DIYbanter - View Single Post - Turning 3 way light switch into 1 motion sensor switch

William Underhill

wrote:

Hello, first time poster at DIY,

My house currently has two 3-way switch controlling a light at the
downstairs entrance. I like to turn one of the 3-way switch located at
the lower floor to a motion sensor wall switch.

I opened up the existing 3-way switch box and it has 3 terminals with
3 wires (black, red, white), how do I hook it up to my motion sensor
switch which has 2 black wires and 1 green (ground wire)? My goal is
to make the top floor switch either an independent switch, or "3-way"
with the motion wall switch.

Any help is appreciated. Thanks.

Peter

Actually, if you look, you'll find your three-ways actually have 4 wires
- red, black, white and bare copper (the ground)

The problem you're going to have is that a three-way switch actually
works like this (bad ascii art coming up)

2 black 4
1 / o-----------------o \ 6 black 7 8 white
black ----o/ \o-----------o(L)o-----------.
o-----------------o |
3 red 5 |
|
white ----------------------------------------------------------'

Consider the "o"s to be the terminals on the switch body; the ground
line is left off since it's not electrically part of the circuit. Each
switch can move to either the 'red' connection (terminals 3 and 5) or
the 'black' (terminals 2 and 4). The idea is if both switches are on the
same line, the light will be on. Since they have to be on one or the
other, changing just one switch can turn the light on or off, at either end.

Now if you replace one of the switches with the motion sensor switch,
since it has only two connections (the black wires; again, the ground
doesn't count, for our purposes), you could connect to either the red
line or the black line, but not both.

2 black blk
1 / o---------x---[M]---. blk black 7 8 white
black ----o/ `-----x------o(L)o-----------.
o---------x |
3 red |
|
white ----------------------------------------------------------'

Let's say it was connected to the black line from terminal 2. A safety
problem then arises, as it would be possible for the switch to be set to
the black line, which would now be energized but unconnected in the
other switch's box. To avoid the safety hazard, you would have to also
disconnect and terminate the red wire at terminal 3 in the remain switch
box - then all you have is a piece of wire running through the walls.

One of the things you don't say is how you want this to work. Do you
want the light to go on and off with the motion sensor detection, but
have the switch override it, so you can turn it on and leave it on? Or
turn it off and leave it off? Or both?

Yours aye,
W. Underhill
--
"Take sides! Always take sides! You may sometimes be wrong - but the man
who refuses to take sides must *always* be wrong! Heaven save us from
poltroons who fear to make a choice!" R.A. Heinlein, "Double Star"