GB886938A - Intruder detecting system - Google Patents

Abstract

886,938. Intruder alarm systems. SPECIALTIES DEVELOPMENT CORPORATION. Aug. 12, 1959 [Sept. 5, 1958], No. 27556/59. Class 118. An intruder alarm system designed to detect footsteps within a perimeter whilst not responding to footsteps or traffic outside the perimeter comprises transducer means capable of receiving energy conducted through the ground and transmitting an electrical output which is applied to electro-responsive means tuned to detect energy only at predetermined frequencies. Transducers 14 (Fig. 3) of the barium titanate piezo-electric crystal type are buried in or placed on the ground at equidistant intervals of say 10-15 feet, the transducer nearest the perimeter being some 20 feet therefrom, and the output from these transducers is fed in parallel to a conventional three-stage tuned-transformer coupled transistor amplifier 15. One of three range settings is chosen by connecting predetermined capacitance across each of the three coupling transformers, the frequency range being in the upper sonic band. Rectified output is used to maintain a relay operated and so keep the alarm 17 inoperative. The higher frequencies of the band generated by a footstep within the perimeter are received with sufficient strength to bias off a final transistor stage of the amplifier 15, releasing the relay and operating the alarm 17. The higher frequencies emanating from activity outside the perimeter are sufficiently attenuated, by the greater distance travelled through the ground, not to release the relay. Fig. 5 shows a circuit designed to avoid false operation from distant disturbances due e.g. to explosions. Such disturbances will be received on each of two transducer networks 14 sufficiently strongly to release both relays RL2 and RL3, thus keeping relay RL4 de-energized and the alarm 17 inoperative. The two transducer groups are spaced apart sufficiently for the footstep of an intruder to be received on the transducer(s) of one group only; thus only one of the relays RL2, RL3 releases, relay RL4 is energized and the alarm 17 operates. The arrangement of Fig. 6 is said to prevent false operation by a bird, mole or worm. Three amplifiers 15A, 15B and 15C are employed, each maintaining energized a corresponding relay RL5, RL6 or RL7. The amplifiers 15A and 15B each have transducers 14 so disposed that the sound of an intruder will be picked up by at least one transducer in each group, thus releasing relays RL5, RL6 and so energizing relay RL4 which operates the alarm 17. Disturbance caused by a mole, worm or bird will affect one transducer only and so fail to operate relay RL4. Amplifier 15C is connected to a single transducer so placed that it will not pick up a local disturbance which affects a transducer in each of the groups connected with amplifiers 15A and 15B, but when a distant disturbance of high intensity affects all three transducer groups then the alarm circuit is broken by the operation of relay RL7.