Simple Metal Detector
The detector in action...
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, s
earch 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.
Steve Hageman 14,Sep, 1997.
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