GB1315684A - Metal detectors - Google Patents

Abstract

1315684 Electromagnetic metal detector E J FOSTER 3 Nov 1971 [10 Nov 1970] 53498/70 Heading G1N [Also in Division H1] In a portable eddy current metal detector the batteries and detector circuits are in separate rigidly connected housings sealed to prevent the ingress of moist air, salt spray or water. A search coil, Fig. 1 (not shown), is releasably mounted on one end of tube 5 which is hollow at the opposite end to hold batteries 10. A second tube 4 houses the circuits which are connected via lead 8 to the search coil. The two housings are held in spaced relation by a block of insulating material 6 to one surface 14 of which is welded tube 4 and in bore 15 of which is welded tube 5. Electrical power to the detector circuit is supplied via screws 24, 25 passing through semi-cylindrical PVC elements 22, 23 welded in the bases of cylindrical bores 16, 17 extending into block 6 from one face thereof and lead wires 28, 29 extending through apertures 19, 20 in the wall of tube 4 and communicating with bores 16, 17. Bores 16, 17 are sealed by PVC mountings through which control knobs pass to connect with the circuit. An earphone is connected via lead 38 passing through an aperture in block 6 to the detector circuit, Fig. 4 (not shown). Circuitry. Fig.5 (not shown). Secondary e.m.f.'s induced in the search coil by eddy currents induced in a metal object by pulses from a pulse transmitter are applied via parallel connected sampling gates to a differential integrator. The gates are gated by signals from a sample pulse generator triggered by the pulse transmitter so as to feed a first sample pulse to one of the gates at a predetermined time after the termination of back current pulse from the pulse transmitter and a second sample pulse to the other gate after a further time delay which is sufficiently long for the secondary e.m.f. to have died away. The output from the integrator is thus free from noise and is applied to the earphone and a meter on housing 4.