Bloody smoke alarms.

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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.

Re: Bloody smoke alarms.
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**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

Re: Bloody smoke alarms.
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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.

Re: Bloody smoke alarms.
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Forget that - it has one of these hidden on the reverse side of the board.

http://www.alldatasheet.com/datasheet-pdf/pdf/170363/FREESCALE/MCZ145010EG.html

Sylvia.

Re: Bloody smoke alarms.
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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.

Re: Bloody smoke alarms.
On Tue, 19 Jun 2012 12:49:23 +1000, Sylvia Else

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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..

Re: Bloody smoke alarms.
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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.



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Well I guess you are right Sylvia, you are just unlucky.

Re: Bloody smoke alarms.



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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

Re: Bloody smoke alarms.
On Tue, 19 Jun 2012 19:42:47 GMT, snipped-for-privacy@trap.ozemail.com.au (Andy
Wood) put finger to keyboard and composed:

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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.

Re: Bloody smoke alarms.


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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

Re: Bloody smoke alarms.
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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.

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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.

Re: Bloody smoke alarms.

"Stupider than Anyone Else"

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** 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



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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.

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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.

Re: Bloody smoke alarms.
"Stupider than Anyone Else"

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**  No-one gives a flying fuck about your insane opinion of anything.



...   Phil










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in QLD it does, in the rental tenancies act.

Re: Bloody smoke alarms.

"kreed"

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** 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



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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.

Re: Bloody smoke alarms.
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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.
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removed from the base,
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order to access the battery compartment.
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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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