View Full Version : My possessed smoke detector. Someone explain this to me...
01-05-2004, 02:44 AM
So I grabbed a smoke detector out of a box of stuff I was moving and put it on the counter to remind me to buy a battery for later.
Later the room was filled with smoke (uh, cooking) and it went off.
I opened it up, no battery. It went off until I took it out of the room.
In #straightdope someone mentioned a capacitor being a possibility, but this had been sitting in storage for at least a year...
So I have no idea.
01-05-2004, 02:59 AM
Good question. One of my smoke detectors started intermintent beeping - usually this means dust or an insect in it and asquirt of compressed air clears it. This time it wouldn't and it began to drive me crazy. I took it down and removed the battery intending to clean it thoroughly. I forgot about until about a week later when it let out one beep. I cleaned it, put the battery back and have had no problems since. I couldn't work out how it beeped with no battery in it.
01-05-2004, 08:53 AM
If lekatt were here, he'd tell you that your smoke detector just had a Near Death Experience.
01-05-2004, 09:07 AM
Presumably the components inside are quite readily visible? (in all the smoke alarms I've seen, the entire front cover flips up to reveal the battery holder and the bare circuit board.
It certainly sounds phenomenal that a capacitor would have held the charge for so long without leakage, but maybe there is a PCB-mounted rechargeable backup battery of some kind - it shouold be easy enough to spot as they are usually quite bulky.
01-05-2004, 04:24 PM
It looks like a regular smoke detector to me. The front cover comes off to reveal a black component with the radioactive sign and a white unit about the same size that I believe makes the earsplitting noise.
I see one really little capacitor on the board, but I doubt it could've held a charge for a year while still being strong enough to set it off for more than 30 seconds.
01-05-2004, 04:31 PM
not a fact, but maybe a chuckle? You should contact the guy that claims candles attract demons. (the thread is in the Pit). If he gets back to you, let us know!
01-05-2004, 05:01 PM
Originally posted by clayton_e
It looks like a regular smoke detector to me. The front cover comes off to reveal a black component with the radioactive signThat'll be the sensor - the small amount of radioactive material inside it ionises the smoke, lowering the electrical resistance of the air.
and a white unit about the same size that I believe makes the earsplitting noise.Yup, that'll be the piezo sounder and horn assembly - in my smoke alarm, this is mounted off the PCB a little and there are a few components underneath.
I see one really little capacitor on the board, but I doubt it could've held a charge for a year while still being strong enough to set it off for more than 30 seconds. Is it possible that there are some other components on the reverse side of the PCB?
The sounder itself and the circuitry that drives it are fairly modest in their power requirements, however, it does seem unlikely that the capacitor would have held a useful charge for so long,
Other rational possibilities at this stage might include:
-Static electricity - anyone? - is there any way that a statis charge could have ended up charing the capacitor a bit?
-The smoke alarm that went off wasn't the one on the counter with no battery. - Sounds stupid on the face of it, but sounds consisting of a single piercing note are notoriously difficult to locate - have you any other smoke alarms fitted and working in the room where this took place, or reasonably near it?
01-05-2004, 09:49 PM
With a hardware technician at my place of employment, I was discussing the heavy duty shock that an unprotected CRT can give the careless person.
He told me that CRTs and capacitors can build charge from ambient static electricity when sitting on a shelf, if the conditions are right. He'd seen a co-worker shocked by a screen which hadn't been plugged in for six months - and which was discharged as a matter of routine when it was unplugged.
01-05-2004, 10:08 PM
hmm my smoke alarm started this stupid intermittent beeping a few weeks a go on a really really hot day, so i took it down removed the battery intending to get another one and it still kept on beeping. So i did what any self respecting aussie would do when faced with a hot day and the choice of thinking through a problem or having a beer.
I put the smoke detector in the fridge to see if heat was causing it and took a beer out just to even out the fridge.
Putting the smoke detector in the fridge stopped it from beeping. the next day i took it out, 2 hours later it started beeping again, so i replaced the battery more beeps, it now lives in my fridge with no batteries in it until i either figure it out, buy a new one, or move out and re-attache it.
beer is much better.
But the idea that bug or dust may be on the sensor is worth looking into i might do that tonight cause my girlfriend gets home tommorrow and she is very Fire safety concsious.
01-06-2004, 04:55 AM
There might be components on the back side of the board.. Even with a small mirror it'd be hard for me to see whats behind there, plastic blocks my view of where a good sized capacitor could go. If I find another detector here in one of these boxes to replace it I might pull it apart and find out. I'll post what I find if I do find one tomorrow.
Anyway, the static idea seems to cover why its doing this.
01-06-2004, 05:47 AM
Don't disassemble the ionising chamber (you'll probably find that a special tool would be required anyway, but still don't crack that bit open).
01-06-2004, 11:29 AM
Yeah.. I'm not going to mess with anything with a nuclear warning sign. I didn't find the other one, so until I do I'll just assume there's a capacitor on the other side.
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