Zephos:
I understand that the Mosfet can act like a capacitor, but what does its resistance have to do with the gate circuit, as the gate is just opening and closing the valve, so to speak, and nothing is actually going through the gate pin other than a small control voltage?
Gate CURRENT flows when that gate capacitor has to charge or discharge.
Once the gate reaches the same voltage as the pin, current stops.
A capacitor is basically a short untill it is charged.
The 220ohm resistor limits pin current to 20mA during that very short charging/discharging time.
Hope this makes it clear.
Leo..
It makes perfect sense now. The voltage is usually not dangerous, but the current IS. The 220 resister keeps that in check, while the 10k resister is simply directing any voltage from the arduino to the gate, so it will actually turn on, while still connecting the gate to ground for the odd times where the arduino doesn't have the pin going to ground or to high.
Not critical. Any common 1Amp diode from the 1N400x family, or a 1N581x schottky.
Full inrush current for the Arduino and attached loads flows through the switch and the diode, untill Q2 takes over.
I doubt that a common 20mA LED will survive that abuse.
Leo..
