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Wire Dual garage motion sensors to a backfed overhead light fixture Answered

My garage has a 2 light switches wired to the main light fixture. Problem is I think the fixture is fed with 2 wire (no neutral) and both light switches wired from it. Circuit looks like this I believe. I don't even think I have a ground wire, its an older circuit and half my house has no ground.

Is there a way I can wire in two motion switches instead? Most I've seen require the ground or can't operate properly with a circuit like this.

My knowledge is weak on the subject and would appreciate any guidance!


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11 months ago

Hi The circuit you have is a Two way switching, It is used to allow a light to be switched ON or OFF from either switch, eg. switch on as you walk in one end of the garage, switch off as you leave from either end. Motion sensors have no need for this facility. They switch ON the light when they "see" you, but only switch OFF when they stop "seeing" you (usually with a time delay switching off) In a garage one well placed sensor should switch the light on as long as someone is in range, then when they leave it will "time out" and switch off.

Jack A Lopez
Jack A Lopez

11 months ago

The switch electricians call a, "three way" switch, is more generally called, "single pole double throw" or SPDT.

This jargon with the number of poles, and number of throws, is explained more completely, in the Wikipedia article for "Switch" in the section titled, "Contact terminology," here:


The picture, wiring diagram, you linked to in the main topic text, is correct... but it is a little bit childish.

I mean, it is one of these diagrams where all the blocks are essentially pictures of the things they represent. You know, the switches look like light switches, instead of little lines and dots.

The reason I call it childish, is because it lacks abstraction.

Like you might write 4+2=6 as,

**** + ** = ******

If you were explaining the problem to a child, or to an extra-terrestrial visitor who had not yet learned our number system.

Actually, I will just attach some equivalent diagrams to this post, and you can figure it out. I prefer diagrams with the switches drawn as these symbols, because I it makes it easier for me to see the connections and continuity, and to follow the currents flowing from here to there.

One of them I found at,


the other at,


Although for the second one, that is not where it was authored originally.

Regarding this idea of replacing three-way switches with motion switches, I can imagine it... if those motion switches are also three-way, aka SPDT, aka {1 common terminal, 2 throw terminals}.

However, I am guessing that most motion switches, are more simple, merely SPST, one pole one throw, equivalently two terminals that get connected together, or not.

In the case you cannot find three-way motion activated switches, it might be possible to build one, especially if the switching is done by a relay. In that case, just swap out the SPST relay with a DPDT relay.

In the case where the current switched to the light is done by silicon, like a big TRIAC, that will probably be harder to modify, and it is not obvious to me how I would do it... except maybe by replacing the TRIAC with a DPDT relay.

Edit: I forgot to mention: It is not obvious to me if this imagined setup will be easy to use and make the garage light turn on and off the way you want it to.


Reply 11 months ago

Thank you very much for your detailed response. Taking your comments in, I decided to draw the circuit out from 1st principles (see attached) and I think I have a handle on what is going on in that room. As you say, I will have to find a 3-way motion sensor switch. Many of the 3-way motion sensor switches I'm seeing online mention they require a neutral wire to function. Does this mean I could feasible only replace 1 of the 3 way switches I currently have? i.e. the one with the neutral wire going to it? Thanks

Jack A Lopez
Jack A Lopez

Reply 11 months ago

I think I see what you meant by calling this a "backfed" light fixture. That is, whoever wired this light fixture, connected the hot wire to one of the lamp terminals, and put the switches in series with the neutral wire.

Also I think I get what you are saying about, "sensor switches I'm seeing online mention they require a neutral wire to function." Another way to say this, is these motion sensor switches are themselves powered, by mains power, so they need both a hot and a neutral.

Also there is a constraint regarding the number of conductors, the number of wires, that run from box to box. Each switch box only has three conductors. (That is unless you want to put more wires in the walls.)

Presumably a three-way, motion sensing switch would need, at minimum, four wires: hot, neutral, and two travelers.

So I guess one possible answer is to use a two-way, motion sensing switch, which uses, at minimum, three wires: hot, neutral, and switched hot (one traveler?).

And what do we want to put in the other switch box?

For now, I am just going to consider possibilities with one two-way motion sensing switch, and one mechanical switch, two-way or three-way, in the other switch box.

Also, for now, I am only going to consider those possibilities that put switches in series with the hot wire, rather than this "backfed" wiring, which is not the usual way things are done.

So far I have imagined roughly four possibilities, but I am trying to narrow that down, to those that are the most easy to use; i.e those that will allow you to control the light the way you want to.

I think the best might be to use a three-way mechanical switch, that switches the hot wire, to one of two places: the overhead light or the hot wire to motion sensor with two-way switch, and when the motion sensor is turned on, it can switch the overhead light on if it wants to.

The problem with that idea, is there is no way to force the light to turn off, unless the motion sensor switch comes with a switch like that built-in.

However, if the box containing the motion sensor switch could be widened, like by installing a box with width for two switches, then it would be a simple matter to just put an ordinary two-way switch in series with the power to the motion sensor switch.

That way there would be a way to turn off the overhead light, and be confident it would stay off without the motion sensor turning it on.

And I included this "off" switch in the attached diagram.

Actually there might be motion sensing switches with this functionality, like some kind of override, or off switch, built-in.

Jack A Lopez
Jack A Lopez

Reply 11 months ago

Could you give me a link, to these "sensor switches [you're] seeing online?

From where I am, I am not sure I am seeing the same ones.