low cost sensing low voltage AC

ok, so this may seem like a silly question, but how do I complete the circuit for the ac sensing?

my circuit is going to be powered off the same 24vac supply, rectified and regulated (yes, the 7805 will have a heatsink!)

see attached ac sensing 'filter' and dc power supply

OK, nothing seems to be your call. All I can say is think liability to the tennants and the city/county/state that has legal authority. I really think you are potentially taking on more liability than you anticipate. This has nothing to do with whatever your parents role in this is.

You should talk to the local agency responsible for code enforcement and see how they feel about this. If this was your own system, it is probably a different situation. You are not the owner, and probably would be legally viewed as an unlicensed contractor screwing with something only possible to a licensed contractor. You could put yourself in a world of hurt. The US legal system takes no prisoners if something goes wrong. Even if a failure is not related to what you do, the cost of proving that may exceed your net worth.

Being a third party is, I believe, a major issue.

if Nigel or another mod can lock this thread, I think i've gotten all the useful information there is to get now that the attorneys have arrived.