| 摘要 |
Proximity of a person (BOD1) causes a change in the capacitance value of a sensor capacitor (CX) having a pair of capacitive plates (10a, 10b). Said change is detected by: - charging said sensor capacitor (CX) by coupling it to a voltage supply (40), wherein said voltage supply (40) is disconnected from a tank capacitor (C2) during said charging, - transferring charge from said sensor capacitor (CX) to a tank capacitor (C2), wherein said voltage supply (40) is disconnected from said tank capacitor (C2) during said charge transfer, - repeating said charging and charge transfer several times, - monitoring the voltage (VX) of said tank capacitor (C2), and -- determining at least one value which depends on the rate of change of the voltage (VX) of said tank capacitor (C2). The capacitance of the capacitive sensor is typically low, typically in the order of 100 pF to 1 nF. The capacitance of the tank capacitor (C2) may be several orders of magnitude higher than the capacitance of the sensor capacitor (CX). The large tank capacitor (C2) acts as a part of a low-pass filter which effectively filters out signal noise.
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