Abstract: A ringback circuit in a security protection system, of the type which monitors a dc current, automatically reports to remote premises that the central station has, at an appropriate time, received an indication that the dc current has been changed to the normal current level for night operation. At the central station, a change in the current level from normal daytime to normal nighttime current is sensed, activating an oscillator. An electronic current limiter in the communications line responds to the oscillator through an isolation circuit, oscillating the current in the communications line, which activates a buzzer at the remote premises.

Abstract: Digital interface circuitry is utilized to analyze the condition of alarm detecting loops. The new circuitry is utilized as an alarm system of the type which has a detecting loop extending between a central station and a remote zone at which is provided a normally open detection switch in parallel with a resistor. At the central station a digital high voltage is supplied to the input loop lead and a resistor is connected to ground from a reference point on the return lead, to provide a voltage divider network with the remote resistor. Two voltage-sampling branches connect to the reference point; the first branch has a comparator or pull-up circuit to indicate digitally whether the voltage level is substantially zero or greater, while the second branch has a level detector and pull-up circuit or a comparator to indicate digitally whether the reference point voltae is greater than its normal level.

Abstract: For monitoring the supervisory current in a communications line between a power source and a protected premises which forms a security protection circuit, monitoring apparatus utilizes sensitive solid state circuitry which appears transparent to the protection circuit. An operational amplifier input circuit protects the circuitry against component damage due to lightning, inherently rejects common mode errors due to noise, transients, and varying power supply voltage, and had an adjustable offset for line length compensation and circuit standardization. Voltage comparator circuitry indicates by digital signals the supervisory current level. After the operator conditions the apparatus for an abnormal current level which has been signalled, a lamp which blinks at a discernable rate is used to serve as a reminder of this state.

Abstract: Each zone of a double-detecting loop type alarm system utilizes a current level detector to produce a digital signal when normal current flow in the detecting loops is interrupted. The digital signal so produced is stored in a holding register, whose output drives electronic shunting switches connected across the ends of the detecting loops. Closing of the switches permits current to again flow to the current level detector and detecting loops, causing the current to return to a normal current level if the interruption was caused by a break or short in a detecting loop; this permits that zone of the alarm system to again function. Since the system is digital, its ouputs and those of the other zones are presented by means of other digital transmitting and monitoring devices, such as a binary multiplexer.