GB1320993A - Intrusion alarm systems - Google Patents

Abstract

1320993 Intruder alarms SYSTRON-DONNER CORP 21 June 1971 [26 June 1970] 28986/71 Heading G4N [Also in Divisions H3 and H4] In an intrusion alarm system in which a Doppler frequency is derived from radiated and received signals due to movement of an intruder, an appropriate spectrum of the Doppler frequency is selected by means of a bandpass filter 17 the output of which is fed to a pair of voltage doubling rectifiers 63, 64 having the outputs connected in opposition and having different charging times so as to have differing frequency characteristics. As described, a supersonic frequency of 19À2 kHz is radiated into the space to be protected, received signals being picked up by a transducer and after amplification in a balanced differential operational amplifier (Fig. 2, not shown) are fed to a bandpass filter 17 comprising an operational amplifier 46 having feedback via a twin-T network so as to pass a frequency band of 40Hz to a further operational amplifier 59. In the two diode pump voltage doublers 63, 64, capacitor 77 is ten times that of 76 whilst resistor 67 has one half the resistance of 72. Movement of an intruder tends to produce Doppler frequencies at the upper part of the band passed by filter 17 whereas unwanted frequencies such as air turbulence provide lower frequencies. The voltage doubler 67 produces an output greater than that of 64 for frequencies at the upper part of the band i.e. due to movement of an intruder so that a net positive output is available at the slider of potentiometer 81 which is integrated at 19. If the signals have sufficient duration, the output of the integrator is sufficient to turn on diode 93 and transistor 96 which turns off transistor 98, releasing relay 99 and operating an alarm. Automatic fault indicators.-Failure of the output from the transmitting oscillator causes transistor 97 to turn off, releasing relay 99 and operating the alarm.