I want to install a 3-way circuit covering a hallway and stairwell. The top of the stairs and hallway would be covered by the motion sensor and the bottom by the mechanical switch. What happens if I turn on the mechanical switch and never trigger the motion sensor? Does the sensor turn the circuit off after a certain time without motion? Or do I need to either trigger the motion sensor on or manually turn off the mechanical switch?

Just to be clear, I understand how a 3-way switch works and how the motion sensor works. It's just the mixing of the motion sensor and mechanical switch that confuses me.


  • I believe the answer will depend on the design of the 3-way motion sensing switch.
    – BMitch
    Aug 7 '15 at 19:25

In this type of set-up, all load power is controlled by the electronic motion-sensing switch. If you use the mechanical switch to "turn on" the light, the motion sensor will turn it off when no motion is detected (after a certain time period, usually selected by the user/installer).

If you look at the wiring instructions for the 3-way motion-sensing in-wall switches you will see that they are wired a bit differently than a traditional 3-way circuit, although the number and type of wires is the same.


To make a true 3-way motion detector switch -- as opposed to a motion detector with a remote, or a motion detector in series with a normal switch -- the motion detector has to be able to monitor whether power is being drawn through either of the travellers and which traveller it is currently connected to.

If no power is now flowing, to turn on flip to the other traveller. If power is flowing, to turn on stay unchanged.

If no power is flowing, to turn off stay unchanged. If no power is flowing, to turn on flip to the other traveller.

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