GB2247135A - Telephone subscriber's instrument - Google Patents

Abstract

A telephone subscriber's instrument is operable in a handset mode and in a hands-free mode. A control circuit (14) provides switching between the two modes and also controls an electronic hookswitch (S3) whereby the instrument is connected to the line during a call. The control circuit includes a bistable latch (TR1, TR2) switchable between a conductive and a non-conductive state via a user operated switch (S2). In the conductive state of the latch the control circuit enables a loudspeaker amplification for hands-free operation. Operation of a mechanical hookswitch (S1) sets the latch to its non-conductive state for handset operation. <IMAGE>

Description

TELEPHONE SUBSCRIBER'S INSTRUMENT This invention relates to a telephone subscriber's instrument and in particular to control arrangements in such an instrument.

As telephone instruments become more sophisticated, a need has arisen for an electronic hookswitch facility to control the connection between the speech circuitry and the line. The provision of an electronic hookswitch enhances the versatility of the instrument by facilitating the introduction e.g. of hands-free operation and automatic time delayed cut-out. This latter feature is described in our co-pending application No. 90 14592.1 (I.W. Jennings 1). An electronic hookswitch facility is currently available on instruments provided with a microprocessor control. An instrument incorporating a microprocessor is however relatively costly.Furthermore, 'retrofit' incorporation of a microprocessor into an existing telephone design is in general not possible without significant modification of the instrument circuitry, nor is it possible to provide a range of instruments incorporating the same basic circuit.

An object of the invention is to minimise or to overcome this disadvantage.

A further object of the invention is to provide a low-cost electronic hookswitch facility.

According to one aspect of the invention there is provided a telephone subscriber's instrument adapted to operate in either a handset mode or a hands-free mode, the instrument including a first hookswitch, a second, electronic hookswitch whereby the instrument may be coupled to the line during a call, a further, user operable switch, and a control circuit, the arrangement being such that the second hookswitch is operated in the handset mode by closure of the first hookswitch, and wherein the control circuit is such that, in the hands-free mode, a first operation of the further switch causes the control circuit to close the second hookswitch, and a second operation of the further switch causes the control circuit to open the second hookswitch so as to terminate a call.

According to another aspect of the invention there is provided a control circuit for a telephone subscriber's instrument and adapted to control the instrument in a handset mode or in a hands-free (loudspeaking) mode, the circuit including a latch having a first (conductive) and a second (non-conductive) state, means for selectively applying voltage pulse signals to the latch whereby the latch may be switched alternately between its first and second states, and first and second outputs, the arrangement being such that, when the latch is in its conductive state, a relatively high voltage is provided at the first and second outputs, and, when the latch is in its non-conductive state, a relatively low voltage is provided in the first and second outputs, and wherein means are provided for selectively applying a relatively high external voltage to the first output independent of the latch condition whereby to enable the first output.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a schematic circuit design of a telephone subscriber's instrument incorporating an electronic hookswitch; and Fig. 2 shows the hookswitch control circuitry of the instrument of Fig. 1.

Referring to Fig. 1, the instrument is coupled to the line or subscriber loop L via a rectifier bridge 11 and includes a dialler 13 and a control circuit 14. The control circuit 14 is continuously supplied with power via a bleed resistor Rl from the positive output of the bridge 11 and is activated either by closure of a hookswitch S1 upon lifting of a handset (not shown) or by temporary closure of a subscriber operated push-button switch S2, the latter function providing for operation in a hands-free mode. Capacitor C10 functions as a reservoir for the power supply to the control circuit 14.

A further electronic hookswitch S3 controlled by the control circuit 14 is connected in the speech path between the line and a two-wire to four-wire transmission circuit 15 which circuit is in turn coupled to a transmitter transducer TX and a receiver transducer RX.

A capacitor Cll coupled to the line L provides a DC block whilst allowing the passage of alternating ringing current to e.g. a tone ringer 16.

Zener diodes D10 and Dll provide an anti-tinkle function when the instrument is used parallel with a similar instrument.

When either the hookswitch S1 is closed upon lifting the handset or the pushbutton S2 is operated, a voltage appearing at the output of the control circuit 14 closes the electronic hookswitch S3 coupling the transmission circuit to the line and allowing the passage of line current. Drawing of line current causes seizure of a line at the exchange. If either switch S1 or S2 has been operated in response to ringing, the increase in line current operates the exchange ring trip to terminate the ringing current so as to receive the call. At the same time the control circuit 14 switches a shunt 17 across the tone ringer 16 to disable the ringer.

Calls are made either by lifting the handset operating the hookswitch S1 or by operation of the switch S2, either operation resulting in closure of the electronic hookswitch S3. Termination of the call is then effected by replacement of the handset so as to open the hookswitch S1 or by a second operation of the switch S2. Either operation reduces the voltage output from the control circuit 14 so as to open the electronic hookswitch S3.

Referring now to Fig. 2, the main part of the control circuit is a latch consisting of complementary transistors TR1, TR2, and resistors R3 and R4. This is an SCR type circuit, current flow through the circuit being decided by the values of resistors R2, R3 and R4.

The switching point of the latch is determined by resistors R3 and R4. The power up state of the latch circuit, i.e. when the handset is lifted to operate S1, is the off -state, the base of transistor TRl being pulled high and the base of transistor TR2 being low, thus both transistors are in a non-conductive state.

Thus when switch S1 is operated to make a call, line current flows via diode D10 to raise the potential at the output E of the control circuit. This increase in output voltage operates the electronic hookswitch and activates the tone ringer shunt.

Alternatively a call may be initiated by operation of the switch S2. In that case the following operating sequence is obtained. Initially transistor TR3 is held in its conductive state by base current drawn via resistor R6 from the negative line.

Transistors TR5 and TR7 are both held conductive by base current drawn from the positive line via resistors R2 and R3, and transistor TR6, which is a pnp device, is held non-conductive. Operation of switch S2 draws a current resulting in a negative-going pulse across capacitor C5. This reduces the voltage at the base of pnp transistor TR1 turning that transistor partly on.

Conduction through transistor TR1 raises the base voltage of transistor TR2 turning that transistor on.

Current flow through transistor TR2 then turns transistor TRl fully on. In this condition both transistors saturate the current through them flowing via transistor TR3 and resistor R7 to ground.

As the collector voltage of transistor TR2 is now low, transistors TR5 and TR7 are turned off whilst pnp transistor TR6 is turned on charging capacitor CM to a potential about 0.8 volts below the circuit rail voltage.

With transistor TR7 turned off, its emitter voltage falls, and one terminal of capacitor C5 is bled to ground via resistors R13 and R14. This maintains a maximum potential difference across capacitor C5.

If switch S2 is now again operated, e.g. to terminate a call, this applies a positive-going pulse to the base terminals of transistors TRl and TR5. This turns transistor TR1 and hence transistor TR2, off and ay the same time, by turning transistor TR5 on, lowers the voltage across the latch reinforcing its switching off.

Switching off of the latch may also be performed by closure of the hookswitch S1 pulling base of transistor TR3 high. This turns transistor TR3 off and stops current flow causing the latch to reset to its original off state.

In the off state of the latch all other conditions are reset. Capacitor C4 being discharged by resistor R12 and the switch side of capacitor C5 being charged via transistor TR7 and resistor R13.

Switching times are determined mainly by the charging/discharging rate of capacitor C4. The switch S2 will, if held down for a long period of time, or if pressed at a rate quicker than the time constant set by capacitor C4, reset itself so that the next press of switch S2 after its release will cause switching of the latch to its opposite mode.

In a situation where there is very little current available and there is a great deal of noise, the value of resistor R10 should be large and the value of capacitor C3 should also be reasonably large. Thus there would then be little continuous current flow, typically about luA, and switching off would still be enhanced by means of the drop in voltage across the latch as it turns off. Also resistance to noise would be present. In certain applications transistor TR5, resistor R10 and capacitor C3 could be omitted.

The required control logic will vary with operating protocol requirements. A typical requirement is provided in the circuit Fig. 2 as an example. This logic function requires additional components comprising TR4 together with the associated resistors R8, R9 and capacitor C2.

With the telephone in its off or on-hook state, pressing of S2 will cause conduction via the latch as described above thus pulling point E high.

This exerts control on the electronic hookswitch S3 switching the telephone on. Current for this initial turn on stage comes from capacitor C1 (Fig. 1). Point L will also be pulled high at this time controlling the peripheral loudspeaker amplifier (not shown) turning it on. Thus the telephone is in call progress monitor mode. Lifting of the handset under these conditions will cause hookswitch S1 to conduct so that the base of transistor TR3 goes high switching that transistor off.

Thus point E will be high, the hookswitch S1 will be on and point L will now be low disabling the loudspeaker amplifier. The telephone will then be in handset mode.

Replacing of handset opens hookswitch S1 ending the call by driving the point E low so as to open the electronic hookswitch S3.

With the telephone in handset mode an alternative is available whereby, pressing of switch S2 after a time period determined by the values of resistor R8 and capacitor C2 causes the latch to turn on, current flowing via transistor TR4. Replacing the handset so as to open the hookswitch S1 will now result in transistor TR3 switching on and the point L going high, switching the loudspeaker amplifier on. As the conduction path is still held by transistor TR4 for a short period due to time delay of resistor R8 and capacitor C2, the latch will not switch off during release of the hookswitch S1. The hands-free mode will now once more be operational. A second operation of switch S2 then switches off the latch to terminate the call.

The preferred position for the control circuit is as shown in Fig. 1, in a parallel position to the dialler. Typically it is protected by Zener diode Zl and capacitor Cl in the on hook position (circuit off), current in this condition coming from the line via resistor Rl. Typically this current is about 30uA. A greater proportion of this bleed current feeds the dialler and other leakage paths. In the off hook condition, the dialler circuit is fed by both the transmission circuit's peripheral power and/or by bleed current from the circuit side of hookswitch S1 via resistor R15 and diode D2.

It will be appreciated that the circuit shown in Figure 2 may be employed as an add-on feature with existing telephone circuitry.

Claims (8)

1. A telephone subscriber's instrument adapted to operate in either a handset mode or a hands-free mode, the instrument including a first hookswitch, a second, electronic hookswitch whereby the instrument may be coupled to the line during a call, a further, user operable switch, and a control circuit, the arrangement being such that the second hookswitch is operated in the handset mode by closure of the first hookswitch, and wherein the control circuit is such that, in the hands-free mode, a first operation of the further switch causes the control circuit to close the second hookswitch, and a second operation of the further switch causes the control circuit to open the second hookswitch so as to terminate a call.

2. A telephone subscriber's instrument adapted to operate in either a handset mode or a hands-free mode, the instrument including a first hookswitch associated with a handset, a second, electronic hookswitch whereby the instrument may be coupled to the line during a call, loudspeaker means operable in the hands-free mode, a control circuit associated with the second hookswitch and with the loudspeaker means, and a further, user operable switch associated with the control circuit, the arrangement being such that, in use in the handset mode, the loudspeaker means is disabled by the control circuit and a voltage is applied via the first hookswitch to the second hookswitch whereby to close the second hookswitch and connect the instrument to the line, and that, in use in the hands-free mode with the first hookswitch open, one operation of the further switch causes the control circuit to apply a voltage to the electronic hookswitch so as to close that switch and to enable the loudspeaker means, a second operation of the further switch causing the control circuit to open the electronic hookswitch and to disable the loudspeaker means.

3. A telephone subscriber's instrument as claimed in claim 1 and 2, wherein the control circuit includes a latch settable to a conductive condition by first operation of the further switch and to a non-conductive condition by the second operation of that switch.

4. A telephone subscriber's instrument as claimed in claim 3, wherein said latch includes a complementary pair of bipolar transisters coupled in a silicon controlled rectifier (SCR) configuration.

5. A telephone subscriber's instrument as claimed in claim 3 or 4, and including delay means whereby a time delay is provided between operation of the further switch and switching of the latch.

6. A telephone subscriber's instrument substantially as described herein with reference to and as shown in the accompanying drawings.

7. A control circuit for a telephone subscriber's instrument and adapted to control the instrument in a handset mode or in a hands-free (loudspeaking) mode, the circuit including a latch having a first (conductive) and a second (non-conductive) state, means for selectively applying voltage pulse signals to the latch whereby the latch may be switched alternately between its first and second states, and first and second outputs, the arrangement being such that, when the latch is in its conductive state, a relatively high voltage is provided at the first and second outputs, and, when the latch is in its non-conductive state, a relatively low voltage is provided in the first and second outputs, and wherein means are provided for selectively applying a relatively high external voltage to the first output independent of the latch condition whereby to enable the first output.

8. A control circuit for a telephone subscriber's instrument substantially as described herein with reference to and as shown in Figure 2 of the accompanying drawings.