GB1267350A - Detection system for indicating the passage of bodies - Google Patents

Abstract

1,267,350. Inductive signalling. M.L. AVIATION CO. Ltd. 24 Oct., 1969 [26 July, 1968; 12 May, 1969], Nos. 35865/68 and 24134/69. Heading H4U. [Also in Division G4] A system for indicating the passage of articles past a given point comprises an inductive loop 2 surrounding the expected path of the article a pulse generator 10 for providing pulses of oscillatory current in the loop 2, a resonant oscillating circuit 12' mounted on the article and a receiver 11 arranged to be inhibited during the pulses from generator 10 but to; receive signals retransmitted from circuit 12 as the article passes through the loop 2. As described, pulses from generator 8 are fed to a Schmitt trigger 9 which turns on a 16 kHz oscillation generator 10-for 5 ms. once every 20 ms. so as to feed pulses of 16 kHz current into the loop 2. Pulses from 8 are fed to a further Schmitt trigger 14 which renders a receiver 11 operative for 8 ms. during each interval of 15 ms. between pulses from circuit 10. Protected articles (e.g. radio or television receivers in a shop) are each fitted with a small pick-up circuit 12 comprising a pair of ferrite rods mounted at right angles to one another, each carrying a pair of windings, one of which serves as a pick-up coil while the other serves as a radiator. Each -pair of coils is associated with a transistor amplifier having positive feedback such that upon reception of a pulse from loop 2, the circuit 12 oscillates for a period at least as great as that during which the receiver 11 is operative. The operative receiver 11 may operate an acoustic alarm 16 or, as in Fig. 3 (not shown), may be arranged to inhibit a photo-electric alarm situated further along the path of the article. Such a system may be used to prevent unauthorized persons (i.e. those not carrying a pick-up circuit (transponder)) 12 from entering a hazardous zone.