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We have two switches (on top of stairs, one bottom) for a light.
(It was set up in 1965, so I don't think there's fancy chipwork.) How
was this set up? I'm thinking of replacing one switch with an infrared
motion detector switch, but I have had some problems with these IR
detectors if the lightwork is too fancy (eg, one LED bulb would never
switch off in IR mode, but would manually switch on/off).




It's simple, it's just a pair of SPDT switches wired together, been used in
just about every multi-floor house since practically the dawn of household
electricity. Most split level houses common in the 60's and 70's have a DPDT
"4-way" switch on the landing so the entryway light can be controlled from 3
locations and you can chain together as many of those as you want.

You can get dimmers, motion detectors and other stuff like that designed for
3 way use though some of it has limitations.