I'm trying to have my arduino turn on by itself when my computer goes into sleep mode. I basically only want one on at a time. My idea is to have an NC relay connected to a 5V line on my computers PSU, so when the computer is on, the arduino is off. My question is, what relay would I need for this and how would I wire it? The arduino has it's own power supply, so I just need to attach that to the relay and have it close the circuit when the computer turns off. This would be my first time working with relays so I figured it'd be better to ask than to smoke test my computer and arduino. Thanks in advance.
Sounds like a simple application. Just get a 5v relay like this maybe: http://www.allelectronics.com/make-a-store/item/RLY-499/5-VDC-SPDT-6A-RELAY/1.html I would wire the relay coil via a spare USB cable, just hack of the end and wire the USB +5vdc and ground wires to the relay coil and plug into your PC. You might want to wire a diode across the coil terminals, anode to the ground wire, cathode to the +5vdc wire, to clamp transients. The six amp contact ratings for that relay should be able to handle just about anything you might have in mind.
Or you could wire the relay coil to a spare internal disk drive power connector's +5vdc and ground pins.
I agree with retrolefty on everything but one. But of course this would depend on your mother board. The molex connector/drive connector would be your best bet and not the usb only because a lot of new motherboards supply 5v even if the computer is in an off state. And that would not trigger the relay like you want. But relay wise when you wire it up make sure the relay is open when there is power and closed when there isn't. Even tho most modern psu shouldn't need the diode. Better safe than sorry.