Bloody smoke alarms.

[Sylvia Else]

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.

 

[Trevor Wilson]

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.

 

[Phil Allison]

"Sylvia Else is So Full of Shit "

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.

** Sure, but they also detect water droplets and dust in the air as well as
smoke.

They may need occasional vacuum cleaning too.

How hard can it be to make one of these that works reliably?

** 99% already do - you bullshitting nut case.



.... Phil

 

[Sylvia Else]

"Sylvia Else is So Full of Shit"


** Sure, but they also detect water droplets and dust in the air as well as
smoke.

They may need occasional vacuum cleaning too.




** 99% already do - you bullshitting nut case.

99% never give a false alarm? I must be extraordinarily unlucky then.

Sylvia.

 

[Sylvia Else]

The electronics seems rather basic.

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.

 

[Charlie+]

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?

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..

 

[MoonShadow]

99% never give a false alarm? I must be extraordinarily unlucky then.

Sylvia.

Perhaps I am extraordinarily lucky. My two battery powered cheapos
(ionisation type) have never false alarmed in the ten years or so years
I've had them, and on the (several) occasions I've tested them with a
visitor's cigarette they've worked.

 

[Sylvia Else]

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.

 

[Sylvia Else]

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.

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.

 

[Franc Zabkar]

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).

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

 

[Sylvia Else]

Condensation as the temperature drops maybe?

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.

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.

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.

 

[Phil Allison]

"Stupider than Anyone Else"

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.

** 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

 

[Phil Allison]

"kreed"

in QLD it does, in the rental tenancies act.

** 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

 

[Sylvia Else]

"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.

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.

 

[Phil Allison]

"Stupider than Anyone Else"

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.

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



.... Phil

 

[keithr]

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.

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.

 

[Phil Allison]

"keithr"

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.

** You are 100% correct.

At my previous address, the installed smoke alarm was of the type that are
somewhat dangerous but have no replaceable battery. One day it went into
slow beep mode, 20 minute intervals.

A sparky came and replaced it and let me keep the old one.

Turned out a 470nF class X2 cap in the AC feed had gone open circuit.

So I fixed it and still have it in a cupboard, too cute to chuck out.


..... Phil

 

[fred]

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.

Can ten kilos of weapons grade uranium even exist in the same post pack
for more than a microsecond

 

[Jasen Betts]

Can ten kilos of weapons grade uranium even exist in the same post pack
for more than a microsecond

critical mass is a bit higher than that
http://en.wikipedia.org/wiki/Critical_mass