This simple circuit works by heterodyning two oscillators together. The fixed oscillator is used to tune close to the search oscillator. A low tone is heard in the earphones. When the search oscillator is brought close to a metal object, it's frequency changes. The resulting tone change is heard in the headphones.

While this metal detector is not as sensitive as the high $$$ models at the outdoor shops, it can find a quarter about 1-2" below the surface of dry sand. It's great fun at the beach or local park.

In the photos above (not a very good photo mind you, but you should be able to get the idea) you can see how we made our detector. PVC pipe was used to make the handle. A small piece of copper tubing was used for the search coil and the electronics is housed in a small Radio-Shack box. The Box has the tune control, volume control and on/off switch in it.

Documentation available is,

• Schematic of the Metal Detector in PDF format.

  This design uses a mechanical 365 pF capacitor. The variable capacitor are getting hard to find, search the web for "365 pf air variable capacitor" or "dual gang variable capacitor".

• Schematic of revised design that orginally appeared in EDN (December 1997) in PDF format: Metal_2.pdf

  The revision replaces the obsolete LM389 with a LM386. This design uses a common varactor diode for tuning.