As a side note, since OP was asking for a "no-powered" solution, I 
wanted to talk about my case - which thing I was going to do soon :) and 
telepathically I got anticipated!

I placed 3 circles of wire, 2mt diameter, of 4mm wire below the surface 
of the entrance of my garage; I planned (not using them yet) to detect 
presence of a car.
My idea was to use the old "dip" method i.e. putting into the wire a 
100KHz - 1MHz frequency and look at its resulting height (once tuned) 
and, when its amplitude lowers, then a metallic-mass "should" be there...


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Ciao, Dario
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Cyberdyne
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