Metal proximity dectors can be very useful in robotics (contactless switch, assorting of metal and non-metal parts, etc.). The figure below shows a simple circuit, built with a TDA0161 chip and a few components:
The data sheet explains the operation. The prototype I built detects even small metal parts like a M4 nut within a max. range of 5 mm. Inductor code calculator goes here.
Can this be beefed up to find metal farther away (underground)?
One of these, a GPS, a little obstacle avoidance on a beach buggy and I could make the LMR Dig Here robot…
Nice easy build. thanks for sharing this.
It’s a proximity sensor, no
It’s a proximity sensor, no typical metal detetctor. The max. range is about 10 mm. But this has also advantages 
mmm i see some potential for
mmm i see some potential for this in the sumo arena 
