Electronic proximity sensor

Claim 1

An electronic proximity sensor for contactless detection of an approaching object, comprising: a free-running oscillator including an amplifier element provided with internal positive feedback; and a negative-feedback path connected between an input and an output of said amplifier element, said path including an external impedance reducible in magnitude under the influence of an object to be detected whereby the amplitude of an output signal generated by said oscillator is diminished by such approach below a normal minimum level, wherein said external impedance is an inverted variable capacitance defined by a monitoring electrode and a reference electrode, wherein said path further includes a sensing transistor with an input electrode and a control electrode bridged by a coupling capacitor forming with said variable capacitance a voltage divider for said output signal, wherein said sensing transistor is an IGFET with a source electrode and a gate electrode constituting said input and control electrodes, wherein with said amplifier element energized by a d-c current supply with a positive terminal and a grounded terminal, said IGFET is of n-channel depletion type having a drain electrode connected to said positive terminal, and wherein said amplifier element is a junction transistor coupled with a tank circuit via a current mirror providing said internal positive feedback, said tank circuit lying between ground and a base of said junction transistor having an emitter-collector network in series with said d.c. current supply.