This circuit represents a brand new genre of metal locator.
Although it borrows from other kinds of locators/detectors, its basic principle
of operation is new.
No, it's not part of the circuit. This view inside the box shows the WHOLE circuit (with the exception of the coils, of course, and the tuning capacitor hidden under the PC board.)
And while this may seem to stretch one’s credulity, the
performance matches that of a budget Induction Balance (IB) detector.
Build it – and you will see!
During testing, it detected an old English penny at 150mm (6")
in air. However, I would only put my neck on the block for 125mm (5"), since a
number of factors influence sensitivity. It roughly matches the performance of
the now sadly misnamed Matchless Metal Locator (SILICON CHIP,
June 2002), while using just one-fifth of the components.
This locator may therefore represent the writing on the wall
for budget IB types and even puts paid to most of the advantages (the few
remaining ones) of Beat Frequency Operation (BFO) detectors.