Patent ID: 7023221

Claim:
An object proximity detector, consisting of a pair of sensor plates connected to a sensor oscillator, sensing proximity of an object by the variation of the capacitance; a sensor oscillator, the variation of the capacitance changes the frequency of said sensor oscillator, the output of said sensor oscillator is connected to the input of a counter; a counter, counting the frequency of said sensor oscillator, the output is communicated with a micro-processor; a time base oscillator, providing system clock to a microprocessor; a microprocessor, calculating and processing the frequency count from said counter to determine an object is in proximity of the detector; wherein the improvement comprising: said sensor oscillator consists of three inverters, a first inverter, a second inverter and a third inverter are in cascaded; a first capacitor is connected between the input of said first inverter and the ground; a pair of sensor plates is also connected between the input of said first inverter and the ground; a compensate capacitor is connected between the input of said first inverter and the output of said second inverter; a feedback resistor is connected between the input of said first inverter and the output of said third inverter; said feedback resistor is used to charge and discharge said first capacitor, said compensate capacitor and said sensor plates at the input of the oscillator; said first inverter is an inverter with Schmitt trigger input, said compensate capacitor is used to reduce the dependence of oscillator frequency on the process parameters.