GB1411683A - Capacitance responsive control circuits - Google Patents

Abstract

1411683 Proximity switching systems WAGNER ELECTRIC CORP 14 Aug 1973 [14 Aug 1972] 39533/73 Heading G1N [Also in Division H3] A proximity switching system comprises a pulse energized oscillatory circuit whose Q factor is reduced by the loading effect of a proximate conductive body, and means to detect the resultant reduction in amplitude (or duration) of the "ringing" oscillations. As shown Fig.2 a pulse generator 20 (comprising Q 1 ,Q 2 ,) energizes in parallel two identical ringing and switching circuits. The upper circuit is described:- RINGING CIRCUIT. To ensure a sharp cut-off of the energizing pulse, the inductor L2 is fed through a bias circuit comprising a capacitor C5 and CHARGE-STORAGE DIODE D2 which blocks abruptly when the capacitor discharges in the reverse direction through inductance L2 after each positive pulse (C5 is not necessary if the oscillator output is negative i.e. below ground potential, between the positive pulses). RINGING ENHANCEMENT AND DECTECTION Q3 is connected in a Colpitts circuit with L2 but does not oscillate in the absence of positive bias. When the L2 circuit rings the positive peaks above a certain level drive Q 3 into oscillation to apply a signal to the output transistor Q 5 . A body proximate the antenna Al loads the ringing circuit so that the amplitude is not sufficient to drive Q 3 . Alternatively, a peak detector circuit may be used wherein the peaks of the ringing signal turn on a transistor to charge a capacitor Fig.3 (not shown).