Security alarm system with audio monitoring capability

A security alarm system comprising, in combination, a remotely located station to be protected, a central monitoring station and electrical transmission lines coupled therebetween, said remote station including output means coupling said remote station to said transmission lines, means for generating a DC transmission signal having a first normal polarity, first relay responsive switching means normally coupling said generated DC signal of said first normal polarity to said output means, a line current detector including a photo-optical coupler having a photo diode coupled between said generating means and said first switching means, said photo diode being responsive to the normal passage of current therethrough for maintaining its phototransistor in a first conductive state and responsive to the absence of current therethrough to switch its phototransistor to a second conductive state, indicator means at said remote station responsive to said second conductive state for providing a visual indication of the failure to transmit said DC transmission signal, a plurality of alarm condition sensors normally maintaining a first normal condition but triggerable to a second alarm condition in response to the detection of said alarm condition, a normally de-energized relay responsive to said second alarm condition for energizing its coil to switch said first relay responsive switching means to reverse the normal polarity of the DC signal supplied to said output means, a manually operable multipositionable switch located at said remote station, said switch being positionable to at least an "on" and an "off" state, microphone detector means including a plurality of individual microphones located about said remote station for detecting audible sounds originating therein, each of said microphones having its own amplifying means and means for adjusting its individual sensitivity, said microphone detector means further including a high gain amplifier for further applying the AC outputs of said microphones, a transformer means for superimposing the amplifier AC audio signal onto the DC transmission signal at said output means, means responsive to said manually-operable switch being in the "on" position for supplying necessary power to said microphones, supplemental second relay responsive switching means responsive to the energization of said relay means in response to the detection of an alarm condition for supplying the necessary power to said microphones even if said manually-operable switch is in the "off" position, closed loop means activated when said manually-operable switch is in said "on" position, for maintaining a first closed circuit state so long as no part of the perimeter security loop is broken but responsive to a break in the security loop for switching to a second closed circuit state, said relay means being further responsive to said second closed circuit state for energizing the relay coil to reverse the polarity of the DC transmission signal supplied to said output means, said central monitoring station comprising a line supervision circuit including a full wave rectifier coupled to said transmission lines for providing a continuous DC signal to a photo-optical coupler for isolating said central station, said photo-optical coupler having a photo diode coupled in series with said full wave rectifier, said photo diode being normally responsive to the presence of DC current for maintaining its phototransistor in a first state but being responsive to the absence of DC current indicating an open circuit or short circuit fault for switching its phototransistor to a second state, means responsive to said second state for indicating the fault condition, an alarm condition monitor circuit including a diode coupled to one of said transmission lines and a photo-optical coupler in series with said diode for isolating said central station, said photo-optical coupler having a photo diode having its anode connected in series with the cathode of said diode such that said photo diode does not conduct when a DC signal of normal polarity is present on the lines so as to maintain its phototransistor in a first normal state indicating the absence of an alarm condition and such that said photo diode does conduct in response to a DC signal of reversed polarity on said lines so as to switch its phototransistor to a second alarm state indicating the presence of an alarm condition at said remote station, means including an SCR responsive to said second alarm state for switching said SCR to a conducting state, means responsive to said SCR being in said conducting state for indicating the existence of said alarm condition, and means for manually resetting said SCR to its non-conductive state after the alarm condition has been corrected at said remote station, and an audio monitoring circuit including a transformer for isolating the central station from the transmission lines, a DC blocking capacitor coupling a first transformer winding to said transmission lines, an audio amplifier having its inputs coupled to the second transformer winding, a threshold amplifier having its input coupled to one of the inputs of said audio amplifier for outputting a gating signal when the audio signal exceeds a predetermined threshold level, an audio gating means having its input coupled to the output of said audio amplifier and responsive to said gating signal from said threshold amplifier for passing the output from said audio amplifier and speaker means coupled to the output of said audio gating means for rendering the audio signals passed by said gating means audible to an operator at the central station so as to enable the operator to listen to the actual audio sounds originating in the protected remote location.