seems a lot of setups have it so you need to set output on the pin to LOW to switch the solenoid on.
1) Two separate power supplies are needed, a 12v one for the solenoid, and the arduino one. I don't want to use the 12v for the arduino because the vreg will get very hot.
Yea, I will have to test it again once I get the fet and solenoid. If it gets too hot I'll just use two supplies, or maybe find a way to heatsink the vreg.
My tests so far with a 12v supply have led to no problems with the vreg even getting warm. It must have been the LCD screen's current draw causing the heatup on a previous project where I attempted to use a 12V supply.
Anyway, I noticed when the power supply is disconnected and the board is only powered through usb, there is still ~50mv backfed through the barrel jack. I'm afraid if the pin switches the fet in this state, the solenoid will attempt to draw a lot of current from the 5v usb supply back through the barrel jack, which might burn something out in the board. Do I need to take this into account, or is there some kind of limiting in the arduino board which will prevent this?