BFO Metal detector

Just a simple BFO (beat frequency oscilator) metal detector based on this one. It is comprised by two LC oscillators, a fixed or reference one and a search one. The coil of the search oscillator is located in close proximity to the ground. When a metal object is close, it changes the inductance of the coil, changing the frequency the search oscillator works at.

In this case, both oscillators work around 200 kHz. Both signals are added together, and that results in an AM signal, which can be demodulated. The audio tone resultant happens to be the difference of the two frequencies. This way, changes in frequency of the search oscillator can be noticed as the sound frequency changes.

Testing and modification of the original design. Notice that AM signal and its demodulation can be seen on the scope:

The frequency adjustment was modified and an audio amplifier was added. Here is the schematic for my version. Capacitor values are marked as they appear on the physical components:

Here is the search coil being wound on the 3d printed frame:

A tin foil shield (connected to ground) is added around the coil. This shield should not completely cover the coil, but leave a small gap on one side. Its function is to prevent ground effect (coil-ground capacitance).

The circuit was build up on a breadboard and enclosed on a 3d printed case. Pots coming out are for coarse and fine frequency adjustment and volume.

Results: While it does work, the detection distance is rather small (below 10 cm on air), and it's rather sensitive to vibrations, as the coil wire moves around, changing the coil's inductance and parasistic capacitance. This could be solved by varnishing the coil so it does not move during operation

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