[tripletao]

> over a few years, the LED gets dimmer and dimmer

That shouldn't happen unless the LED is driven near the top of its current rating, which shouldn't be necessary unless you're pushing the limits of its rise/fall times (in which case a different part would be advisable as you say).

A random app note shows 95% of initial current transfer ratio after 25 years at If = 5 mA, and depending on the necessary bit rate we could probably design for at least 2x initial margin on that CTR. Such a design would last effectively forever.

https://www.we-online.com/catalog/media/o303314v410%20ANO006...

I think the galvanic isolation is mostly a feelgood here, allowing people to say it's "air-gapped" even though that's not directly relevant (since Wi-Fi is also "air-gapped"). A simple gate or level shifter can also enforce unidirectional data flow as you say.

[zephen]

Upvoted for:

> which shouldn't be necessary unless you're pushing the limits of its rise/fall times

Right, I should have clarified this. To make them go fast, you can often use more power (to a point), and that can shorten the LED lifespan. (To be fair, there are techniques to give you a bit faster speed without making them too bright, like pulse-shaping, but it didn't appear anything that fancy was going on there.)

And, unfortunately, "fast" for the optoisolator isn't very, in any case. The cut-off frequency for the first datasheet I found corresponding to that app note was 80 KHz.

> I think the galvanic isolation is mostly a feelgood here,

And...

I don't get this.

If it does nothing useful, why bother?

IMO, the primary good use for an optoisolator these days is either for something analog-ish like the feedback for a switch mode power supply, or for when you're breadboarding something with really high voltages and don't want to bother with SMT devices.

[tripletao]

I think those optoisolators are indeed sold mostly for switching power supplies. That's probably why someone cared enough about aging to write an app note, since the ambient temperature is high there and the exact CTR matters more when it's in that analog feedback loop. I've also seen them for digital inputs in industrial control systems, where speeds are slow and the wires might be coming from far away on a noisy ground.

That said, I believe optical isolation is typical for these "data diode" applications, even between two computers in the same rack. I don't think it provides any security benefit, but it's cheap and customers expect it; so there's no commercial incentive to do anything else.