GB1376358A - Induction-keyed control circuit with keying network having variable resonant frequency - Google Patents

Abstract

1376358 Inductive control systems WAGNER ELECTRIC CORP 12 May 1972 [20 May 1971] 22352/72 Addition to 1350706 Heading H4U [Also in Division E2] In an electronic lock arrangement in which the oscillation of a resonant circuit is modified by the proximity of a key circuit to produce a modified output controlling a lock, the possibility of picking the lock is precluded by making the lock operating circuit active only when the oscillator is rendered rapidly intermittently oscillatory by a cyclic spontaneous variation of the natural resonant frequency of the key. The key 10 comprises a capacitor 14 shunted by a resistor 12 in series with a diode 16 across the pick-up inductor 20. If desired, a tuning capacitor 18 may be included. The oscillator tank circuit comprises a capacitor 22, which may be adjustable and have inductors 24, 26 in series, and it is maintained by transistors 36. When the coil 20 is placed near coils 24, 26, it draws enough energy from the network to modulate its oscillation. However, as capacitor 14 fills up, the effective capacity of diode 16 and capacitor 14 is reduced, thus checking the resonance of the key circuit, until the charge on the capacitor 14 is discharged by resistor 12, whereupon the sequence starts again. Thus the resonance of oscillator I is cyclically modulated, and this frequency is detected by diodes 52, 54 in detector II. The low-frequency output is amplified at III and rectified at IV to feed a load, such as a relay at 88.