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

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

 RJT> And that implies software,  and that the chip has some code in 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 the
 GM> thing wasn't working.

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

That should'be be that hard to do.

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

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

 GM> The requirements are:

 GM> 1. Battery powered

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

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

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

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

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

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

Why I specify an alkaline,  and CMOS.

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

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

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

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