GB1139600A - Electrical protection system - Google Patents

Abstract

1,139,600. Burglar alarms. AMERICAN DISTRICT TELEGRAPH CO. 15 Sept., 1966, No. 41328/66. Heading G4N. [Also in Division H3] An intruder alarm, e.g. for detecting attacks on a vault or like structure comprises a microphone feeding a first amplifier, the output of which is divided into two channels, the outputs of which being combined at an integrator. The first channel carries low level signals whilst the second channel is provided with a threshold device responsive only to high level signals above a predetermined value. As shown, a plurality of piezo-electric microphones 20 are sealed in epoxy resin and are mounted on the internal surface of the vault. Their outputs are coupled by a transformer TF1 to an amplifier T1 tuned by transformer TF2 and capacitor C6 to the natural frequency of the microphones 20, the peaking curve being asymmetrical so that its higher frequency side embraces minor peaks in the microphone output. The output of TF2 is amplified at T2, the output of which is passed via a sensitivity control 27 and a further amplifier T3 to a voltage-doubling integrator C1, C11, R13, REC6, REC7 having a time constant of 15-25 minutes. A further output from TF3 is rectified at REC9 and feeds a threshold device comprising transistors T4, T5 which are arranged to pass only signals 25dB (adjustable at R14) above the usual alarm signal level. The output of the threshold device is passed via diode REC10 to the integrating capacitor C11. When the output of the integrator reaches a predetermined level, unijunction transistor T6 becomes conductive, energizing relay 35 which operates an audible or visible alarm. REC 8 provides temperature compensation. Power supplies.-A bridge rectifier 43 arranges the supply to the transistors to be independent of the polarity of the supply voltage at 41, 42. A Zener diode REC 5 protects the system from high voltage surges. REC 11 and a potential divider 36 are arranged so as to prevent spurious low-level triggering of transistor T6 due to supply voltage dips. The supply is smoothed at C3, C4; REC 12 preventing these capacitors discharging via the potential divider 36.