“With an Arduino, 10m of wire and a 100 Ohm resistor you can build a metal detector in 10 minutes! It is based on sound physics and works for a large range of coil sizes and shapes. The sensitivity is not enough for treasure-hunting, but it can be made into a small hand-held device that is very useful indoors to check for the presence of metals. It will help you find nails inside wood or heating pipes in the wall, and to check the composition of tools and furniture. The method can also be used to integrate as a sensor, integrated with more elaborate projects.
You will need a coil of at least 20 windings but it works better with 50 or more windings. The detector is sensitive to items that are larger than about one tenth of the coil diameter, so smaller coils are better. Finally, make sure that the resistance is less than 10 Ohm. Here I describe the coil I made, but anything will go, even a 10m roll of breadboard wire.
I used AWG26 enameled wire, which has a diameter of 0.4mm and a resistance of 0.13 Ohm per meter. The wire was wound 60 turns around a candy can with a diameter of 6.3cm. Leave one turn extra length for connection leads. The wire comes at ~12m and the resistance 1.5 Ohm. I put some electric isolation tape around it and then squeezed it in an oval shape and completed covering it in isolation tape. It protects the coil and keeps it stable. Remove the isolation at the last few centimeters with sandpaper.”