I've been thinking of ways to do home automation that is somewhat future proof (hardwired); and the best idea I have is to use relays for lights on a 3 way switch circuit. Put in a dual pole dual throw relay and have it emulate a 4 way switch. Also add a current sensor so I can tell if the light is on or off (secondary goal). All the lighting circuit wiring is in the attic (1 story house) so I'll splice the box in above. Due to the nature of the way a 4 way switch works this should be passive and allow for simple switches to be used unlike what I saw here Any obvious problems with this DIY home automation?. Also means that if the automation logic is removed the switches will work as normal. I can put these in now and worry about hardware/software later; having all control and sensor wires run back to a central location.
Question is what relay do I need to get, and how would I wire it up in order to accomplish this task? I was thinking of using 24vac coils because they are very common (in homes) but that makes it harder to hook up to a logic board as I'll need another relay to convert DC to AC for the coil on the 120v (USA) relay.
Pretty sure I would need a DTDP (or 2x STDP) latching relay but I'm not sure which one to get and how to wire it up so that the circuits cross like X or goes parallel like = when given a switch signal.
Making all switches be 3 way wouldn't be that hard to do in this house; so I could do this for every light switch without too much hassle.