Just a few points on that first schematic posted.
It is quite a poor circuit:-
- Why use all those resistors in series when a 180R would do instead. In fact as it is an LED in the opto isolator any value from 100 to 330R will do.
- There is absolutely NO POINT in using an opto isolator if you are going to connect the grounds of both sides together. Either don't do it or don't use an opto isolator.
Just a point from later:-
Would I be correct in assuming that SSRs have opto-isolators built in
Yes all the ones i have seen have opto isolators built in, but just check the data sheet to make sure.