GB677773A - Improvements in and relating to metal detectors - Google Patents

Abstract

677,773. Locating conductors. KERR, B. G., and HISCOCK, D. July 29, 1949 [June 29, 1948], No. 17443/48. Class 37. In a metal detector, the responsive device is affected only by changes in the state of balance of a bridge when a metal object passes the detector coils and not in accordance with the permanent state of balance, and so is unaffected by slow changes due to ageing &c. of the circuit components. As shown, the bridge comprises four coils L1-L4, of which L3, L4 are the search coils. The unbalance A.C. voltage after amplification is rectified by a diode V3 followed by a filter R5, C2, which suppresses residual A.C., and is fed to the following stage by R.C. coupling R6, C3, so that there is no input to the next stage except when the bridge output is varying and no effect is produced by slow changes such as caused by drift. To ensure equal response to unbalance voltages of any phase, caused by ferrous or non-ferrous material, an initial unbalance is introduced by a resistance R3 in parallel with coil L3. The change in unbalance voltage may thus be positive or negative, and is fed to an indicator stage comprising two R.C.-coupled valves V4, V5, the latter including a relay winding A/2 in its anode circuit, which releases on receipt by V4 of a positive signal, or on the termination of a negative one, from diode V3, and closes contacts A1, A2 to lengthen the period during which the signal is effective.