[snip]

2a) One example might be the X-10 gadget that plugs into the wall for the
"high voltage" side and has some screw terminals for the "low voltage" side.
These have options including send "ON" when an external contact closes and
"OFF" when the external contact opens. Other X-10 devices would switch the
lamp load. This is the quick and dirty "off the shelf" approach.

Doesn't X-10 all depend on radio transmissions.

No. Most doesn't.

I'm going to use that
to flash the porch light when the alarm goes off, but that's not
critical. If at all possible, I like to use hard-wired things.


X10 is a power line carrier system, where the signals are modulated on
121KHz bursts that are sent along the power line during zero-crossing.
This is an old, slow protocol with little error correction.

There are related devices that transmit through the air, but normally
X10 does not.

Because of the powerline dependence, you will not be able to use X10
across power transformers, and may have problems controlling devices
on the other AC phase.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Properly read, the Bible is the most potent
force for atheism ever conceived." -- Isaac Asimov