Remove all the outer screws and the hard drive. Use a paint knife or other thin flat pry tool to open the top lid. It’s attached with tabs located mostly in the corners.
There are four large screws in each corner. Remove them and you will be able to pry the ring of plastic that makes the sides of the housing off. The bottom face plate is attached with more snap tabs. That will leave you with just the inner metal enclosure left.
Remove all the screws on the outside of the metal enclosure, it will come apart in two halves. One half is connected with only outer screws, the other half is also screwed to the circuit board. There is also a metal face plate screwed to the circuit board protecting the connectors that you will have to remove.
There are two places on the circuit board that you have to solder wires to. One is where the voltage and ground is connected, the other is where the the power button is connected.
Replace the faceplate and thread the power and ground wires through the slot to make sure that they don’t deflect the contact between the faceplate and the circuit board. Remember that the outer plastic casing is a very tight fit and any deflection in the metal casing will cause the plastic to not snap together correctly
Make a three position cable terminated in a female header to connect the IR sensor to. Also solder some short pieces of wire to the Arduino. The raw power coming into the computer is over the maximum that the Arduino Pro can handle, I use a rectifying diode to drop the voltage to an acceptable level. A voltage regulator would be more appropriate if the voltage would have to be dropped more than 0.3v Insulate the Arduino with heat shrink tubing Refer to the code for pin mapping. The wires between the IR sensor and the Arduino have to be soldered together because they are on separate sides of the metal casing. There is a place in the metal housing to route the wires coming from the IR sensor and power button over to where the Arduino is located. Solder and insulate the wire stubs of the Arduino to the routed wires from the sensor/power button.
Replace all the screws for the metal casing, don’t forget the screws that connect one half of the casing to the circuit board. Add the bottom face plate of the plastic casing.
Measure and drill two holes in the side piece of the plastic casing. Glue the IR sensor in place where the holes are. Attach plastic side to metal casing, while doing so you need to carefully connect the IR sensor to it’s wiring harness.
Add top face plate of plastic casing and replace all outer screws and hard drive. And you’re done!
Here’s a pdf version of these instructions:
Here’s the code that you’ll need to program the Arduino: