| 摘要 |
PROBLEM TO BE SOLVED: To eliminate a peak value holding circuit for a long period, to reduce the number of comparators for a reception wave detecting part, and to make an analog circuit work only in reception wave detection to reduce a current consumption. SOLUTION: The reception wave detecting part is constituted of an amplification degree-variable amplifier for amplifying a signal from a receiver, the comparator having two reference voltages, the comparator for zero-cross detection, and the like. In a reception wave after the amplification, a reception wave detection signal is output in a zero-cross point when the third wave thereof exceeds the reference voltages of 200 mV and 500 mV at a stretch as shown Fig. (a). The reception wave detection signal is output hereinafter at the zero- cross point near to an arrival time at this time. The reception wave detection signal is not output when the third wave does not reach 500 mV. An amplification degree of the amplifier is changed to retransmit and rereceive an ultrasonic wave.
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