Security system with plural sensors connected in parallel across the transmission line

Claim 1

In a security system for seismically detecting intrusions into a physically defined enclosure and including a plurality of seismic sensors arranged in a spaced distribution throughout the enclosure, a two wire electrical transmission line, means individually connecting each sensor in parallel across the two wires of the transmission line, and a readout device connected to one end of the transmission line, the improvements characterized by: a. the connecting means each including an amplifier having its input connected to a sensor and its output connected to the transmission line, whereby each sensor is always active and on-line, each amplifier having a high output impedance with respect to the impedance of the transmission line whereby each sensor and amplifier combination has a negligible influence on the impedance of the transmission line and does not provide any substantial attenuating shunt path for the signals from the other sensor and amplifier combinations, b. readout impedance means connected across said one end of the transmission line, the value of said impedance means being equal to the characteristic impedance of the transmission line at the seismic frequency range within which the system operates to thereby avoid any signal reflections, c. two resistors connected in series across the other end of the transmission line, the value of the resistors being high relative to the characteristic impedance of the transmission line whereby the resistors do not provide any substantial attenuating shunt path for the signals from the sensor and amplifier combinations, and d. means connecting a junction point between the resistors to the readout device to thereby implement the monitoring of the transmission line for grounding, shorting and cutting.