-=> Jasen Betts said to Greg Mayman
 -=> about "" on 09-02-04  15:34.....

 JB> here's a simple one.

 JB> .--- themostat --- electric nosise-maker -----.
 JB> |                                             |
 JB> `---------- battery --------------------------'

 JB> actually I can't improve on that.... any sort of solid-state sensor in
 JB> coing to be a continuous draw on the battery andif you powerit from the
 JB> mains some fool may knock the plug out, ot pull the plug to plug
 JB> something "more important" in.

I agree you can't run this kind of thing from the mains power.
One of the things you're guarding against is a blown house fuse
that cuts off the freezer and everything else on that circuit.

 JB> temperature sensors are easy , thermistors are gettign harder to find
 JB> but but the new semiconductor temperture sensors are more stable, and
 JB> even give linear response

Indeed they do, but they pull enough current to drain a small
battery in a matter of a couple of weeks.

 JB> it's not like there'd bemuch advantage to checking the temperaturein
 JB> there more often than once every 5 minutes,

Yes, that's what I felt too.

 JB> heck if youv'e got the hardware to do it go with the "lightest"
 JB> microcontroller you can find and power the sensor (and associated
 JB> divider chains, adjustment pot, comparator) off one of the outputs or
 JB> mosfet driven by one of the outputs

The 16F84 microcontroller pulls only about 6 uA in the SLEEP
condition. And it can be set to wake itself up when an internal
timer overflows.

 JB> Those piezo nosimakers used in smoke alarms etc don't use much power.

That's true.

 JB> if it's a one off: I don't know what you'd pay for a new freezer
 JB> thermostat, but a used one can probably be had cheap if you can trust
 JB> it. noise makers can be had for free from expired smoke alarms.

My experience with thermostats is that the contacts go high
resistance if they aren't cycled regularly.

And contacts designed for mains switching usually fail quicker if
they don't have heavy current and voltage to blast the dirt off
them.

 JB> not looking for a place on "new inventors" are you?

LOL! It's a thought.... but no, not really 

   ,-./\
  /     \ From Greg Mayman, in beautiful Adelaide, South Australia
  \_,-*_/    "Queen City of The South"    34:55 S  138:36 E
       v

... * You can't hide a piece of broccoli in a glass of milk.
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