Hello Roy.

04 Sep 04 12:06, you wrote to me:

 JB>> with a cmos/thermistor setup the biggest quiescent drain is
 JB>> possibly going to be the thermistor and the trimpot.

 RT> Probably.

 JB>> what's the capacity of an 9v alkaline anyway? 200mAh ?

 RT> According to my little chart here it's 500mAH,  but I have my doubts
 RT> about that.  They give "typical drain" as being 15mA...

so, if you want it run for three months non-stop (amd leaving no margin to
sound the alarm) that's approx 2000 hours, so from 500mAh it's 250uA, from
9V that's 36 megaohms resistance.  which is near the edge of CMOS
performance, but with a resistance like that it's not something you can
build on veroboard.

if the sensor can be gated somehow, and run part time, that could do it....
if the timer that switches the sensor can be built to use less than 250uA

Possibly an old smoke detector could be modified, and the "sign of
life" pulse (which flashes the LED once per minute) could be used to
trigger the temperature sensor. you get a low battery alarm this way too.

Jasen

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