| 摘要 |
A triangular wave generating circuit ( 21 ) generates a sweep voltage of a triangular wave, a VCO ( 22 ) generates a sweep of a frequency according to the sweep voltage, and a driving coil ( 4 ) is driven by the sweep voltage. From the current flowing through the driving coil ( 4 ), an interference component is extracted and rectified by a rectifier circuit ( 27 ). A peak hold circuit ( 28 ) holds a peak value of an interference voltage within a period corresponding to one sweep. The peak value thus held is compared by a comparator circuit ( 29 ) with a predetermined value and accordingly, a relay circuit ( 30 ) outputs a signal indicative of whether an object is present or not.
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