Thanks!
First, let me say that I did this a few months ago, and I don't quite remember everything perfectly. But I think I documented it pretty well in this thread. So alot of the info you'll have to pull out from there.
But feel free to ask questions about anything you don't understand. That's fine.
And, I'm going to assume you have a basic knowledge in electronics.
I went to Radio Shack and I got an IR LED emitter and dector. They both came in one package and I think Radio Shack only has one anyway. Next, I wired up the IR LED just like you would wire up any standard LED. I got the voltage from the 5v connector from the junction of the Molex pin and the PCB of the optical drive. Ground also came from here. But After the IR LED (aka the emitter) is wired up I had to use a bit more complex circuit for the detector.
Cuzza kindly found the circuit that I needed. (great thanks to him!) I've reproduced his circuit below with my comments in red, just clarifing the parts.
http://img.techpowerup.org/100113/irleddiagram.png
One thing I
think I did differently was that I used a 1000 ohm resistor instead of the 500 ohm one. The schematic diagram above is pretty self explanatory. You have the two contact points from the switch and a ground from the Molex. When the two contacts of the switch are connected (like when you press the button) the drive opens. This circuit works the same way, but uses a transistor as the switch.
When the IR detector 'sees' IR light, it allows current to pass through it. This completes the circuit of the Collector and Base of the transistor, causing a bit of current to pass through. When the transistor senses this current across its Collector and Base, it connects the Collector to the Emitter, which closes the circuit across the optical drive switch, which opens the drive.
You need a multimeter to determine which contact of the optical drive switch has voltage on it. One contact does, one doesn't. The one that does is Vin and the one that doesn't is Vout.
Hopefully that made sense. If not, feel free to ask questions.
Edit: This pic may help too:
http://img.techpowerup.org/100113/irledrealpic.jpg