Telephone ringing extender

DETAILED DESCRIPTION In the drawing, a subscriber telephone loop shown as ring conductor 10 and tip conductor 11 is connected from the central office to the left of the drawing to the telephone subscriber telephone sets to the right of the drawing.
Bridged between conductors 10 and 11 are a power supply 13, a filter circuit 14 and a threshold detector circuit 15.
Power supply 13 provides a balanced voltage source of preselected magnitude for powering the electronic circuits of the interface units.
Filter circuit 14 is an active filter designed to be responsive to ringing signals (nominally 20 Hz) while at the same time rejecting dialing signals and power frequency voltages.
Filter 14 provides the input to threshold circuit 15.
Threshold circuit 15 compares the output of filter 14 with a reference voltage and, when the filter output exceeds the reference voltage, provides a control signal on output lead 16.
Lead 16 is connected to a switching circuit 17 which, when operated by a signal on lead 16, connects tip conductor 11 through a resistor R16 to ground potential.
Protector circuit 18 monitors the voltage on tip lead 11 and monitors the current through resistor R16.
When either this voltage or this current exceeds a preselected maximum, protector circuit 18 operates to ground tip conductor 11 directly to protect the electronic components as well as service personnel working on the loop.
It can be seen that the arrangement in the drawing provides a two-wire interface to the subscriber telephone sets and, at the same time, permits a large number of ringers to be operated on long subscriber loops.
Power supply 13 includes a surge-limiting resistor R17, a direct current blocking capacitor C7 and a varistor CR12 which has a high impedance at talking voltages to minimize the insertion loss during talking.
Diodes CR13 and CR14 provide full wave rectification of the voltage across conductors 11 and 12.
Capacitors C8 and C9 filter out the alternating current components of the line voltage, while zener diodes CR15 and CR16 limit the voltages on leads 19 and 20 to preselected values (e