No wonder we keep hearing of cases where people die in fires where the batteries have been removed from smoke alarms. The damned things go off so often even in the absence of smoke.
How hard can it be to make one of these that works reliably?
Sylvia.
**Agreed. They're hopeless. But they are cheap. Mine are mains wired units. Originally, they were ionisation types, but prone to false alarms. I replaced them with photo-electric types. Two have failed in six years. Failure means that I couldn't shut the damned things off. Since they were mains powered and failure occured in the middle of the night, I have to get the ladder out and disconnect the mains wiring. A real PITA. The units were moderately expensive ones. They cost me $80.00 trade price. Not impressed. The $15.00, non mains powered one in the workshop has never given me a moment's trouble.
-- Trevor Wilson www.rageaudio.com.au
I've just today replaced an optical one that I previously bought for about $50, in the (forlorn) hope that it would eliminate the false alarms I was getting from the ionisation type.
I took it apart. No evidence of the obvious culprit - dust in the sensor chamber. The electronics seems rather basic. I wonder whether it's simply suffered drift in some component values - particularly the several electrolytic capacitors on board.
I want one with two chambers with smarts that require both chambers to indicate smoke before it triggers, and a failure warning mode that operates only during daylight.
Sylvia.
Forget that - it has one of these hidden on the reverse side of the board.
Sylvia.
On Tue, 19 Jun 2012 12:49:23 +1000, Sylvia Else wrote as underneath my scribble :
Never had any trouble with the cheap ones, except bipping when the battery needs replacing, a trick is to not screw them onto anything, leave on a high shelf etc in free air, then if you burn the toast etc, just remove unit to a lower area for a few minutes to stop the shouting..
If it were just nuisance trips from real smoke, then I wouldn't mind. But I've had these things go off in the early hours of the morning, more than once, with no smoke, no one using a shower, nothing being cooked in the kitchen, and no one stirring up dust.
It doesn't exactly make for a restful night's sleep.
Sylvia.
Hmm....
OK, lots of speculation here.
The IC contains almost everything one needs to control the function of a smoke detector, but one thing it noticeably lacks, and which no smoke detector is complete without, is a mute button [*].
To get round this the manufacturer has added a circuit that for a period of time holds the anode of the sense diode at 9V. This prevents the strobe pulse from traversing the diode even if light is reaching it.
Indeed, not even the IC's periodic test pulses are sensed, which results in the alarm making a periodic beep indicating that the test has failed.
The anode is pulled high through a diode, the other side of which has a very long trace to the other end of the board where it's connected to a transistor.
I speculate (did I mention that I was doing that), that this trace is picking up interference which is then capacitively coupled to the sense circuitry, making it appear that the sense diode is conducting when it's not.
Perhaps all I need to fix this is a pair of wire snippers.
Sylvia.
[*] Despite the fact that the thing is almost always out of reach, and if one's got the ladder out to access it, one is more likely to take the battery from it, or a hammer to it.
Well I guess you are right Sylvia, you are just unlucky.
Condensation as the temperature drops maybe?
Temperature drop does seem to be the reason why the darn things choose to start chirping at 3am to warn that the battery needs to be replaced (battery voltage drops as the temp goes down).
We had an ionisation detector that went off a couple of times during the night. I vacuumed it, and that seemed to help for a while, but when it did it again I replaced it. It was way past the 10 year nominal lifetime anyway. No problem with the new one so far.
The detectors we have are mains powered, with battery backup. Those can be a double nuisance if they go off for no good reason, as you may not be able to easily (or legally) disconnect the power to shut them up. That was something I looked out for when choosing replacement detectors. The ones I got are constructed such that when you open them, the mains power is cut off - the mains connection is via a small plug that pulls out as you flap the cover open.
Of course, mains powered detectors are supposed to be installed by a duly licensed person. That is fair enough, but when I was investigating the detectors, I was amazed to find that for most of them, the manufacturers tell you that replacing the backup battery must also only be done by a licensed sparky! The ones I got were the only ones I saw where that was not the case, perhaps due to the way the mains gets unplugged as you open them.
The other thing that gets me about smoke detectors, is how the suppliers can give a very long warranty, knowing full well that you are probably not going to make a claim on that. To make a claim, they ask you to return the faulty device to them in outer woop woop. Ionisation detectors cannot be posted, because some rule-making idiot does not know the difference betweens a tiny speck of Americium and 10 kilos of weapons-grade uranium.
Andy Wood snipped-for-privacy@trap.ozemail.com.au
On Tue, 19 Jun 2012 19:42:47 GMT, snipped-for-privacy@trap.ozemail.com.au (Andy Wood) put finger to keyboard and composed:
I have a smoke detector that is currently behaving in exactly the opposite way. During the day it is OK, but at night when the air conditioner (heater) is switched on, the unit emits a low-battery chirp.
- Franc Zabkar
-- Please remove one 'i' from my address when replying by email.
When I had a battery-only detector that started to chirp in the middle of the night, I did an experiment with it. In the morning, it was not complaining. I sealed it up in a plastic bag, and put it in the fridge. After only a few minutes it started to chirp. I took it out and after it warmed up a bit it stopped.
That could mean that the circuitry itself was temperature sensitive. I did check the battery voltage, and noted it went down as the temperature fell, but I have forgotten the details now.
Andy Wood snipped-for-privacy@trap.ozemail.com.au
Perhaps, though I'd be surprised if the inside of the house reaches the dew point, and I've certainly never seen mist inside the house, nor noticed condensation on any surface outside the bathroom, other than window panes.
I don't think the law has anything to say about replacing batteries in such detectors. More likely the manufacturer just doesn't want to get sued if someone electrocutes themselves.
Sylvia.
"Stupider than Anyone Else"
** Mains powered detector/alarms are installed with permanent connection to the AC supply and all the internal circuitry is live at 240 volts, so it is not safe or permissible for un-unlicensed person to work on them.Usually, switching off the relevant lighting circuit at the fuse box renders them harmless to handle but in law you should not do this.
In some examples, replacing the ( 10 year lithium ) battery involves a soldering operation.
.... Phil
in QLD it does, in the rental tenancies act.
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I have a couple of the lithium ones at home. They have 3 X 1/2 AA lithium c= ells that are soldered in. One problem is that the case on these is not in= tended for disassembly / reassembly, but that shouldn't worry anyone here.
Note that if these cells "leak", (rare) a white powdery film will form on t= he PCB near one of the terminals. the good news is that it doesn't seem to = corrode components like normal batteries, the bad news is that heating this= with a soldering iron when desoldering can make it explode, and while not = a massive explosion, it caused minor stinging on my hand from the flying pa= rticles so these could damage your eyes, so cover up if you are going to un= solder old lithium cells.
"kreed"
** The regulations in Qld include an obligation on tenants to replace flat and nearly flat batteries in smoke alarms.However, it can be taken for granted that the particular alarm must have a user re-replaceable, readily available battery AND that there is no unusual hazard involved for the user in so doing.
AC powered alarms with back up batteries that are live at 240V AC when the case is opened are not part of the deal.
... Phil
Whether it's safe is one thing. Whether it's permissible is another. In NSW, for example, it would depend on whether replacing the battery falls within the definition of electrical wiring work in the Electricity (Consumer Safety) Act 2004. I contend that it does not, at least in the non-soldering case.
Sylvia.
"Stupider than Anyone Else"
** No-one gives a flying f*ck about your insane opinion of anything.... Phil
They are in my opinion dangerous, unless the AC part uses transformer isolation, and is insulated so it cant be touched when opened, then they are a risk to the consumer, even more so as they can't be unplugged like normal appliances.
Fortunately the ones I have, and have seen to date will "unplug" safely when removed from the base,
They have to be removed from the base (disconnecting mains in the process), in order to access the battery compartment.
isolation, and is insulated so it cant be touched when opened, then they are a risk to the consumer, even more so as they can't be unplugged like normal appliances.
removed from the base,
order to access the battery compartment.
If they have a replaceable battery then it should be possible to do it safely without having to turn the power supply off. There are numerous ways of doing this, it ain't rocket science.
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