[HH] CO detectors self-destruct

[Tom Metro]

Federico Lucifredi wrote:
> My assumption is that either 1) the sensor self-tests and EOLs when
> the test fails (I think this is unlikely)...

I agree this is unlikely, given the only way to actually test the sensor
is to generate CO in its presence.


> ...or 2) the sensor has a timer running internally, since the time of
> manufacturing, that runs to the actual lifespan (not the warranted
> life, the longer design life) before alerting with EOL.

If by "timer" you meant some means to test the age of the sensor, then
this might be plausible if there is a secondary characteristic of the
sensor that is measurable. For example, measuring the resistance of the
activated carbon or the reaction liquid. If that, or some other easily
measured characteristic, is reliably correlated with the age of the
sensor, then yes.

If you're thinking a digital countdown timer with an independent power
source, then extremely unlikely. Take a closer look at the bill of
materials in that CO alarm teardown. The manufacturer wouldn't even
splurge on a micro with an analog input, despite how it could be useful.
These devices are designed to be manufactured for as little as possible.
So if they need an EOL timer, they'll use software in the main micro.
(They probably hate having to pay for a micro with Flash storage to
preserve the EOL counter across battery changes. A cost they can avoid
in the permanently powered units.)

You could always answer this question by doing a destructive disassembly
of a CO sensor.

 -Tom