Apparatus is disclosed for providing all of the functions of battery-feed, isolation, and supervision for a line circuit in a communication switching system. The apparatus comprises a primary and a secondary winding coupled inductively via a magnetic core which includes an air gap. A reed contact pair is positioned across the air gap to operate and give indication of the presence of magnetic flux across the air gap resulting from current flow in the primary winding.

A circuit which performs both the supervisory functions and transmission functions of a line circuit under control of a Hall effect device. A Hall effect device operates in the magnetic field generated by the transmission transformer to produce a proportionate signal. This signal is used by the Pulse Code Modulation (PCM) processing circuitry of the line circuit for transmission signals and it is used by a level discriminator to generate supervisory signals.

The inputs of an operational amplifier are connected across a telephone line and bridged by a capacitor. The output of the operational amplifier drives the coil of a relay via an intermediate amplifier. The relay is of the make-before-break type, and is interposed between an audio source, audio speakers, and dummy load resistors. When the voltage across the telephone line indicates an on-hook condition for a telephone, the audio source is connected to the speakers through the relay. When the vol...

A circuit which can perform a plurality of sensor functions relating to telephone subscriber supervisory circuits. These functions can occur before and after ringing current is applied to the subscriber line facility. Coils function as magnetic field generating devices and a Hall effect device is used to perform the sensory functions. A programmable comparator is used to compare detected conditions with predetermined thresholds.

Monitoring apparatus for an electrical circuit, as a telephone circuit, operable to discriminate between various signals and a particular signal different from any thereof and for utilizing the detection of the particular signal to perform a useful function. By way of example, the monitor is useful to discriminate between the various normal signals typically involved in the operation of telephone circuitry and the "on hook" and "off hook" signal inherently generated as an incident to reseating o...

A filter (45) is passive and has a complex impedance which gives good impedance matching to the complex impedance of a transmission line. Because the filter is passive, it does not need to be powered and can thus be placed in locations that lack a power supply. The filter can also function in the event of a power failure. Because the filter has an impedance which can be well matched to the complex impedance of a transmission line, problems relating to echo and side tones can be minimized.

A tractor with two spaced front wheels and a trailing tail wheel has an operating lever controlling two sets of hydraulic pumps and motors independently driving a respective front wheel. The operating lever controls the pumps and motors through a linkage system by movement in a vertical longitudinal plane and the turning of the tractor by clockwise or counterclockwise rotation of the lever. Releasable friction means hold the lever in a given position in the longitudinal plane.

A current detector circuit, preferably for use in telephone equipment, to detect the on-hook and off-hook conditions of of such equipment, includes two equal transformers which have primary coils mounted in series and secondary coils mounted in series opposition, but in series with a circuit within which it is desired to detect the current. The primary coils constitute the load of the collector of an NPN transistor. Such collector is connected by means of a resistor to the base of a PNP transist...

An electronic circuit is disclosed for detecting when a telephone handset is lifted in response to ringing tone and for providing a voltage which operates a relay to interrupt the transmission of ringing current. The circuit includes an oscillator controlled by the d.c. potential difference across a resistor in series with a wire carrying ringing current to the telephone line. The oscillator is coupled through transformer windings to the circuits to be controlled.

In order to discriminate between true and spurious key signals in a telephone system in which a d-c subscriber line is independently closable by separate hook switches in series with a dial-type and a pushbutton-type digit selector, an exchange serving this line includes a low-level current sensor and an intermediate-level current sensor working into a coincidence circuit. In the presence of an output from the intermediate-level sensor with simultaneous absence of an output from the low-level se...