The original application of this 1967 Ferranti circuit can be changed so that it functions as a pass sensor shield for gears and other moving objects.
This alarm with sensors in series and parallel was found in the 1995 Encyclopedia of Electronic...
This integrated amplifier was used in televisions from the 70s, with good sensitivity...
This source, found in a publication from the 1980s, uses an integrated circuit that is uncommon today....