PLL Metal Detector

by Kamil Kraus
This article describes a PLL-based detector which uses the LM565 phase-locked loop. Technically, it is not a BFO, nor an IB; however, a metal target does change the inductance of the search coil which effects a change in frequency, therefore it is a frequency shift detector much like the BFO.

The article was published in the September, 1992 issue of Electronics World [+ Wireless World], a popular British electronics magazine that is published by Reed Business Information.
If the VCO output of a phase-locked loop is phase shifted and taken back to the input, the loop locks to itself and runs at whichever frequency causes 90 degree phase shift in the network. This principle is used to make a metal detector here, but has many applications in measurement systems.

When the search coil is within 75cm of a metal object, the VCO increases its frequency for a nonferrous metal and decreases it for ferrous objects. Loop output on pin 7 of the 565 is compared with the pin 6 reference voltage, the long-tailed pair Tr1,2 amplifying the difference.

My search coil is 50 turns on 50mm diameter to give an inductance of 0.5mH. The VCO frequency is around 1kHz.

Kamil Kraus
Rokycany, Czechoslovakia
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