how do I use an optoisolator or optocoupler to run more hard drive LEDs?

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This is another bona fide frequently asked question. I typed up this lengthy response tonight and posted it in a forum.

OK. Let’s roll up the sleeves…

Why not just use a FET?

Because the best reson to use an optoisolator is the total electrical isolation between your motherboard and your circuit. Now, if you are a serious electronics gawd, feel free to use whatever interfacing strategy you like. But the opto is cheap and gives your expensive mobo a lot of protection from things like short circuits.

I recommend a 4N27 optoisolator. This item should cost about $0.35 from an electronics discount house like Hosfelt.

The basic theory of operation is that the HDD activity signal activates an infrared LED. This is coupled optically to a phototransistor, so that the transistor conducts when the HDD activity signal is ON.

Here’s the pinout:
1: input LED anode
2: input LED cathode
3: no connection
4: output emitter
5: output collector
6: output base

The pinout for an optoisolator

We don’t need the base connection here, we’re switching not amplifying.

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