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1,185,265. Intruder alarm systems. WESTINGHOUSE ELECTRIC CORP. 10 May, 1968 [24 Aug., 1967], No. 22306/68. Heading G4N. An intruder detector comprises two parallel flexible fluid-filled tubes buried in the ground along the boundary of the area to be protected. Each tube is provided with a transducer sensitive to pressure pulses transmitted through the fluid in the tubes resulting from movement of the intruder, the transducers being connected together to operate an alarm when these outputs differ. As described, tubes 31, 33, Fig. 2, e.g. of rubber and filled with, e.g. water or water and glycerine, are buried in the ground with their common plane parallel to or at right angles to the surface of the ground. The tubes are connected at one end to chambers 81, 79, Fig. 4, each divided by a diaphragm 83 provided with a piezo electric crystal 91 sensitive to stresses in the diaphragm produced by the pressure pulses. The crystals are arranged and connected so that their outputs are in opposition so that the combined transducer assembly only produces an output when an intruder is present. In a modification the pressure transducer may include an LVDT or capacitor. To check that the system is operative the other end of each tube is provided with a similar transducer which is supplied with an intermittent pulse of electrical energy to thereby generate a positive and negative pressure pulse in each tube. Alternately the pressure may be produced by movement of the core (151), Fig. 17 (not shown), of a coil (149) when the coil is energized either by closing a switch (160), Fig. 16 (not shown), connecting the coil to a D.C. battery (162), or from an oscillator. When the checking transducers are operated continuously to produce pressure pulses, 180 degrees out of phase, in the two tubes at a frequency of, e.g. 60 c/s., the output from the two crystals is fed, after preamplification by a circuit including F.E.T's. F1, F2, Fig. 14, transistor Q3 and integrated circuit MO to the base of transistor Q5 via filter FI which suppresses the 60 c/s. signal and to amplitude limiting circuit including transistors Qi, Q2 which produces a 60 c/s. signal of constant amplitude. This signal is fed to the base of transistor Q6, rectified by diodes D20, D21 and supplied to the base of a transistor Q4 which is thus maintained in its conductive state so that relay R is energized to close switch R6 of an indicator which indicates that the system is operating. The 60 c/s. signal from transistor