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Anybody who?s seen a G4 knows it. That power LED that fades in and out. Figured that'd make a sweet addition to a modded case, especially since it's less then $3 in parts. Besides, why should them Mac people have all the fun? 
What you'll need:
- LM1458N op-amp (Digi-key Part #LM1458N-ND)
- 2N3904 NPN amplifier (Digi-key Part #2N3904-ND)
- 22uF 100V capacitor
- 4x 47K ohm 1/4w resistors
- 2x 100K ohm 1/4w resistor
- 100 ohm resistor
- Molex connector
Our parts
You should be able to find at least the resistors and caps at your local electronics shop, if you're missing something, can always buy it online.
What does it do?
Basically, the LM1458 op-amp will take in a fixed voltage (in our case, 9 volts), and spit out a voltage between 2-7 volts, depending if we're fading or brightening the LED. The 2N3904 runs as a voltage comparator, which lets the LM1458 know when to reverse it's direction. Here?s how you wire it up:
Wiring Diagram (Click to enlarge)
I built it on a tiny chunk of veroboard, will be using double-sided tape to stick it to the inside of the case (high tech stuff):
Puttin it together
One thing to note about soldering: Most of these parts (the op-amps in particular) can be pretty sensitive to heat. Make sure when you're soldering wires, don't hold the iron on their pins for any long period of time. Wouldn't want to find out you killed something after putting the whole thing together.
Now all that?s left to do is mount the board in the case, hook it up to an extra PSU connector and fire it up. I added a regular motherboard LED connector to the circuit so I can change out LEDs pretty easily. I tried both the blue LEDs and some cheapie green LEDs, both worked great. My cheap-ass red LEDS didn't seem to like the varying voltage too much.
(Click to see it in action!)
There ya go. Yet another small add-on to separate your case from all those 'other' modders.