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Step 1: Materials and Tools
-PC cooling fan, must run on 5 volts, any size 
-39 Ohms +/-5% Resistor (Orange, White, Black, Gold) 
-NO/NC (SP/DT) Switch 
-USB Cord 
-Hole Saw (optional)
-Screws and Screw Driver (optional)
The reason for the "(optional)" is because there are many ways you can build this project, chose the way that works best for you.
Step 2: Modding the Cabinet
Once the holes are drilled all you need to do is insert the fan and screw/glue it in place; be sure the air flow arrow (located on one side of the fan) is pointing out of the cabinet. The NO/NC (SP/DT) switch will need to be glued next to the side of the cabinet close to the inside edge of the door. The purpose of this is so that the lever on the switch is pressed down when the door is closed, it will turn on the fan and off the LED's and vice versa for when the door is open.
The wires going to and from each component can prove to be troublesome. I suggest you glue them to the sides of the cabinet in neat organized lines, you can even paint them black to hide them. As for the USB plug you can simply lead it to the USB port on the system and plug it in. Make sure you measure out all the wire you need before soldering it all together, as you don't want to come short and have to make more connections.
Step 3: The Circuit
Step 4: Putting It All Togther
This is where the fun starts. Chose a corner for where all your connections will meet (Ex: USB power, Fan wires, LED's, etc.). Then once that place is found check if all other wires can reach their place easily; so see if the LED's can reach the front of the cabinet and if the fan can reach the hole you drilled for it. Because my cabinet had a bit of a ledge inside, I had to use a scrap piece of wood to keep the LED's and switch level with the top of the door. I would suggest that you check your cabinet and be sure a small issue like this doesn't get in your way. Once you have checked all of this its time to glue/nail it all down. I used No more Nails for this simply because its strong, durable, and wont cause any short circuits. After the glue has dried or your finished nailing, double check your circuit to see if its still functioning, if so congratulations! You are finished!
Step 5: Testing, Testing, 1, 2, 3...