I have a hard wired smoke detector that is connected to the same circuit breaker as the two hallway lights. The two switches at the end of the hallway do not turn the smoke detector on and off, however before I added a three-way dimmer to the circuit, the alarm would chirp whenever switching the hallway lights on and off. I didn't quite understand what was happening, so I went ahead with my plan to install a three-way dimmer. Now with the dimmer switch installed, the smoke alarm turns on when I'm at any setting other than full brightness. I believe this can be explained by a voltage drop causing the alarm to trip.

This perplexes me because no switch position can turn the smoke alarm on or off, however it's acting like one end of the circuit connection is experiencing extra resistance whenever the dimmer is activated, or as previously when the circuit was inturrupted briefly when the switch was flipped.

The hallway switches behave as you would expect, turning alternately on and off when one or the other is flipped.

I was hoping for some suggestions as to how the alarm or switches might be wired to explain this, and how I could fix the problem so that the alarm doesn't do that.

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    Are you really 100% sure the smoke detectors aren't on the switched part of the circuit? Because it sure sounds like they are. Commented Apr 9, 2021 at 22:33
  • Yes, it definitely stays on when both the switches are fiddled with. Commented Apr 9, 2021 at 22:51
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    @WilsonMalone -- it's closer to a PWM device, but not really PWM, look up how phase control of AC loads works Commented Apr 9, 2021 at 23:36
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    The smoke detector will have a battery as well - so if you temporarily remove battery (if possible), you can determine which circuit it is attached to. Looking like detector is attached to switched circuit. Commented Apr 10, 2021 at 1:11
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    I'm going to be a bit crude here, but either somebody didn't know what they were doing, or some lazy ass electrician connected them to the light fixtures. Smoke detectors have battery backup so they will stay operational in the event of a power outage. @PolypipeWrangler made a good suggestion to remove the battery and see if it stays on. Alternatively, if you are fairly handy, respect electricity and have a multimeter, you could remove the SD and test for voltage with the switches in various positions. Commented Apr 10, 2021 at 12:43

1 Answer 1


I think I have figured it out. One of the guys in the comments suggested that it chirps every time it switches between current and battery. What I think was happening was not that the lines were directly connected to the switches, but that they were getting EM interference from the current changes on the switched lines. I took apart the old detector and determined that it was too old to keep using. I installed a new one, and the problem disappeared. I would need an oscilloscope to confirm the interference theory, but it makes the most sense.

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