Re: [HH] CO detectors self-destruct

[Greg London]

On Tue, April 18, 2017 11:27 am, Tom Metro wrote:
. The manufacturer wouldn't even
> splurge on a micro with an analog input,

Two gpio pins, a resistor, a capacitor,
and a digital counter can get you a cheap
approximation of an ADC in a digital only micro.

Put cap and resistors in series between ground and gpio.
Use gpio to discharge cap then pull it high.
Use another gpio as an input tied to the point between r and c.
Count how long it takes for gpio to go to 1.
A bit of calculation will tell you R of thing you measured.

I imagine a smoke co detector sample rate is extremely slow
And the accuracy is really just good/notgood
So it could work.


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Re: [HH] CO detectors self-destruct

[Bill Bogstad]

On Sun, Apr 16, 2017 at 9:33 PM, Tom Metro  wrote:
> Greg Rundlett wrote:
>> I bought a house 6 years ago, and I was perplexed this week as ALL the
>> First Alert combo smoke + CO detectors started chirping and went bad.
>> The First Alert smoke detectors have and "End of Life" feature...
>
> As Federico pointed out, this is due to the limited lifespan of the sensor.

[Pure speculation on my part]

I can imagine sensors which might last a very long time when not being used
but have a finite life when activated.   Perhaps using chemicals that are
aerosolized when warmed (by electricity).  Without knowing the actual
physics/chemistry
involve in the sensor component itself, this all seems like
speculation to me.  As others
have said, this is a relatively inexpensive safety item for a very
expensive asset,

Bill Bogstad
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Re: [HH] CO detectors self-destruct

[Greg Rundlett (freephile)]

On Mon, Apr 17, 2017 at 1:33 PM, Tom Metro 
wrote:

> Federico Lucifredi wrote:
> > This excess life span I detected in use leads me to think the sensor
> > chemistry decays even when warehoused and not in powered use.
>
> Sure, that makes perfect sense given what we know about the sensors. The
> author of the teardown you referenced speculated that the limited
> lifespan was due to evaporation of the liquid and accumulation of
> contaminants in the activated carbon filter.
>
>
> > I think the lifespan runs from the date of manufacturing, and exceed
> > the warranty by an unknown amount...
>
> Yes, I assumed as much. As with any engineering parameter, the system
> should be designed with sufficient margin in excess of what is needed.
> And with a life safety device, even greater margins.
>
> But the issue at hand - with respect to having all your alarms die at
> once - is not the actual life of the sensor, but the duration of the EOL
> timer.
>

>From documentation that I found, the EOL timer is activated upon first
"power" - so installation by electrician if powered with battery backup, or
activation by consumer.  In my house, the detectors were all installed
during construction of the house.  They all died 6 years to the day from
the time they were installed.
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