| 摘要 |
An intrusion alarm circuit including a single electroacoustical transducer, such as a diaphragm-supported piezoelectric element, connected to an amplifier in a positive feedback loop configuration. The transducer functions as both a sound pickup and sound generator. When the ambient sound level exceeds a preselected threshold level, the resulting vibration of the transducer generates a voltage which activates the amplifier, whereupon the transducer vibrations are sustained and amplified in the manner of an oscillator, thereby producing an audible alarm.
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