I've been thinking it might be interesting to try to make a metal detector using an Arduino, and it seems like a VLF sensor would be the best way to go; outputting signal at ~10KHz should be feasible. My question is, what would be the best way to go about making the actual sensor?
I know that the general design is two concentric loops of wire, with signal being 'written' to the outer one and 'read' from the inner one. But how big do the loops need to be? How circular? Should they be single lengths of wire, or coiled like a homemade inductor?