Capacitive HF proximity switch - has HF oscillator and capacitive sensor whose capacitance to earth changes when object approaches

The object to be detected is non-metallic, and the sensor capacitance change is evaluated to determine the distance of the object. An amplitude stabilised oscillator is connected to an h.f. transformer with two symmetrical output windings; one terminal is connected to the sensor or to a reference sensor, and the other to a capacitance current or voltage converter, whose outputs are connected to a difference forming stage. The two output windings have tappings forming the other terminals, and the other winding ends are earthed through neutralising capacitors; capacitances to earth of the reference sensor and of the test sensor are approx. equal.