Key Telephone System

1. Field of the Invention

This invention relates to a key telephone system and more particularly to a circuit for controlling a line circuit in a key telephone system.

2. Description of the Prior Art

A key telephone system generally comprises a plurality of key telephone sets, a service unit provided with a line circuit and a common line circuit. A key telephone system employs service features for incoming calls such as line holding, transfer and intercom using locking-type keys in the form of a group of manually operable switches having a common latch arrangement for maintaining one of the switches in the actuated condition until another switch is actuated. The status of the service features is indicated by a visual indicator comprising lamps or light emitting diodes (LEDs), buzzers and the like. Generally, incoming call indication is performed by an intermittent audible tone of a buzzer in synchronism with lighting or extinguishment of the visual indicator representing the presence of an incoming call. Line holding indication is performed by lighting or extinguishment of the visual indicator at a cycle time different from that of an incoming call indicator. Talking indication, representing the in-use status of a telephone line (i.e., "busy" status) is performed by continuous illumination of the visual indicator. Conventionally in order to satisfy these service features, four lines are required, as shown in U.S. Pat. No. Re 26,722. For example line No. 1 in that patent is connected between each line circuit of the service unit and the key telephone sets. Two speech lines T and R, one visual control line LL and one holding control line AL are also provided between each line circuit of the service unit and the key telephone sets.

However, modern key telephone systems are becoming large in size, causing increases in the number of corelines required in cables connected between the service unit and key telephone sets and in the number of corelines in telephone set and terminal connector cables which increases the cost of installation and parts for connection. Furthermore, the increase in the number of corelines and connection parts raises the risk of equipment failure and maintenance difficulties in key telephone systems.

Accordingly, one object of the invention is the provision of an improved key telephone system.

Yet another object of the invention is the provision of an improved key telephone system of lower cost, simplified structure and requiring fewer interconnecting cables.

Still another further object of the invention is the provision of a novel full service key telephone system with circuitry for reducing the number of required interconnecting cables.

Another object of the present invention is to provide a key telephone system with visual or audible indication and line holding performed by two speech and one control line.

Briefly, these and other objects of the invention are achieved by providing a first relay, normally energized, and a second relay for speech supervision in a line circuit of a key telephone system, wherein the operation of the first relay establishes a circuit for holding a state of the second relay. The first relay is released by the voltage drop of a control line connected normally to a line holding control circuit and a visual indication circuit respectively at the manual operation of a non-lock type hold key to thereby establish a circuit generating signal intermittently for a line holding indication. A switching transistor is provided with a circuit comprising the first relay so that the first relay is not actuated by the signal intermittently sent to the visual indication circuit, thereby controlling the line holding control circuit and the visual indication circuit with one control line.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of the key telephone system of the present invention;

FIG. 2 and FIG. 3 illustrate other embodiments of the line circuit in the key telephone system of the present invention; and,

FIG. 4 illustrates another embodiment of key telephone set in the key telephone system of the present invention.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, L.sub.1, L.sub.2 represent line connection terminals, while T, R, L and B indicate cable and connection terminals between key telephone sets and a service unit. T and R are the speech lines. The L line is a control line which controls both speech supervision such as line holding and visual indication of the use status of lines. Line B is a control line which controls audible indication, by buzzer or the like, when an incoming call signal is received. In a line circuit No. 1 of the service unit, R represents a relay for detecting incoming call signals, R(I) is a primary winding receiving signals from the line, R(II) is a secondary winding forming a self holding circuit, r.sup.0 . . . r.sup.3 are contacts of the relay R. H is a relay for speech supervision and line holding which includes two windings (I), (II). The contacts of the relay H are denoted by h.sup.0 . . . h.sup.3. T is a relay for supervision of both stoppage of electric current and speech which includes two windings. The contacts of the relay T are denoted by t.sup.0 . . . t.sup.3. Transistor Tr.sub.1 normally assumes the on-state due to base-current flowing through a resistor R.sub.1 to thereby supply to the relay T. Transistor Tr.sub.2 assumes the on-state during line holding in order to control a visual indicator, while transistor Tr.sub.1 assumes the off-state, thereby to cut off power source E supplying relay T. Resistor R.sub.2 is a resistor limiting the base-current of transistor Tr.sub.2. Resistor R.sub.3 is a resistor employed in order to form a direct current loop during line holding to the C.O. or P.B.X. D.sub.0 is a diode which makes the relay T restore after a delay to the relay H when line talking is terminated. D.sub.1 - - - D.sub.8 are diodes which prevent reverse current. INT.sub.1 is a common circuit of the service unit and is an oscillating or intermittent circuit with a frequency of 2.5Hz for flashing or cycling both the visual indicator and the audible indicator when an incoming call signal is received. The circuit INT.sub.1 comprises coils and capacitors.

Contacts of the oscillating circuit INT.sub.1 are denoted by int.sub.1A and int.sub.1B. TM is a timer circuit for closing the self-holding circuit of secondary winding R(II) of relay R for a predetermined time when an incoming call signal is received. The contact of the timer TM is denoted by tm. INT.sub.2 is an intermittent circuit with 1Hz frequency for lighting or extinguishing a visual indicator during line holding. The contact of the intermittent circuit INT.sub.2 is denoted by int.sub.2. In telephone set No. 1, N indicates a telephone circuit network, HS.sub.1, HS.sub.2 are switch-hooks, LK.sub.1 is a line key corresponding to line No. 1 for selecting the line corresponding to each line circuit, wherein HK is a non-lock type key for line holding. The hold key HK is operated in association with the operation of the line key Lk.sub.1.

BL.sub.1 is an indicator lamp, EL is an electric source for the lamp BL.sub.1, BZ is a buzzer for indicating incoming calls and E is an electric source. R.sub.4 is a resistor connected to the line L when switch-hook HS.sub.2 is closed, wherein the resistor R.sub.4 is employed in order to change the voltage or potential in the line L. A zener diode may be used in place of the resistor R.sub.4.

Operation of the present invention will now be described with reference to FIG. 1, it being understood that the relays shown in the drawings are released as shown in the drawings.

When electric power source is switched on, base-current of transistor Tr.sub.1 in the line circuit flows through resistor R.sub.1 so that the transistor Tr.sub.1 assumes the on-state. Relay T is energized by current flowing in the primary winding T(I), so that contacts t.sup.0 - - - t.sup.3 are operated. The relay T is energized through a circuit comprising ground, contact h.sup.2,primary winding T(I), transistor Tr.sub.1 (C-E) and electric source E.

When an incoming call signal is received at lines L.sub.1, L.sub.2 of line No. 1, contact r.sup.0 of primary winding R(I) of relay R is energized and closes so that the oscillating circuit INT.sub.1 and the timer circuit TM operate in a common circuit. Accordingly, intermittent contacts int.sub.1A and int.sub.1B and contact tm operate. At the same time, a self holding circuit for the second winding R(II) is formed by the following circuit path to hold the operating states of the relay R and of the receiving circuit. The circuit path includes ground, contact r.sup.3, contact h.sup.3, contact t.sup.3, contact r.sup.1 , secondary winding R(II), contact tm and electric power source E. As a result thereof, an indicating lamp BL.sub.1 of a key telephone set lights or is extinguished in response to intermittent operation of the intermittent contact int.sub.1A by the following circuit path; ground, intermittent contact int.sub.1A, contact r.sup.2, diode D.sub.3, line L, lamp BL.sub.1 and electric power source E. At the same time the buzzer BZ sounds intermittently controlled by the following circuit; ground, intermittent contact int.sub.1B, line B, buzzer BZ and electric power source E. At this time current flows in primary winding H(I) and secondary winding H(II), but relay H is not actuated because the primary winding H(I) and the secondary winding H(II) are connected in the following circuit so as to cancel the magnetic forces generated by each other; ground, intermittent contact int.sub.1A, contact r.sup.2, diode D.sub.4 in parallel with diode D.sub.3 and diode D.sub.2, primary and secondary windings H(I) and H(II) of relay H and electric source E.

When the hand-set of one of the key telephones is removed from its cradle after a locking type line key LK.sub.1 is actuated, the telephone circuit network is connected to the line circuit through speech lines T and R so as to put the hand-set into communication condition. At the same time, the indication lamp BL.sub.1 continues to light because line L is grounded through the hook-switch HS.sub.2 and a contact of the line key LK.sub.1(8-9). At this time, it is indicated that line No. 1 is busy. At the same time, when line L is grounded, relay H of the line circuit is energized along the following circuit path in response to opening of the intermittent contact int.sub.1A ; ground, hook switch HS.sub.2, hold key HK.sub.1-2, line key LK.sub.1(8-9), line L, diode D.sub.2, primary winding H(I) of the relay H and electric power source E. At this stage, even if the line L is grounded, current does not flow in the secondary winding H(II) of the relay H because of diode D.sub.2. When the relay H operates, the primary winding R(I) of the relay R in the receiver circuit is disconnected because of the operation of contact h.sup.0 and contact h.sup.3 of relay H. At the time contacts r.sup.0 - - - r.sup.3 of relay R restore because the secondary winding R(II) of the self-holding circuit is disconnected from ground. As a result, the intermittent circuit INT.sub.1 and the timer circuit TM deenergize so that the buzzer BZ stops sounding due to restoration of the intermittent contact int.sub.1B. The primary winding (I) of the relay T remains actuated while the line is in use even if contact h.sup.2 is operated as ground line L is connected to the primary winding T(I) of the relay T through diode D.sub.1.

In performing line holding during communication, when the non-locking type hold key HK is depressed, the potential of line L drops because a resistor R.sub.4 is connected in series with the line key LK.sub.1 and the switch-hook HS.sub.2. However, at this stage, if the relay H is set to hold by way of primary winding H(I) and the voltage potential of the line L is set at the line holding value so as to be lower than that of the common connecting point of the primary winding T(I) and the secondary winding T(II) of the relay T in relation to the total resistance values of the primary winding H(I) of the relay H, the primary winding T(I), the secondary winding T(II) of the relay T and the resistor R.sub.4, the current does not flow through diode D.sub.1 in the forward direction. As a result current flows in the relay T by way of the following circuit path; ground, contact h.sup.2, secondary winding T(II), primary winding T(I), transistor Tr.sub.1(C-E) and electric power source E. Accordingly if the number of windings of the primary winding T(I) of the relay T is the same as that of the secondary winding T(II), magnetromotive forces of the primary and secondary windings are cancelled so that the relay T restores, as the same current flows from the starting point of the primary winding T(I) and from the end point of the secondary winding T(II). As a result the relay T restores. At this time the relay H continues to hold itself by way of the following circuit path; ground, contact h.sup.2, contact t.sup.2, diode D.sub.8, primary winding H(I) of the relay H, electric power source, and the line is held by inserting the resistor R.sub.3 through the contact t.sup.0, contact h.sup.1 and contact t.sup.1 between line L.sub.1 and line L.sub.2. On the other hand, the intermittent circuit INT.sub.2 in the common circuit operates by way of the following circuit path; ground, contact h.sup.2, contact t.sup.2, diode D.sub.7, intermittent circuit INT.sub.2 and electric power source E. When the hold key HK is released, the line key LK.sub.1 having a mechanism in association with the hold key HK is restored from the locking condition so that telephone circuit is opened and the line L is disconnected from ground through the line key LK.sub.1(8-9).

Accordingly, indicator lamp BL.sub.1 lights and extinguishes in synchronism with the intermittent period of the intermittent contact int so as to indicate the line holding status by way of the following circuit path, ground, intermittent circuit int.sub.2, contact h.sup.3, contact T.sup.3, diode D de D.sub.5, line L, lamp BL.sub.1 and electric power source EL. In this stage, when the relay T is energized by the ground connected to the line L because of the intermittent operation of the intermittent contact int.sub.2, the line holding condition is released. In order to prevent the line holding condition from releasing, transistor Tr.sub.2 assumes the on-state through both a diode D.sub.6 and a resistor R.sub.2 by way of ground potential when the ground is connected to the line L by the operation of the intermittent contact int.sub.2 so that transistor Tr.sub.1, usually assuming the on-state and allowing the primary winding T(I) of the relay T to supply electric power switches to the off-state, as the base-emitter path of the transistor Tr.sub.1 is short-circuited. As a result the relay T is prevented from operating.

In the case of answering again a line being held as mentioned above, when a hand-set is taken up from its cradle after the line key LK.sub.1 is depressed, a telephone circuit is connected to the line circuit through the speech lines T and R so that a talking circuit is established. On the other hand, when the intermittent contact int.sub.2 in the common circuit is opened, the transistor Tr.sub.2 assumes the off-state so that the transistor Tr.sub.1 assumes the on-state. Accordingly, the relay T is energized by way of the following circuit path; ground, switch-hook HS.sub.2, line key LK.sub.1(8-9), line L, diode D.sub.1, primary wind T(I), transistor Tr.sub.1(C-E) and electric power source E. Accordingly, in response to the relay T being energized by the intermittent circuit INT.sub.2, the line being held is restored by the opening of the contact t.sub.2 and a loop circuit comprising the resistor R.sub.3 is disconnected from the lines T and R. The indicator lamp BL.sub.1 continues to light because the line L is connected to the ground. In this situation, operation of the relay H is held by the primary winding H(I) being connected to the ground.

When hand-set is put on the cradle upon termination of talking, the switch-hook HS.sub.1 is opened so that the talking circuit is disconnected from the line. At the same time, the L is disconnected from the ground. As a result current is cut off in the primary winding H(I) of the relay H and in primary winding T(I) of the relay T. At this stage, the relay H restores first. The primary winding T(I) is connected in parallel with a diode D .sup.0. . . Accordingly, when the relay T restores, an instantaneous current flows through diode D.sup.0 due to the reverse electromotive force induced in the primary winding T(I) so that the relay T restores with a delay. As a result, the relay H restores earlier than the relay T. When the relay H restores, the operation of the relay T continues in a normal state again by way of the following circuit path; ground, contact h.sup.2, relay T, transistor Tr.sub.1(C-E) and electric power source E.

Another embodiment of the present invention will now be described referring to FIG. 2 and FIG. 3. FIG. 2 illustrates a circuit similar to that of FIG. 1 with the exception that a transistor Tr.sub.3 is provided for controlling the operation of the relay T. A diode D.sub.9 is provided for preventing reverse current from flowing. R.sub.5 and R.sub.6 are resistors for limiting the base-current of transistor Tr.sub.3. The operation of the relay T will now be explained for the situation when the line is held.

In the talking condition of line No. 1, the line L is connected to ground through switch-hook HS.sub.2 of the key-telephone set so that the transistor Tr.sub.3 assumes the on-state. As a result thereof, the relay H and the relay T are energized respectively by way of the following circuit path: as to the relay H; ground, line L, transistor Tr.sub.3, diode D.sub.2, primary winding H(I) of the relay H and electric power source E and as to the relay T; ground, line L, transistor Tr.sub.3, diode D.sub.1, primary winding T(I) of the relay T and electric power source E. At this stage, when the line is held, the operation of the relay H continues as explained in the embodiment of FIG. 1, but the relay T restores due to the voltage drop of the line L. At the same time, the contacts - - - t.sup.0 t.sup.3 of the relay T restore so that the intermittent circuit INT.sub.2 for line holding is actuated and the indication lamp BL.sub.1 for line holding lights or extinguishes so as to indicate the status of line holding, for the line L is connected to ground by way of the intermittent operation of the intermittent contact int.sub.2. The transistor Tr.sub.3 switches to the off-state because the base of the transistor Tr.sub.3 is connected through diode D.sub.9 and resistor R.sub.5 to the ground intermittently so that no potential exists between the base and emitter of the transistor. As a result, the operation of the relay T is prevented due to the line L being connected to ground.

Another embodiment of the present invention will be explained with reference to FIG. 3, wherein all parts except the circuit as shown in FIG. 3 are the same as the circuit of FIG. 1. In FIG. 3 Tr.sub.4 is a transistor which controls the operation of the relay T, D.sub.10 is a diode for preventing current from flowing in the reverse direction and R.sub.7 is a resistor for limiting the base current of the transistor Tr.sub.4. When line holding is performed from the communication condition, the transistor Tr.sub.4 assumes the on-state because the base of the transistor Tr.sub.4 is connected to the ground through intermittent contact int.sub.2, diode D.sub.10 and resistor R.sub.7. As a result, both ends of the primary winding T(I) of the relay T are shorted so that operation of the relay T due to the line being connected intermittently to ground is prevented.

FIG. 4 shows another embodiment of a lamp circuit for a key telephone system connected to the line L as shown in FIG. 1. As shown in FIG. 4, a transistor Tr.sub.5 is provided for controlling indicator lamp BL.sub.1 and making the impedance of the lamp circuit connected to the line L higher, for there is the possibility of affecting on the operation of relay H and relay T when indication lamp BL.sub.1 lights due to the low impedance of the lamp circuit shown in FIG. 1. In FIG. 4 R.sub.8 is a resistor for limiting the base-current of the transistor Tr.sub.5.

Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.