Hi,
I have noticed in newer homes that they have smoke detectors that take
both a battery and are plugged in to the AC. I think the wires running
between them seem to have an additional conductor and if one smoke
detector goes off and starts beeping, shortly thereafter, the other
ones start beeping as well. How does this work? If you were to mix
and match between different smoke detector vendors (all having the
interconnect feature, of course), would they all work the same? Is
this an industry standard connection? I assume it is an additional
conductor that either gets shorted to ground or hot by the tripped
smoke detector and when the other ones detect that short to ground/hot,
they start beeping as well.

Does any company make something that could tap in to that extra wire so
that, say, if I wanted to turn on emergency lighting or something like
that, it could trip a relay to do that? I'm thinking something that
wires in like a door bell transformer where the line voltage AC is all
in the box and the relay contact screws are outside where they belong,
not sharing a box with the line voltage at all.

Of course, if company XYZcorp makes an "interconnect smoke detector
relay module" that is UL etc approved and can be legally connected to
the smoke detector wiring harness in a standard single gang box and
will trip a NO/NC relay when the detectors are beeping, I'd like to
know about that.

Since IMO, smoke detectors are life safety, I'm not thinking about
modifying anything or experimenting on my own. It is more just a
curiosity on how the thing works and if there is an industry standard
or not. I am sure some of you have modified smoke detectors to do
something like this, but I would prefer if these mods were kept out of
this thread since I think there are many that feel this is no problem
and there are many that feel that modifying a smoke detector is a
dangerous thing to do. I, personally, would rather not be lead away in
handcuffs after explaining to the fire investigator the cool mod I did
to my smoke detectors that caused them not to work when I needed them
most and, golly, I don't see why it didn't work right, should I get a
lawyer now? the circuit really should have worked....

Thanks