Abstract: In a metal detector, such as may be used in a bakery, an adjustable phase shifter is provided to adjust the detectors for different products. The adjustable phase shifter is controlled electronically from the output signal so that the arrangement is self-adjusting.

Abstract: Metal detection apparatus includes an oscillator coil 101 driven by an oscillator 101, which induces in detector coils 103,104 equal and opposite E.M.F.s such that a nominally zero output is produced at terminals D.E. When metal is detected the E.M.F.s in coils 103,4 move out balance to produce a voltage at D.E., which is detected by circuit IC11 and fed to output circuits 111. Drifts in the nominally zero output at D.E. are corrected by combining quadraphase balancing signals on lines 109,110 with the detector coil output by means of amplifier IC10. The combined signal from IC10 is fed to phase sensitive detectors IC1,2 which respectively detect the quadraphase components of the combined signal from IC10. The outputs of IC1,2 are stored by capacitor circuits 112,113 and used to adjust variable capacity diode bridges 106,7 which adjust the amplitude of the balancing signals to maintain a nominally zero output from IC10 when metal is not being detected.

Abstract: Metal detection apparatus includes an oscillator coil 101 driven by an oscillator 101, which induces in detector coils 103,104 equal and opposite E.M.F.s such that a nominally zero output is produced at terminals D.E. When metal is detected the E.M.F.s in coils 103,4 move out balance to produce a voltage at D.E., which is detected by circuit IC11 and fed to output circuits 111. Drifts in the nominally zero output at D.E. are corrected by combining quadraphase balancing signals on lines 109,110 with the detector coil output by means of amplifier IC10. The combined signal from IC10 is fed to phase sensitive detectors IC1,2 which respectively detect the quadraphase components of the combined signal from IC10. The outputs of IC1,2 are stored by capacitor circuits 112,113 and used to adjust variable capacity diode bridges 106,7 which adjust the amplitude of the balancing signals to maintain a nominally zero output from IC10 when metal is not being detected.

Abstract: An electric sensing circuit, typically a metal detector includes a bridge including inductors. The out of balance condition of the bridge is stored on a capacitor and a balancing signal is fed to balance the bridge in dependence upon the stored value of the charge. The stored value of the charge is only updated in response to relatively slow changes in the out of balance condition of the bridge. Relatively rapid changes in the out of balance condition are used to provide an output signal for alarm or measuring purposes, and the arrangement is thereby so arranged that the balancing signal does not attempt to automatically balance out the output signal giving the alarm or measurement.