2Switches - one light - how?


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).
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snipped-for-privacy@at.BioStrategist.dot.dot.com wrote:

"Three-way" switches can be wired in several ways, depending on physical layout. see http://www.the-home-improvement-web.com/information/how-to/three-way-switch.htm
Are you using a *3-way* motion sensor switch like this one: http://www.acehardware.com/product/index.jsp?productId 94389
--reed
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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.
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L1 L1 0===========0 0===========0============= Line | \ / | C 0================================O C \ / \ / 0===========0 0===========0============= Switch return L2 Optional L2 Intermediate
If you wanted to replace a two way switch with an IR operated one it would need to have a Single Pole Double Throw relay etc. Ie, three switch ^ ^ ^ ^ terminals.
If you wanted to *add* it to an existing two way switched circuit it would need a Double PDT relay wired as per the intermediate above, or the wiring altered to take a discrete intermediate switch in one of the original positions and the IR one as a two-way at the end of the altered circuit.
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