An Electronic Eddy Current Sensing Circuit
U.S. Patent 3,609,580 Figure 5, sheet 2 of 2
The schematic at left
presents Figure 5 of U.S.
Patent 3,609,580.  The
circuit is a three transistor
oscillator circuit that is
designed to detect eddy
currents caused by a cold
rolled steel pole piece that
is inserted in the ID of the
oscillator coil shown as
item 14.  
The coil inductor located in the top left side of this picture and shown as a 176 uH inductor in the
schematic can be designed to generate a voltage at the output of the circuit indicating a linear travel
position movement or point.  The coil in this picture will generate a linear travel position indication of .5 to
4.5 Volts which represents a linear position travel  from 0 to .75 Inches when one inserts a round cold
rolled steel core into the coil ID.
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