Communications – electrical: acoustic wave systems and devices – Echo systems – Presence or movement only detection
Patent
1977-11-17
1980-02-19
Caldwell, Sr., John W.
Communications, electrical: acoustic wave systems and devices
Echo systems
Presence or movement only detection
73290V, 331 65, 340565, 340621, G08B 2100, H03B 500
Patent
active
041897226
ABSTRACT:
The present apparatus is designed to determine the presence of an intruder in a swimming pool by detecting a predetermined change in water level caused by displacement. The surface of the pool water forms the lower boundary of an air filled acoustic cavity, and cavity resonance is excited therein by a sonic transducer coupled to the cavity and also to a signal amplifier, causing the cavity to determine the frequency of oscillation of the feedback amplifier. A rise in water level caused by intruder displacement decreases the vertical dimension of the cavity so that the cavity has a different resonant frequency than before intrusion, and the amplifier therefore oscillates at a different frequency. The predetermined change in amplifier signal output frequency is sensed by an electrical network and converted to an alarm device activating signal to indicate an intrusion.
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patent: 3058101 (1962-10-01), Malvini
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patent: 3693445 (1972-09-01), Johnson
Caldwell Sr. John W.
Myer Daniel
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