Patent ID: 7102366
Filing Date: 2006-09-05
Classification: A47K,H05F

Abstract:
1. A proximity detection circuit for detecting the presence of a moving hand, said circuit comprising: an antenna with which is associated a fixed time constant determined by a fixed internal capacitance and a fixed resistance, as well as a variable time constant that is longer than the fixed time constant by an amount determined by an external capacitive load, said variable time constant being on the order of twice said fixed time constant when the external capacitive load is a hand of a person in proximity to the antenna; an oscillator circuit adapted to provide a periodic charge to the antenna; with a periodicity greater than said fixed time constant; a first operational amplifier being operated as a unity gain follower and receiving from the antenna a periodic antenna signal having an exponential waveform that has a longer time constant and a lower amplitude when said external capacitive load is in proximity to said antenna, the waveform of the antenna signal being thus representative of changes in the external capacitive load on the antenna; a detector circuit including a peak averaging capacitor responsive to the periodic exponential waveform of the antenna signal via the first operational amplifier and adapted to output a detection signal representative of a low frequency component of the antenna signal; a low-pass filter coupled to an input of a second operational amplifier operated as a gain and offset amplifier for amplifying said low frequency signal component and rejecting a higher frequency noise component; an auto-compensate capacitor responsive to the amplified and filtered low frequency signal component output by the second operational amplifier for providing a compensated detection signal with increased sensitivity to transient signals representative of a waving hand in proximity to the antenna; and a comparator receiving the compensated detection signal and being adapted to generate an output signal in response thereto.