You would need something to amplify your LEDs so you would get like 5v from a molex.
This is a circuit made by Linear. The 4N22 is an Optoisolator, that would protect your HDD mobo header. R is the limiting resistor for the LEDs. I haven't tried this circuit yet but I will for my next project. Now the limiting resistor can be computed using the standard LED resistor calculation (see Brutal's site) but I have also seen a formula that also involves the number of LEDs to be used so I'm unsure.
http://www.geocities.com/_x24/HDD_LEDs.txt">
Here's the formula for R
R = (Vs0.2Vf)/(n*If)
where
Vs: Supply Voltage
Vf: Forward/Operating Voltage of the LED
If: Current rating of the LED (usually 20mA, use 0.02 in the formula)
n: the number of LEDs connected in parallel
the 0.2 is a loss of voltage in the optoisolator (as Linear said) so he put it there to be on the safe side. About the number of LEDs, I'm not sure until how many you can place in that cicuit. I'll let you know when I do.
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