@sylvia: "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: "<quote>
**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 <url>"
    @sylvia: "<quote>
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."
    @nospam: "<quote>
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: "<quote>
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: "<quote>
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 carmenmorgan@example.org. 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."
    @fzabkar: "On Tue, 19 Jun 2012 19:42:47 GMT, <email_address> (Andy
Wood) put finger to keyboard and composed:
<quote>
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."
    @sylvia: "<quote>
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."
    @phil_a: ""Stupider than Anyone Else"
<quote>
** 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_a: ""kreed"
<quote>
** 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: "<quote>
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_a: ""Stupider than Anyone Else"
<quote>
**  No-one gives a flying fuck about your insane opinion of anything.
...   Phil"
    @user: "<quote>
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_a: ""keithr"
<quote>
** 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: "<quote>
Can ten kilos of weapons grade uranium even exist in the same post pack
for more than a microsecond :-)"
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