I'd like to be able to wake a PC when someone enters the room, what would be the easiest and cheapest way to achieve this? I'm thinking a PIR sensor hooked up to the Arduino and set the machine to wake via USB, does that sound possible? I've never done anything like this before, but I can follow instructions! Any advice would be much appreciated.
Depends on the type of sleep. Some computers require you to press the power button again to wake it up. This could be achieved with a simple relay on the power switch. To wake up my desktop I usually just hit the spacebar. You could either make the arduino act like a keyboard, and have it "press" a key a few times to wake it up, or you could take a cheap keyboard and have the arduino short one of the keys a few times.
What do you mean with "waking up"?
Booting or restoring from screensaver/sleep mode?
If you want to Boot the PC:
Look for the cable of the power-button inside the tower. Just cut it and bypass it in paralell with a small reed relay.
Replacing the powerline-switch (you mean the one outside the pc @robtillaart? ) won't do the trick, because turning the power on doesn't mean the pc is booting up by itself...
Sorry, yes, I mean wake from an S3 or S5 sleep. Would I have to switch off the sensor somehow once the PC was awake, otherwise it would keep sending a signal every time the sensor got triggered? Is there a specific PIR sensor I should get? Also, can anyone recommend some guidance to learn how to do this? I don't have an Arduino, it's something I would have to get and figure out specifically for this application and it looks a bit complex. Thanks.
Off the top of my head, I would think you accomplish this with out an arduino.
Sketched out, I would simply have a PIC, a switching device (Maybe a Normally Open relay) and an old keyboard.
The 'switching device' being an object that when the PIC goes LOW. will create a contact closure.
Rip apart the old keyboard until you just have the circuit board (usually containing the Num, Caps, and Scroll Lock LED's) Figure out what combination of pins will result in a 'Left Shift Key' or 'Right Shift Key'.
Then with some wiring, When the motion sensor is tripped, it will go through the switching device. and trigger the 'shift' key. Only glitch to all this is every time the motion sensor is tripped, the shift key will be pushed. But you can always wire it to a random other key like F12.
All, thanks for the input. It's clear it can be done, but I think it's something that will have to go on the back burner for me. I just don't have the time at the moment to learn what I'd need to in order to accomplish this. I'll keep my eyes open for something off the shelf (which I doubt is available) or perhaps try and find someone to build it for me. Is there anyone who would take on work like this? I couldn't afford a great deal, particularly for something that's a 'nice to have' rather than a need, but I'd be open to chat. Thanks.