-=> ROY J. TELLASON wrote to GREG MAYMAN <=-

 RJT> 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.

 RJT> 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

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

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

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

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

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

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

 RJT> 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.

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

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

 Use a 4093 quad 2-input NAND schmitt trigger and the thermistor
 you suggested.

 Anything from a 3.6 volt litium battery to 15 volts will work fine.
 
 One gate has the thermistor and potentiometer on an input calibrated
 to go off when the voltage hits 2/3rds of the battery supply.
 (tie the other input high)

 Calibrate with some lightly salted ice water for margin.

 One gate operates as an oscillator and another as a short
 beep asymmetrical driver for the oscillator thus minimizing 
 current drain.

 Not operating life should be shelf life of the battery as the 
 CMOS remains static with outputs being high output impedence
 until actuated.
 I'd expect several weeks operating time activated.

An added advantage would be it would have to reach 1/3rd power supply
at input to turn it back off so it'd not be constantly going on and off
and a short power outage to the freezer would turn the unit back off.

You can use a Thermistor, FET as temperature sensor, 1n914, dedicated 
temperature I.C. or whatever floats your boat biased accordingly.

With the 4093 this even leaves one gate free for anything you might care to 
use it for.
Tie the inputs together and use it as an inverter in conjunction with
positive feedback with the temperature activated gate and the alarm
will NOT shut off after dipping under freezing then refreezing.
Used this way an input resistor to the sensor gate of 1 meg and feedback 
resistor of 220K from second inverting gate might be appropriate.

Just another idea from the dust bin ;-)
 
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