Is it possible to use a capacitor to make an LED fade out, such as a HDD activity light, so that it slowly dies out when there is no activity, rather than just turning off? Would it require seperate power circutry(transistors, an optoisolator perhaps?), so that its not being drawn from the motherboard, to avoid damage? Thanks.
1/(2pieRC) thats the time formula. As to wether you need a driver, such as an opto-iscolator, depends on how much current your trying to draw, and then it seams to be mostly guesswork. i would recomend a cheap opto-iscolator + a resistor, cpemma says the resistor is superfurlous, but i don't like to take the chance over a 0.5pence resistor.
Also as an addendum: The larger capacitor you use, the longer the fade out time BUT the longer the first initial charge for the capacitor thus resulting in a delay for the LED because of forward voltage that must be surpassed. A capacitor, when first "completing" a circuit, will short a circuit and thus "suck" almost all the current leaving the LED with nothing until the capacitor reaches is satiated.
would an outside source of power, that was only activated by the HDD activity, rather than supplied by it, help the capacitor to charge faster?
I really don't know if having a larger voltage power source would help the capacitor charge faster, though I do know that having a less resistive capacitor will decrease the amount of time for th capacitor to charge. Usually smaller capacitors will have a smaller ESR rating and thus be applicable; the charge time for capacitors may be of a few milliseconds come to think of it so it's best to just try it out and see.
