Instructables

Step 1: Powering the arduino

now obviously we cant get power from the usb port when the pc is off!

you will be tapping into the +5v standby pin on your computers power supply.
the easiest way to do this is simply to take the male end of your female to male jumper wire (or any wire) and stick it down the back of the connector and secure it there.
this is a constant 5v source when the pc is off and on.
then you just run that wire to the 5v pin on your arduino. dont use vin unless you have a 3.3v board (like my pro mini).
the ground can come from anywhere be it the 24 pin atx connector, a hard drive connector, or like what i did an unused usb header.

Step 2: The other connection.

there will be two wires from your power switch to your motherboard.
unplug them both and connect your jumper wire to the ground on your arduino being powered by your computer.
touch the other end to one of the two pins. if the pc starts to shut down or power on (depending on which state its in) then thats the pin you need if not connect it to the other pin and connect it to adruino digital 13
 
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I don´t get this completely. you say: "there will be two wires from your power switch to your motherboard.

unplug them both (note from me: unplug them from where? from the power switch or the motherboard?? I guess it should be the power switch since the arduino will replace the power switch?) and connect your jumper wire to the ground on your arduino being powered by your computer. touch the other end to one of the two pins. (those two pins on the motherboard right? if I don´t get the correct pin at the first time will I fry my arduino board or something? aand the last question, once I get the pin that will turn on/off my computer, I connect the other end of the wire to the pin 13? or I keep it at GND and connect the other pin to the pin 13?).

thank you!!!

Hi, this project is great! I have a question though, do you know if it would be possible to do with a laptop computer?