>4) In order to maximize battery life, I intend to fire this
>metal detector every 30 seconds or so and take a read. I am
>also concerned about how much each firing will drain.

I suspect you will need to do it more often than that. Once the culprits
realise what the boxes are for, then future hits may well try and open the
door, slip inside, shut door, sort out what they want inside the box, then
open door and exit, all in the hope of minimising the chance of being
detected, and having the logger figure it was just a bump in transit.

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