We were trying to see how inexpensively we can build a reliable/decent PI metal detector. 14 years ago we found a copyright expired design, my fatherinlaw (Elec. Engineer) added a way to switch to headphones and add a meter for visual detection. We used an old meter from an analog voltmeter, old metal box we had, and some knobs off a receiver, telephone solid core wire for coil- not counting the 12v lead acid battery- the total cost was $35. Components for circuit from digikey, and circuit board etched from Canada. It is limited in depth by the coil diameter, but we detect pennies down about 10". The coil design does really well at pinpointing objects. As you can see it is a bit janky(a term my kids use), so we are going to do some structural integrity modifications soon. This is addicting and fun with the kids!