| 摘要 |
1284674 Ultrasonic Doppler systems WALTER KIDDE & CO-Ltd 23 April 1970 [15 May 1969] 19630/70 Heading H4D [Also in Division G4] In an ultrasonic Doppler-effect intruder alarm, false alarms due to air turbulence, Fig. 2 (not shown), are reduced by eliminating low frequency Doppler beats by a filter 17 and adjusting the level of the residue to a magnitude such that only signals thereabove actuate lamp or bell alarm 54; such selection is achieved in differential amplifier 21, the threshold being set by diodes 44, 45 and potentiometer 16; the latter is set such that with no intruder but with air turbulence present no alarm is raised. An integrator 24 prevents an alarm in response to transient signals, e.g. due to water hammer in pipes or line voltage surges. In a modification a feedback loop between the output of detector 22 and diodes 44, 45 reduces false alarms due to abnormal air turbulence, e.g. due to very large fans, Fig. 3 (not shown). |
