| 摘要 |
1,126,996. Intruder alarms. VANDALARM SECURITY SYSTEMS Inc. 28 March, 1967 [25 Aug., 1966], No. 14094/67. Heading G4N. An alarm system comprises two input signal channels 11, 12 each modulated on to a common, swept, carrier signal from oscillator 31. The resulting signals are fed via bandpass filters 27M, 27C to detectors 28M, 28C, the outputs from which are fed to an inhibit gate 14. The inputs to the two channels are derived from microphone 24M, 24C in the area to be protected. The microphone 24C is placed near a source of normal sound, e.g. a refrigerator, so that any sounds from this source picked up by microphone 24M are prevented from initiating an alarm. Sounds from an intruder are picked up only by microphone 24M and are passed by the gate circuit 14 to the alarm circuit 17-23. Details of this circuitry are described with reference to Fig. 3 (not shown) and include a capacitor (35) which is charged at a predetermined rate by pulse input signals, and at a slower rate by continuous tone signals until the voltage across it is sufficient to trigger Schmitt circuit 22 and initiate the alarm.
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