Oscillator circuit and proximity switch using

A proximity switch comprising:

• a switching circuit having an output which switches when an input to said switching circuit crosses a threshold value;
• a first series circuit with a resistor R1 and a capacitor C1, said first series circuit being connected between said output of said switching circuit and a ground terminal;
• a second series circuit with a detector coil L and a capacitor C2, said second series circuit being connected in parallel to said capacitor C1;
• and
• wherein a capacitance of said capacitors C1 and C2 and a resistance of said resistor R1 are selected so that a ratio of change in oscillating frequency of said oscillator circuit is constant regardless of the type of metal of said object to be detected.

A method of forming a proximity detector for detecting both magnetic and non-magnetic materials. The method comprising the steps of:

• routing a first side of a detector coil to an input of a threshold responsive switching circuit and an output of said switching circuit to a second side of said detector coil through a series resistor;
• placing a first capacitor between said first side of said detector coil and ground and a second capacitor between said second side of said detector coil and ground, thereby forming an oscillator;
• and
• selecting values of said resistor, detector coil and said capacitors such that a frequency variable with a distance between an object to be detected and said detector, varies by approximately the same amount for magnetic and non-magnetic objects to be detected.