GB954536A - Intrusion alarm system - Google Patents

Abstract

954,536. Radar. SYLVANIA ELECTRIC PRODUCTS Inc. Aug. 7, 1962 [Aug. 15, 1961], No. 30246/62. Headings H3F, H3T and H4D. [Also in Division G4] In a system for detecting the presence of a moving target (e.g. a person) in a predetermined area by means of a Doppler-effect beat note in a receiver 2, Fig. 1, produced by mixing the direct unmodulated microwave energy from a transmitter 1 and the target-reflected energy, the receiver incorporates a transistor band-pass (B.P.) amplifier, Figs. 2, 4, and a circuit which derives the time-integral value of the amplified note and actuates an alarm when such value exceeds a predetermined value. Receiving apparatus, Figs. 2, 3.-The mixer 8, Fig. 2, comprises a low-Q cavity, Fig. 3, formed by an outer conducting shell 25, and an inner conductor 26 is connected to a diode 27. Screw 28 permits adjustment of the cavity to the transmitter frequency. All A.C. components except the beat frequency are by-passed to earth by capacitor 20, Fig. 2, the D.C. component being passed via amplifier 13, voltage comparator 14 and OR gate 62 to the alarm circuits 63 . . . 65, thereby monitoring the system. To define the boundaries of the system the B.P. amplifier has sharp upper and lower cut-offs, and comprises stages 30 . . . 33 with the upper B.P. limit controlled by capacitors 36 and 40, and the lower limit controlled by capacitors 15, 38 and 48. The B.P. amplifier output is fed to a D.C. restorer and integrator circuit 46 which inhibits response to incoherent signals, e.g. noise and signals from small animals, weeds &c. Zener diode 52 ensures constancy of D.C. level at the junction of capacitor 48 and diode 50. The output of circuit 46, if sufficient, actuates a Schmitt-like trigger circuit 56 the triggering level of which is set by resistor 60. The trigger circuit output actuates an alarm bell or light 65 via OR gate 62. Protected areas, Fig. 6.-The protected area may be a room, Fig. 8 (not shown). By disposing several systems " back-to-back," an outdoor perimeter may be protected, Fig. 6.