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220V live Wire-in-Wall scanner:
This little circuit will help you to scan line of a 220V live wire in
wall. FET gate is connected to a capacitive metal sensor (which is usually a
simple metal plate). When you hold sensor close to wall, LED will
indicate current flow around where it detects wire. Works on 9VDC. (added
4/05)
Jetco Treasure Hawk: Department store BFO from the 70's. Schematic for
commercially-produced metal detectors (added 2/08)
Metal detector: A reasonably effective bfo (bifilar oscillator). Its performance
is not comparable to more advanced commercial products, of course, but it still works.
Anyway, during WWII metal detectors based on the same principle were utilized my
combat engineers of many armies to clear mines.
The main idea is really quite simple: build two identical oscillators and adjust them
to the same frequency. One of the oscillators uses the search coil while the second
one incorporates a variable inductor. When both are operating at the same frequency,
the output is zero. If the search coil moves near any metal, however, frequency of the
first oscillator shifts and an audible tone is heard in the headphones. (added
10/05)
