Iwasaki Tsushinki Kabushiki Kaisha

Kouichi Sekiguchi

Hikaru Takematsu

Kazutada Katsukura

Toshiichi Takano

A key telephone system in which a key service unit is provided for selectively connecting a plurality of key telephone sets to at least a pair of CO lines through at least one control line provided in correspondence to the CO lines and a common line. The key service unit comprises a DC power source for producing two outputs respectively having certain positive and negative potential differences with respect to a reference potential, at least one first detector provided in correspondence to the CO lines and selectively connected to the control line for detecting the state in which one of the two output voltages is applied across the control line and the common line to flow a current of predetermined value, at least one second detector provided in correspondence to the CO lines and selectively connected to the control line for detecting the state in which the other of the two output voltages is applied across the control line and the common line to flow a current larger than a certain value, and at least one switching means provided in correspondence to the CO lines for disconnecting the first detector from the control line and for connecting the second detector to the control line when the first detector produces an output.

Key telephone system

A key telephone system, in which a key service unit is provided for selectively connecting a plurality of key telephone sets to a CO line by two control lines and a talking line. The key service unit is comprised of a DC power source having two terminals of a positive output voltage and a negative output voltage, a first detector connected to one of two control lines for detecting a state where one of the two output voltages is applied across the two control lines to flow a current of predetermined value therethrough, a first switch for connecting the other output voltage across the control lines in response to the output of the first detector, a second detector connected to the other of the two control lines for detecting a state where the other of the output voltages is applied across the two control lines to flow a current larger than a certain value therethrough, and a second switch for connecting the talking line to the CO line in response to the output of the second detector. Additional circuitry is further provided to perform the intercomm. talking operation, the holding operation for the CO line, and the privacy operation and other functions.

A key telephone system, in which a plurality of key telephone sets are selectively connected to a plurality of CO lines by the use of a plurality of talking lines under control of a key service unit and a plurality of control lines. In order to seize a CO line, a CO line seizure signal from one of key telephone sets is time-divisionally transmitted at a low speed and received by a speed change memory of the key service unit. A high-speed output from the speed change memory is added with dial or hook information and then space-divided for seizing a desired one of the CO lines. A talking operation, a holding operation, a privacy operation and a synchronizing operation in this system are performed on the base of the above operation principle.

A two tone generator using switching transistors in two LC resonance circuits each composed of a coil and a capacitor for producing a multifrequency signal in the form of two-tone signal by switching the value of the inductances of the coils in response to the control of push-button switches, in which only one make contact is actuated in response to the depression of each push button, so that the ON-OFF output of the contact is employed to control selected ones of switching transistors in each of the two resonance circuits for determining the oscillation frequencies of a two-tone oscillator associated with the two LC resonance circuits. The switching transistors comprises a first group of either NPN transistors or PNP transistors and a second group of NPN transistors or PNP transistors. The switching transistors are coupled with the two LC resonance circuits to switch the value of the inductance of each coil in response to the turning ON of the collector-emitter path thereof. Each of the switching transistors is turned-ON by flowing a base current more than the peak value of the AC signal developed in the resonance circuit without flowing any DC current in the collector-emitter path thereof.

Two-tone generator using switching transistors in LC resonant circuits

A circuit for preventing impulse generation caused by operating a hook switch in a telephone set, in which the hook switch has a make contact and a break contact which are make-before-break contacts in going to the OFF-hook condition, and are break-after-make contacts in going to the ON-hook condition. A capacitor is charged through the make contact when it is closed in the OFF-hook condition. A transistor switching circuit is provided to turn ON by the charge on the capacitor so as to short-circuit between telephone line terminals of the telephone set when the break contact is closed in the ON-hook condition.

Circuit for preventing impulse generation by operating a hook switch in

An AC wave switching circuit in which the collector-emitter path of a switching transistor is inserted in series in the circuit loop of an AC signal to be switched. A control voltage source capable of flowing a DC current greater than a peak value of the AC signal current is connected across the base-emitter path of the switching transistor, so that the switching of the AC signal is performed by the voltage of the control voltage source without flowing any DC current in the collector-emitter path of the transistor. The collector-emitter path of another switching transistor may be connected in series or in parallel with the collector-emitter path of the first-mentioned switching transistor. At least one of a diode and a resistor may be included in the base-emitter path circuit of each switching transistor. The circuit loop may provide an output by transformer coupling or the capacitor coupling.

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Asaka, Japan

Kouichi Sekiguchi