We have an wired smoke detector that is right outside a bathroom. Often, when we turn on the lights in the bathroom, the smoke alarm will chirp. The bathroom used to have a fixture with 4 40-watt incandescent bulbs, now it has one with 3 13-watt CFL bulbs. It chirped with the old fixture and the new one as well.

What causes the chirp? Can it it be fixed?

The smoke detector is not on a switched circuit. It also has no battery (at least no user replaceable one)

It is probably close to 10 years old.

  • Have you managed to get rid of the problem? And if so, then how did you do it? Commented Apr 24, 2012 at 22:47
  • I replaced with a new battery operated unit. Commented Apr 26, 2012 at 1:19
  • This is happening to me. I'm thinking that the light switch causes a voltage spike on the line, this assumes that they are wired on the same line.
    – user46066
    Commented Nov 29, 2015 at 20:23

9 Answers 9


You don't say how long it's been there, but if there are any doubts the unit you might want to simply replace it. Most smoke detectors have a limited lifespan, usually around 10 years.

And unless it's wired into a household alarm system, you can probably replace it with a simple battery operated unit.

  • 2
    I believe some building codes require hardwired units, it may become an inspection issue when you sell the house. I would replace with a hardwired unit that has battery backup.
    – TomG
    Commented Feb 22, 2012 at 1:49
  • A hardwired unit would definitely be preferable, but don't most places grandfather existing units? For my locality, you either have to sell with hardwired units or new units with ten-year batteries.
    – Kris K.
    Commented Feb 22, 2012 at 14:19

I don't think I've seen a smoke detector that only chirps when it's light out, but I'd love to get one if that's the case. Mine always seem to chirp at night (typically because it's colder and batteries fail in the cold).

I have seen smoke detectors with a light sensor used in place of a test button, but this typically requires a flashlight shined directly on it.

My guess is that you don't have a battery in there at all and when the lights come on the smoke detector is getting reset and doing an activation chirp. Replacing or installing a battery would fix this. But I would double check that the smoke detector is working when the lights are off, on the off chance someone screwed up and wired it into the switch.

My second guess is that the voltage drop from the lights turning on, combined with a borderline low battery, is enough to cause the low battery chirp to trigger. But this would get worse as the battery dies until it's constantly chirping.

  • Ooh good thought - a smoke detector on the switch is a baaaad thing. Commented Oct 4, 2011 at 19:55
  • 5
    Obviously you don't want the power to the detector to be switched but it is very common to put the smokes on a common light circuit to increase the chances you would notice that the breaker has triped. It is certainly possible someone screwed this up when trying to hook the detector into the "light circuit".
    – auujay
    Commented Oct 4, 2011 at 21:30
  • I agree, it sounds like it's being controlled by the switch. Most wired detectors I've had chirp when power is first applied. It's quite possible that this can be fixed by changing the wiring in the switch or behind the fixture, it probably got mistakenly hooked up to the switched side at some point.
    – gregmac
    Commented Oct 5, 2011 at 5:48
  • It's not on a switched circuit, but it was a good suggestion to check for that. Commented Oct 8, 2011 at 4:13

I do not know if my problem is the same as what OP asked, but I have a similar problem and thought it would be a good idea to post the solution here. The CFL light in my family room triggers an adjacent battery-operated First Alert Smoke and CO alarm to beep. I found the answer here (it has to do with the remote-control testing feature of the unit):


I was running into the exact same issue as mentioned in this post, and between the info in this post and a bit of investigating I was able to fix the issue.

To recap a bit of this posting, yes turning on a CFL sets off a First Alert smoke/carbon monoxide detector. In my case it was model SC07. As mentioned in this post, both a regular IR remote control and turning on a CFL can set off the test feature of the detector.

The First Alert product documentation states that the IR remote test feature can be disabled by opening the battery door, pressing and holding the test button, then closing the battery door while holding the test button until the device chirps. I did this on all of our detectors and have not been able to set them off with a remote control or CFL since.

One note, if you replace the batteries or open the battery door for any reason you will need to go through this process again.

It is ridiculous that First Alert released a product with these issues.

Here is the exact wording from the user guide:

HOW TO DISABLE THE IR REMOTE CONTROL FEATURE You may need to disable the IR Remote Control feature because your alarm is located in an area that is in the path of your remote control and is setting off the alarm unintentionally. If alarm cannot be re-located to an optimal location, follow these steps:

1) Open the battery drawer. 2) While holding down the test button, close the battery drawer. You will hear a chirp. Your IR Remote Control feature is now disabled.

Note: You can re-enable your IR Remote Control feature by opening the battery drawer and closing it WITHOUT holding down the test button. Remember, when you change batteries, you must follow the steps above to keep the IR Remote Control feature disabled.

  • I've seen ones that activate the test with a bright flashlight (great for testing on a high ceiling), but didn't know about IR ones. Great tip and details. +1
    – BMitch
    Commented Feb 22, 2012 at 2:10

I'm reaching here, but there are smoke detectors that are based on photodetectors instead of radiation sources. In such a detector, I could see where a change in ambient light might produce a momentary false reading.


Typically the chirp in a smoke detector means "low battery". Its usually a single chirp every 30 seconds or so. I wonder if your smoke detector has some kind of light sensor that keeps it from chirping in the dark to avoid keeping people awake and preserving battery life until it thinks someone is there to hear it.

Try changing the battery, see if that helps.

  • agreed on the chirp normally being low battery ... but it was mentioned that this was a hard wired detector.
    – Joe
    Commented Oct 5, 2011 at 14:09
  • 1
    Doesn't matter. Hard wired detectors have battery backups for power outages. Commented Oct 5, 2011 at 14:10

I have been told by the installer of my smoke alarms, an electrician, that dimmer switches on the same circuit can cause the chirping, which I've experienced. I have dimmers and smoke alarms on same circuit, and chirping off and on. When dimmer lights are off and dimmer up all the way, no chirping as constant voltage on the circuit exists.


This is a long shot.

Some (maybe all) hardwired detectors can be wired together so that any one detector in the house will set off all units. It's possible that the wiring for the light fixture runs close enough to the detector and/or the linking wire to be creating a small current that is confusing the detector into thinking a linked unit has alarmed. Once the inrush (higher current as the fixture comes on) passes after a few milliseconds, the false signal goes away, so you just hear a chirp.

Removing the bulbs and testing to see if the chirping behavior stops should confirm or disprove this theory (if there is no load on the light fixture, there will be no current flowing to set off the detector).

A different detector might not have this behavior, and as Chris notes, it's a good idea to replace one that age anyway.


If it is photoelectric, it may think reflections of light from your fixture is light reflections from smoke, or faulty. Try covering the alarm with something light cannot get through and turn on the lights. If it does not beep, the reflections are your problem. Remember to take off the cover so that if there is a fire it can still warn you.


Take an air compressor and blow it out then put in a new battery it works every time .They get loaded up with dust.

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