While I have no idea how they do it, I used to do it with one of these: https://en.wikipedia.org/wiki/Opto-isolator https://en.wikipedia.org/wiki/Opto-isolator

While I have no idea how they do it, I used to do it with one of these: https://en.wikipedia.org/wiki/Opto-isolator https://en.wikipedia.org/wiki/Opto-isolator One circuit emits light, the other circuit listens to the pattern.