> I believe Adam's idea below was probably the best to solve this problem.
....
> Any comments ?

The proposed 30 seconds between tests may be long enough for a savvy person
to open the door and place a suitable piece of metal against the oscillator
case. It should be easy enough to make a continuously operating oscillator
and detection circuit low power. You are not really limited by space
constraints so batteries can be as suits. If you had 2 amp hours available
(AA Alkaline) you can provide 10 mA for 200 hours / 8 days or 1 mA for 80
days. Something in that range should be doable. Using C or D cells increases
the time.

You can still use metal sensing without using an oscillator in the strict
sense. Measuring the inductance of a sense coil does the same job.
Inductance sensing can use many means (some of which uses oscillators). You
can also buy commercial inductance sensors fwiw.



        Russell McMahon

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