The two - wire inductance sensor is used to measure the pump and rotary speed. Internal circuit is composed of oscillation, detection, and the output stage. Oscillation coil produces alternating magnetic field. When there are metal objects near the coil, magnetic field produces vortex in the metal objects, thus oscillator stops due to energy losses. After rectification, the output stage changes into a higher level. If there is no metal objects in the approaches, the output is on the low level. When metal is close, the indicator light is on; when metal is not close, the indicator light is off. This sensor has the characteristics of wide voltage range, high repeated positioning accuracy, fast frequency response, long service life, vibration resistance, corrosion resistance and waterproof.