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

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So pins one and two connect to your HDD header. You load goes in the emitter circuit (+12V connects to collector, emitter connects to +ve side of the load, -ve side of the load connects to ground.

Here’s an example with 3 parallel LEDs:

An example optoisolator circuit

I’d pay attention to collector current rating (4N27 offers you 50 mA). The above diagram shows a 60 mA load (3 * 20 mA). You could arrange the three LEDs in series and keep the load current to 20 mA. You could get six LEDs by two strings of 3 in parallel, etc. And don’t forget that you can use 12V at the collector to make it easier to drive series loads.

The collector-emitter junction will drop about 0.2 V when ‘ON’, so deduct 0.2V from Vs when you calculate the value for your current limiting resistor.

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