GB1088919A - Proximity detecting apparatus - Google Patents

Abstract

1,088,919. Electro-physical measurement. TATEISI ELECTRONICS CO. Dec.13, 1965, No. 52881/65. Heading G1N. [Also in Division H3] A proximity detector of the capacitative probe type for detecting, e.g. the level of powder in a tank, comprises a transistor connected in a form of Colpitts oscillator, the effectively inductive circuit of which is shunted by the variable capacitance of the probe so that the oscillator ceases to oscillate when this capacitance increases beyond a set value. The effectively inductive collector/ base circuit of the oscillator 12 comprises an inductance 24 shunted by the capacitance (represented at 29) of the probe 28, this parallel tuned circuit being in series with a capacitor 26. The two capacitative branches of the oscillator circuit are respectively, the capacitor 13, and the capacitor 20 in series with a parallel tuned circuit 14, 16. It is shown that by suitably proportioning the several tuned circuits so that the normal frequency of oscillation is slightly lower than the resonant frequency of inductance 24 and probe capacitance 29, oscillation will cease sharply as the increase in probe capacitance causes these two frequencies to approach coincidence. In Fig. 1 (not shown) the output of the oscillator is amplified (42), and rectified (46) to key a transistor (48) on or off. A low-voltage discharge tube (38) is connected between probe 28 earth to protect the oscillator from static charges induced on the probe.