Capacitance measuring circuit for a capacitive sensor

A circuit for measuring a capacitance of a capacitive sensor, comprising:

• charging circuit means for charging the capacitive sensor with a voltage, wherein the charging circuit means comprises a voltage source providing said voltage, and an electrical path connecting the voltage source to the capacitive sensor;
• discharging circuit means for discharging the capacitive sensor, wherein the discharging circuit means comprises a ground and an electrical path connecting the capacitive sensor to the ground, the electrical paths of the charging and discharging circuit means having a common portion connecting with the capacitive sensor, wherein the charging and discharging circuit means comprise a common resistor having an end connected to the capacitive sensor, and an opposite end;
• trigger means for triggering the charging circuit means and the discharging circuit means in alternation into play as a potential of the capacitive sensor drops and rises to predetermined lower and upper levels respectively, wherein the trigger means comprises a switch selectively opening and closing the electrical path of the discharging circuit means with the ground and an output connected to the opposite end of the resistor, the output of the trigger means being alternately set at said voltage and to ground as a function of the potential of the capacitive sensor so that the capacitive sensor is charged or discharged respectively through the resistor;
• wherein the trigger means further comprises a pair of comparators connected to said switch, wherein said comparators have first inputs connected to the capacitive sensor and second inputs to receive respective reference potentials, and outputs producing comparison signals that control the selective operation of said switch, the reference potentials representing respectively the lower and upper levels;
• output means for providing a signal having a low level during operation of one of the charging and discharging circuit means and a high level during operation of the other one of the charging and discharging means;
• and
• whereby the signal has a period defined by a same level repetition which is indicative of the capacitance of the capacitive sensor.