0

The lights in my garage are controlled by two 3-way light switches. One in the hallway leading to the garage door and the other in the garage itself. For the last 5 years, the lights would be off if both switches were pointed down in the “off” position (I know they don’t say off with 3-ways). A couple days ago without changing things, it changed so now one of the two switches has to point up for the lights to turn off.

How would the circuit have changed so that the switches now have to be in different positions?

2
  • That's not really how 3-way switches work. If both are in the down position and you flip the hallway one to the up position to turn the lights on and then flipping the garage one to the up position will turn the lights off. Have you, perhaps, been consistently using one to to turn the lights on and the other to turn them off when you leave and doing the opposite when you come home? That would lead to them always being in the same configuration after a full leave-arrive cycle, but any deviation from that routine would leave the switches in a different configuration. Commented May 22 at 3:59
  • There's a third switch you don't know about and have never had occasion to use. It got thrown. Commented May 23 at 22:26

2 Answers 2

4

There are really only two ways this can change:

  • Work and/or prank - someone opened up a switch and either rotated it 180 degrees or moved some of the wires.
  • 4-way switch - If you have two switches controlling one light, they are called 3-way switches. If you have three or more switches controlling one light, the first and last are called 3-way switches and the middle ones are called 4-way switches. All the switches will look the same from the outside. So there may be a third switch somewhere - possibly behind a shelf or some other place - that is not normally used. Someone flipped that other switch.
2

That behavior indicates the lights are controlled by at least three switches, as shown in the image below (adapted from Wikipedia).

Light controlled by three switches

The two single-pole double-throw (SPDT) switches at the end can be used without the middle double-pole, double-throw (DPDT) switch. if wired directly to each other; then the switches will always behave the same, i.e., when both are up, or both are down, the light is on; with one up and the other down, it's off.

The middle DPDT switch acts by flipping the two potentially hot wires. That allows the third switch to control the lamps, effectively changing the setting of one switch to invert it; i.e., making up act as down.

So to get the two end switches to behave as before, find that DPDT middle switch and flip it back to its previous state.

Note that there can be any number of DPDT switches in the middle! For example, in a long hall, there might be 10 DPDT switches by 10 doors, and each could turn hall lights on, if off or off, if already on (and of course a SPDT switch at each stairway, at the end of the hall, to do the same).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.