Hello Roy.

01 Sep 04 12:09, you wrote to Greg Mayman:

 RT> Greg Mayman wrote in a message to Roy J. Tellason:

 RT>  -=> Roy J. Tellason said to Greg Mayman
 RT>  -=> about "UNKNOWN IC" on 08-26-04  12:06.....

 RJT>> And that implies software,  and that the chip has some code in
 RJT>> it...

 GM>> Almost certainly these days.

 GM>> CHANGING THE SUBJECT....

 GM>> Does anyone have a circuit of a deep freeze failure alarm? I
 GM>> recently had to empty about $60 worth of stuff into the garbage
 GM>> when we found the power plug on the freezer had been bumped and
 GM>> the thing wasn't working.

 GM>> An alarm that went off before the stuff thawed out would have
 GM>> saved it all.

 RT> That should'be be that hard to do.

 GM>> I thought I remembered seeing such a gadget as a project in one
 GM>> of our electronics magazines some years ago, but I can't find it
 GM>> in any of the copies I've saved.

 GM>> I have some ideas of my own, but I thought I'd toss the matter
 GM>> into the ring to see whether anyone has already got something.

 GM>> The requirements are:

 GM>> 1. Battery powered

 RT> I'd use a 9v alkaline -- they have a pretty good shelf life (several
 RT> years) at this point.

 GM>> 2. Alarm to go off if temperature rises above a preset limit.

 RT> Use a thermistor and a resistor to bias one input of a comparator,
 RT> maybe use a CMOS op amp,  for super low drain.  Or a logic gate,
 RT> perhaps,  though CMOS logic tends to eat more current when biased into
 RT> the linear mode as it would be for the oscillator.  Two or three gates
 RT> for an oscillator,  one for a comparator...?

 GM>> 3. Preset limit to be adjustable between 0 deg C (32 F) and -20 C
 GM> GM>> (-4 F) or thereabouts.

 RT> Another resistor or two and a trimpot,  perhaps a 10-turn unit.

 GM>> 4. Long battery life in the non-alarmed condition.

 RT> Why I specify an alkaline,  and CMOS.

 GM>> 5. Alarm to be able to operate for several hours without draining
 GM> GM>> the battery, in case the house is unattended when the alarm
 GM>> goes off. Alarm must be very low drain or operate in short beeps.

 RT> Ok,  so that gets a *little* more complicated,  then,  you need a
 RT> gated oscillator.

 RT> Maybe a couple of the CMOS versions of the 555,  what's that a 7555?
 RT> Do they make a dual versoin of that part?

I've not seen a 7556 at any retial outlet. but all you gain with a 556 over
a 2 555s is a slightly smaller footprint and  hahving to drill 2 fewer
holes in the PCB

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

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

Jasen

--- GoldED+/LNX 1.1.4.7