The circuit can be *much* simpler by using an opto like the PC814. This chip has 2 diodes in anti-parallel at the input, so the output triggers for both, the positive as well as the negative half-cycle, without needing a bridge at the input. Just use a 47K resistor at the input.
And Betwixt, you do not need to have the grounds shared between the input and the output; you do have true isolation.
Incidentally, I have been using this in my X10 receivers for years.
Regards,
Anand Dhuru