HDD LED for add-on cards

Among the many little things that irritate me, it’s when you have a computer case with multiple drives in it, attached to multiple add-on cards as well as the motherboard, but (of course) there’s only one hard drive LED on the case — so you get to pick a controller card to drive the lights on the case, and … well, I suppose you can be happy with that, especially if you don’t pay it much attention.

When looking for a simple cable to combine the inputs, I realized that I may be part of a subset of the larger population, the subset who wants their leds to behave themselves whether it really matters or not. Either my skills with Google are not up to snuff, or there is no such animal commercially available.

Partly it’s because if you’re hapless enough to simply connect them all in parallel, you’re likely to burn out your case led the moment there’s activity on more than one controller card. So slightly more complex circuitry is necessary to make the lights work.

Lying around, I had a tiny case, optocouplers, and resistors, so I had to get power connectors, header connectors, and a breadboard — but there’s really not much more to it than that.

Case open

To the left, the circuit assembled on a breadboard in a little case — to the right, is a sketch of one of the optocoupler circuits. And here’s what it looks like with the case closed: