The initial process is really simple and can be done in a matter of minutes. There are instructables that can show you step by step if you should have any trouble. After I converted the power supply I then added a "kill" switch and led to each output enabling me to turn any combination of 3V, 5V, and 12V on or off without turning the main power to the unit off. This is a project that has been done for about a year now, it is very simple to do and if there are enough interested I will do a step by step instructable explaining the process from start to finish.
I hope you found this little modification interesting and useful.
Here is the schematic for this project. I will take pictures of the inside of the power supply later. As I said this is a very simple straight forward project. If you get stuck there are many tutorials on converting an ATX power supply.
3 red banana plug receptacles ( I used the ones you can unscrew and insert wires)
3 black banana plug receptacles
3 latching push buttons ( i used square ones because I liked the look)
4 LED ( i used red and white)
3 led holders( they come in packs of 2 at radioshack )
4 resistors ( I used 100OHM )
piece of clear plastic, plexi glass (optional)
If you look at the wires coming from the power supply you will notice multiple,yellow, red, orange,and black wires. These are the ones we are concerned about. You will want to lay out your output locations and your button/led locations, you do not have to put them in the same places that I have. Drill the appropriate holes for the receptacles and the buttons, leds. Now you can group your wires together. Yellow is generally 12V, Red is generally 5v and Orange is generally 3.3V. There should be a single blue(-12V) wire and a single white ( -5V)wire and purple(5Vstby) you can cut those and put heat shrink over the ends.
Solder the green wire to any common and heat shrink this will tell the power supply to come on.
Now that you have your wires grouped by color cut to length and solder to appropriate receptacle, reserve 4 of the 3,3V (orange) lines for the Leds.
Connect a 3.3V line to your "power on led" anode and the the cathode to a common. Be sure to use your resistor(see schematic)
Connect the 3.3V lines you reserved to your output Leds. Be sure to use your resistors(see schematic)
Now your commons will be what is will be broken by the switch(s), also known as low side switching. split your common wires and solder to one side of the switch(s) run a wire from the other side of the switch to both your outputs common side and to the cathode of your output LEDs (the power on led will not be switched)
The plexi glass is what I used to cover the hole the harness came through, you can do whatever you choose.
The fan had to be located on the outside of the case due to the switches and leds, I then added a chrome grill that I had laying around.
Now you should have everything installed and wired up. Plug in the ac and be very careful not to touch the pcb itself, you can slide the cover on temporarily and test your outputs with a meter.Once everything is working you can button it all up and viola!
Some things to consider:
Some power supplies may require a load before they power on, this can be fixed by placing a resistor between the 5V line and common (10OHM 10W power resistor) mine did not require this.
Some power supply manufacturers may not use the standard wire color code.
I hope this is somewhat clear, I am typing and holding a sleeping baby(multitasking)
Any questions feel free to ask.