| 摘要 |
PURPOSE:To more accurately detect only an illegal entrance into the interior. CONSTITUTION:When an ultrasonic wave is sent out into the interior R from a transmitter 3, a reflected wave of the ultrasonic wave is received by a receiving sensor 4, and is inputted to a comparator 6 through an amplifying circuit 5. The comparator 6 compares a signal from the amplifying circuit 5 with a threshold level from a level setting circuit 12, and when a signal level exceeds the threshold level, it outputs a signal to a mixing circuit 7 every time. The mixing circuit 7 inputs a beat created by a difference in a frequency between a signal from an oscillating circuit 1 and a signal from the comparator 6 to a frequency counter 8. The frequency counter 8 outputs a signal to a judging circuit 9 in the beat counting timing. Only when input from the frequency counter 8 in prescribed time (t1) set by a timer circuit 10 is uncertain, the judging circuit 9 outputs a command signal to an alarm output circuit 11. |
