SWITCHING OSCILLATOR CONTROLLED BY A MOVING METAL PIECE

A proximity switch using an oscillator with a divided oscillation winding into which a metal control piece is moved to start and stop oscillations for changing the level of current in the circuit. The center tap of the two parts of the winding is connected to a DC source, a first parallel circuit comprising an oscillation transistor, and a bias circuit; and the opposite ends of the coils are connected across the oscillation transistor and DC source so that the degree of inductive coupling between the coils as controlled by the penetration distance of the metal piece between the coils produces abrupt start and cessation of oscillations in the circuit and corresponding changes in circuit current. Temperature induced hysteresis fluctuations are compensated by use of a temperature responsive bias resistor, an oscillation winding core with a temperature-responsive permeability characteristic, or bimetal flux-shunting strips adjacent to the oscillation windings.