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I want to put a motion sensor light in my breezeway. Right now the light is controlled by a three way switch. Will I ab able to do this installation.

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Sure. The three way just allows two switches to turn a light on and off. The motion sensor limits the controlled light, if it is set to go on, only when it senses motion.

Effectively, the three way switches prime the light to go on and the motion triggers it. The switches determine if there is power to the light socket. The motion detector (which controls power after the switches) then either turns the light on or off based on motion.

The problem is that it will be difficult to determine when the light is primed to go on. Regular switches (single pole, one way) are on when up and off when down. However, for three ways, up on a given switch an be on or off depending on the orientation of the other switch.

How do you know if the motion sensor controlled device is ready to go? If you can see both three way switches at the same time, a practical way to do this is to set up the switches so that they are on when both are up or down and off when they are not in the same position.

UPDATE: If you can't see both switches at the same time, you cannot know the motion controlled device is ready (ON) unless you set up some sort of pilot light. There are three way switches that are illuminated when off, such as this one.

illum switch 3way

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No, don't do it that way. It will be way too confusing, just as Bib says. You would have to memorize the switch pattern in order to know if the motion sensor will activate.

Run the switches and the sensor in parallel.

This way either can control the light. Well, either can turn the light on, but not off if the other is on.

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Putting a single-pole motion detecting switch in the middle of a 3-way circuit (or even in parallel) would result in non-intuitive behavior as the other answers already describe.

However, instead of putting a motion detecting switch in the middle of a 3-way lighting circuit, consider replacing one of the current 3-way switches with a 3-way motion-detecting switch (this an example of one).

This would give typical 3-way switch behavior and also a light that automatically turns on in the breezeway which may meet your needs depending on the desired functionality and the current locations of the switches.

E.g. Assuming that you want the light to come on automatically when you enter the breezeway from the garage: If one of the 3-way switches is in the garage near the breezeway entrance, you can replace this 3-way switch with a 3-way motion detecting switch and the light will now turn on automatically as you enter the breezeway from the garage.

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Some of the responses above are over-complicating this very simple project.

While standing next to one of the switches, can you see the other one from across the room? If so, (1) single-pole motion sensor switch will be sufficient. Buy a blank cover plate to put on the wall where the other switch used to be, as you'll remove it to avoid confusion.

If you have to go around a corner or through a door way or some other obstacle to see the other switch, then you may want to buy (2) SINGLE-POLE motion sensor switches. This is only necessary if you want the light to come on when movement is seen on both sides of the obstacle.

Remove both of your 3-way switches. There were two "traveler" wires A and B that connect the two switches to each other. Typically these are black and red. You will no longer need one of these wires, so cut the tip off the end of the red wire at both switch boxes to eliminate its bare copper portion. Cover each end of this wire with a small wire nut or securely tape over its end with electrical tape, and tuck it back inside the back of each box in the wall.

Install a single-pole motion sensor switch in the switch box that would give it the best view of the area you would like it detecting motion. It will connect to the remaining two wires that used to connect to your old 3-way switch. Because it will need a little bit of power to operate the sensor itself, the motion sensor switch will also need to be connected to the white neutral wires. You'll probably find a couple of white wires tucked back behind the old switch with a wire nut on them. Remove that wire nut and connect the white wire from the motion sensor switch to it and then replace the wire nut.

If you need a second motion sensor switch at the other location to sense movement on both sides of some obstacle, install it in exactly the same manner. Make sure they are both SINGLE-POLE motion sensor switches. DO NOT buy the 3-way motion sensor switch model version as those are only necessary in extremely rare complex scenarios which simply isn't this. If there's no obstacles and you do not need a second sensor, then connect the other two wires together that were previously connected to the old switch in the other box and put a wire nut on them. Tuck the wires all back into the box and put a blank cover plate over it.

Consult with the manual to configure how many minutes you want the light to remain on after motion has ceased.

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  • Note that with 2 single pole motion sensors in parallel, you'll get an "OR" function. This is mostly fine, but could lead to confusion if you try to use the manual override functions on both sensors. Commented Mar 7, 2021 at 1:35

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