quote: "Just use the 12V connectors (the ones that feed the HDD and CD/DVD drives. Choose the wires (as there are 4 of them in each connector) to have 12Vcc on a pair (usually red and black."
claude: FWIW, you've only mentioned using the 12V output (which are YELLOW and BLACK, not RED and BLACK), when every other output of a PC supply COULD be used. Essentially each power supply output needs a different resistor value. But wait before you jump all over this! There are some issues that need to be discussed.
Common PC power supplies offer +5@20 to 45+ Amps, but the 12V outputs are usually limited to only 5 to maybe 14 amps (barely 100 Watts). Just using only the 12V output is not really taking full advantage of the 300+ Watts which almost any PC power supply could be used for.
->Think about that: at 4 watts per LED lamp, a 340 Watt PC power supply could power maybe 80 lamps - enough lamps for a whole house, certainly support for a lot of lamps per room! Let me add that running long wires to illuminate a house is not a good idea, here.
There is a possibility for using the 3.3V outputs, but I'd suggest not getting into a discussion about using 3.3V since 3.3V USUALLY draws from the 5V supply, and that gets very difficult to explain.
Don't forget that oftentimes in PC power supplies, there are additional outputs of +5V, +12V, -12V; but again, like the 3.3V output, I'd suggest users should stay away from outputs labeled '5VSB' as that output is usually too limited for more than a few lamps.
In my experience, the negative output voltages tend to be a bit sloppier compared to the positive voltages.
Normally, a PC power supply needs some minimum amount of power to be drawn from a particular output in order to get accurate voltages on all the other other outputs. users might want to plan on using the specified output as the output which has the first LED string.
Don't forget that PC power supplies expect to be in a room where humans would reside - putting a PC power supply in an attic is not a great idea because in the summer, almost any attic I can think of would get very hot and possibly cause the power supply to either fault or fail completely.
Lastly, ATX class PC power supplies need to be enabled properly or they will not turn on. Connect the green wire which is in the motherboard power connector (the connector which has something like 16 wires) to a black wire in the same connector, and it all starts running.
Be careful, but have fun!