Would a relay work just as well or is there a "absolute must use" reason for an optocopler/opto-islolator?
Well, I'm not sure what you're using it for but.. I'm know the major benefit of an optoisolator is that it completes removes all electrical connections between the two loads. Which is perfect for isolating LEDs and other logic etc from your sensitive motherboard. Now, I'm not 100% positive on this, but I think that relays may still have the risk of an electrical contact from possible surges over the allowed current. But again, I'm not sure. The benefit of relays though is that they can handle more current than an optoisolator.
Relays are very slow-acting compared to opto-isolators. Takes from 1mS to over 20mS for the electromagnet to move the thingy & the switch to close. They also wear out.
