GB2391424A - Detecting presence of headset connected to telephone - Google Patents

Abstract

A detecting circuit 14 detects the presence or absence of a headset electrically, connected to a jack socket 7 of a telephone 1 by detecting current flow through an earpiece speaker of the headset. A switch 5 selectively and alternately couples the jack socket and a handset 3 to a telephone circuit 2 of the telephone, so that in the absence of a headset being electrically connected to the jack socket, the handset is automatically switched to the telephone circuit. A pilot voltage (VCD, fig.2) is applied through resistors (R1 ,R2) to lines (19,20), which couple the earpiece speaker through the jack socket (7) to the switch circuit (5). The voltage at a node (33) on the line (19) is applied to the inverting input of an op-amp (35) which is configured as a comparator, and which compares the voltage at the node (33) with a reference voltage. On an earpiece speaker being electrically connected into the jack socket (7) the voltage on the node (33) is high, and in the absence of an earpiece speaker, the voltage on the node (33) goes approximately to ground. The op-amp (35) outputs a low on a line (15) to a microprocessor (6) on the node (33) being high, and the op-amp (35) outputs a high when the node (33) is low. The 20 switch circuit (3) is operated under the control of the microprocessor (6) in response to the status of the line (15).

Description

iA detecting circuit for detecting the presence of a headset connected to

a telephone, and a method for detecting a headset connected to a telephone" The present invention relates to a detecting circuit for detecting if a headset of the 5 type composing an earpiece speaker is electrically connected to a telephone, and the invention also relates to a method for detecting if such a headset is electrically connected to a telephone, and in particular to a detecting circuit and a method for detecting If a headset is electrically connected to a telephone of the type which comprises a handset and a terminal through which a headset may be connected to lo the telephone. The invention also relates to a telephone comprising the detecting circuit. Telephones of the type which are typically used in conjunction with a private branch exchange (PBX), In general, comprise a telephone circuit which includes speech it, channels through which speech signals are passed through the telephone to the PBX A handset which comprises an earpiece speaker and a mouthpiece microphone is connected to the telephone circuit of the telephone, typically, by being hardwred to the telephone, or plugged into the telephone Where the handset Is plugged into the telephone, it is generally plugged in using a standard RJ11 plug and 0 socket arrangement. A quick release Jack socket provided in the telephone is coupled to the telephone circuit for facilitating connection of a headset to the telephone circuit. The headset jack socket is suitable for receiving a Jack plug of the headset, and the jack plug and socket are of the type which facilitates quick and easy disconnection of the Jack plug from the jack socket, in order to permit an as operator to disconnect the headset from the jack socket, so that the operator can temporarily leave the telephone with the headset in place on his or her head. Such headsets are generally of the type which comprise one earpiece speaker and one mouthpiece microphone, and typically require a four-wre connection to the telephone circuit, two wires being required for the earpiece speaker, and two wires JO being required for the mouthpiece microphone.

The handset and the headset socket are coupled to the telephone circuit through a switch which facilitates selective and alternate coupling of the handset and the

headset socket to the telephone circuit. The switch is set to normally connect the headset socket to the telephone circuit. A micro-swtch is located in the headset socket for mechanically detecting the presence or absence of the Jack plug of the headset in the headset Jack socket. The switch is responsive to the mcro-switch 5 detecting the absence of the headset Jack ping from the headset Jack socket for soiating the headset jack socket from the telephone circuit and for switching the handset to the telephone circuit. Thus, on an operator disconnechog the headset from the headset socket the handset is automatically switched to the telephone circuit, so that the telephone may be answered by a temporary replacement for the lo operator using the handset This arrangement has many advantages in that it permits a telephone operator to take a break from telephone answering duties and to keep the headset In his or her possession. This is particularly important for hygiene reasons. The temporary I, replacement of the operator, if he or she has their own headset, he or she can connect the headset into the jack socket of the telephone and continue to answer the telephone using the headset, and operate the PBX through the telephone Alternatively. if the temporary replacement does not have a headset, the telephone can be answered and the switchboard operated through the handset, which as 20 discussed above in the absence of a headset plugged into the headset jack socket is automatically connected to the telephone Circe it.

However, such a rrangements of a telephone with automatic switching of the F telephone circuit between the headset socket and the handset suffer from one 5 serious disadvantage The more recent generation of headsets which include an earpiece speaker, a mouthpiece microphone and a lead extending from the speaker and microphone to a jack plug for connecting to the headset jack socket of such a telephone, also include an additional Jack plug and socket for releasably connecting the lead extending to the Jack plug to the earpiece speaker and the mouthpiece So microphone. This facilitates disconnection of the headset from the lead, so that an operator can disconnect the headset from the telephone adjacent the headset, without having a long trailing lead extending from the headset when disconnected from the telephone.

The normal procedure adopted by operators with such headsets Is to disconnect the headset from the lead which extends between the headset and the Jack plug.

However7 operators tend to leave the Jack plug connected into the headset Jack socket of the telephone Thus, In such cases. although the headset has been electrically disconnected from the telephone circuit, the Jack plug remains engaged In the headset jack socket The micro-swtch In the headset Jack socket of the telephone detects the Jack plug. and thus outputs a signal to the switch Indicative of the headset being connected to the telephone. The switch therefore continues to 10 connect the jack socket to the telephone circuit with the handset isolated from the telephone circuit. Accordingly, In order for a temporal replacement for the operator to be able to use the handset, the jack plug of the headset must first physically be disengaged from the headset jack socket. This causes considerable nuisance and inconvenience. Indeed. a temporary replacement who did not have his or her own headset, and who was unfamiliar with the telephone, would find the handset dead, and would assume there was a fault In the telephone or the PBX There is therefore a need for a detecting circuit for detecting If a headset is electrically connected to a telephone, and there is also a need for a telephone To incorporating such a detecting circuit Further, there Is a need for a method for detecting if a headset is electrically connected to a telephone circuit of a telephone.

The present invention Is directed towards providing such a detecting circuit and a telephone, and the Invention is also directed towards providing such a method for - detecting If a headset is electrically connected to a telephone.

According to the Invention there is provided a detecting circuit for detecting if a headset of the type conposng an earpiece speakems electrically connected to a telephone, the detecting circuit comprising a monitoring circuit for monitoring a 30 characteristic ndcaDve of current flowing through the headset, a comparing means for comparing the monitored characteristic with a reference value for determining If the headset is electrically connected to the telephone, and for outputting an output signal for indicating at least one of the presence or absence of the headset being

electrically connected to the telephone.

In one embodiment of the Invention the output signal outputted by the comparing means Is Indicative of the absen ce of the headset. Preferably the output signal outputted by the comparing means Is of alternate values for indicating alternately the presence and absence of the headset In one embodiment of the Invention a pilot voltage is applied across a pair of headset terminals of the telephone into which the headset may be electrically connected for lo applying the pilot voltage to the earpiece speaker of the headset when the headset is electrically connected Into the headset terminals, and the monitoring circuit monitors a voltage resulting from the pilot voltage Preferably, a means for applying the pilot voltage to the headset terminals is provided.

15 In one embodiment of the invention the pilot voltage is a DC voltage. Preferably, the pilot voltage Is a low level voltage. Advantageously, the pilot voltage does not exceed ten volts, and ideally, the pilot voltage does not exceed five volts.

In one embodiment of the Inventor the pilot voltage Is applied to at least one of the JO headset terminals through an impedance element, and preferably, the monitoring means monitors the voltage developed across the Impedance element resulting from the pilot voltage. Advantageously, the Impedance element Is connected to ground, so that the voltage developed across the Impedance element resulting from the pilot voltage is referenced to ground in one embodiment of the inverton the comparing means comprises a comparator which compares the monitored voltage developed across the impedance element resulting from the pilot voltage with the reference value, which Is provided by a reference voltage.

In another embodiment of the Invention the comparator outputs the output signal indicative of one of the presence and absence of the headset when the monitored voltage crosses the reference voltage

- In a further embodiment of the inventon the comparator is implemented by an operational amplifier (op-amp) configured as a comparator Preferably, the monitored voltage Is applied to the Inverting Input of the op-amp. Advantageously, the reference voltage is applied to the non- nvertng Input of the op-amp i In one embodiment of the invention the reference voltage Is derived from the pilot voltage through a voltage divider circuit. Preferably, the value of the reference voltage is below the value of the monitored voltage which would be ndcaLve of the lo presence of the headset. Advantageously, the reference voltage does not exceed 50% of the value of the monitored voltage which would be Indicative of the presence of the headset.

Preferably, a pair of impedance elements are provided' and the pilot voltage is 1- applied to the headset terminals of the telephone through the two Impedance elements Preferably, one of the Impedance elements couples one of the headset terminals of the telephone to ground, and the other impedance element couples the other of the headset terminals of the tel ephone to the pilot voltage. Advantageously, the impedance elements are of similar i mpedance value.

In one embodiment of the invention each impedance element Is provided by a resistor. In another embodiment of the invention the detecting circuit is adapted for detecting -.

as the presence or absence of a headset of the type comprising an earpiece speaker having an Inductive coil through which a current flows In response to a voltage applied thereto.

The Invention also provides a telephone comprising a telephone circuit, a handset to and at least two headset terminals for receiving a headset of the type compnsng an earpiece speaker, a switch means for selectively and alternately connecting the headset terminals and the handset to the telephone circuit, and a detecting circuit according to the invention for detecting at least one of the presence or absence of a

headset electrically connected to the headset terminals, the switch means being responsive to the output signal from the detecting circuit for selectively connecting the handset to the telephone circuit In response to the output signal from the detecting circuit indicating the absence of the headset being electrically connected to the headset terminals Further, the invention provides a telephone comprising a telephone circuit, a handset and at least two headset terminals for receiving a headset of the type comprising an earpiece speaker, a detecting circuit for detecting one of the presence or absence of If) a headset being electrically connected to the headset terminals, the detecting circuit comprising a monitoring circuit for montonng a charactenstc indicative of current flowing through the headset, a comparing means for comparing the monitored charactenshc with a reference value for determining if the headset Is electrically connected to the headset terminals, and for outputting an output signal for indicating at least one of the presence or absence of the headset being electrically connected to the headset terminals, and a switch means for selectively and alternately connecbog the headset terminals and the handset to the telephone circuit, the switch means being responsive to the output signal from the comparing means for connecting the handset to the telephone circuit, when the output signal Is indicative 30 of the absence of the headset.

The invention also provides a method for detecting the presence of a headset of the type composing an earpiece speaker electrically connected to a telephone circuit of a telephone, the method comprising the steps of applying a pilot voltage across a pair of headset terminals to which the headset may be connected for connecting the headset to the telephone circuit, montonng a characteristic indicative of current flowing through the headset in response to the pilot voltage, and comparing the characteristic with a reference value for determining if a headset is electrically connected to the telephone, and for outputting an output signal indicative of at least 30 one of the presence and absence of the headset being electrically connected to the telephone circuit In one embodiment of the Invention the pilot voltage is applied to one of the headset

terminals through an Impedance element, and a voltage developed across the Impedance element resulting from the pilot voltage Is monitored for determining at least one of the presence and absence of the headset being electrically connected to the telephone circuit.

- In another embodment of the nvenhon the mpedance element to whch the plot voltage Is applied is connected to ground.

In a further embodiment of the Invention the monitored voltage is compared with the reference value provided by a reference voltage, and on the monitored voltage crossing the reference voltage the output signal Indicates one of the presence and absence of the headset.

The invention will be more clearly understood from the following description of a

1 preferred embodiment thereof, which Is given by way of example only, with reference to the accompanying drawings in which: Fig 1 is a block re presentation of a telephone according to the invention, 3 Fig. 2 Is a circuit d iagram of a portion of a circuit of the telephone of Fig. 1 illustrating a detecting circuit also according to the invention for detecting the presence or absence of a headset, fig. 3 is a circuit diagram similar to Fig. 2 of a portion of a circuit of a telephone according to another embodiment of the invention, and Fig 4 Is a circuit d iagram similar to Fig 2 of a portion of a circuit of a telephone according to a further embodiment of the invention.

-.() Refernng initially to figs 1 and 2, there Is 1iustrated a telephone according to the invention indicated generally by the reference numeral 1. The telephone 1 Is of the type which is typically used In conjunction with a private branch exchange (PBX) (not shown) and comprises a telephone circuit 2 which includes speech channels through

which speech signals are passed through the telephone to the PBX. Signals from a keyboard (not shown) on the telephone 1 are also passed through the telephone urcut 2 to the PBX for signaling to the PBX extension and exchange lines which are to be connected. A handset 3 Is connected to the telephone 1 through a plug/socket 4, a nd Is selectively coupled to the telephone circuit 2 through a switch means, namely, a four track two-way switch 5. In this embodiment of tine invention the plug/socket 4 is an RJ11 type plug/socket. A microprocessor 6 controls the operation of the telephone 1 A Jack socket 7 having four terminals 8, 9, 10 and 11 Is provided In the telephone 1 for receiving a jack plug of a headset (not shown). In JO this embodiment of the invention the Jack socket 7 Is an RJ11 socket. The jack socket 7 is selectively coupled to the telephone circuit 2 through the two-way switch 5. A detecting circuit also according to the invention indicated generally by the reference numeral 14 is provided for detecting the presence or absence of a headset electrically connected to the Jack socket 7, and for outputting an output signal on a l: line 15 to the microprocessor 6 fomndicatng if a headset Is electrically connected to the Jack socket 7. The two-way switch 5 Is normally switched for connecting the jack socket 7 to the telephone circuit 2, with the handset 3 Isolated from the telephone circuit 2 The switch 5 is responsive to the microprocessor 6 for selectively switching the handset 3 to the telephone circuit 2 in response to the output signal from the So detecting circuit 14 Indicating the absence of the headset. When the handset 3 is switched to the telephone circuit 2, the Jack socket 7 is isolated from the telephone circuit 2 Before describing the detecting circuit 14 in further detail, the type of headset to wh ich the detecting circuit 14 Is particularly adapted for detect) ng will first be descnbed. The headset is of the type which comprises a mouthpiece microphone (not shown) and an earpiece speaker (also not shown). A Jack plug (not shown) is provided at the end of a lead from the headset for connecting the headset Into the Jack socket 7. When connected into the Jack socket 7, the mouthpiece microphone 30 Is connected to the telephone circuit 2 through the terminals 8 and 9, while the earpiece speakems connected to the telephone circuit 2 through the terminals 10 and 11. Additionally, the headset is of the type which permits the earpiece speaker and the mouthpiece microphone to be disconnected from the lead extending from

the Jack plug so that the Jack plug with the associated lead can remain In the jack socket 7 with the headset, the mouthpiece microphone and the earpiece speaker electrically disconnected from the jack plug. Typically, a jack plug and socket are provided adjacent the headset for facilitating disconnection of the headset from the lead and the Jack plug The earpiece speaker composes an inductive coil having a resistance of the order of a few hundred ohms, thereby facilitating current flow therethrough when a voltage Is applied tc' the earpiece speaker Refemng now to Fig 2, the detecting circuit 14 according to the Invention will now be JO described The Jack socket 7 Is connected to the switch 5 through a four wire bus 16, comprising lines 17, 18, 19 and 20 The lines 17 and 18 connect the terminals 8 and 9 to the switch 5 while the lines 19 and 20 connect the terrnnals 1 0 and 11 to the switch 5. A bus 21 comprising four corresponding lines 22, 23, 24 and 25 extend from the switch 5 to the telephone circuit 2. A bus 26 composing four corresponding lines 27, 28, 29 and 30 extend from the switch 5 to the handset 3.

A low level DC pilot voltage, in this embodiment of the invention, a five volt pilot voltage VCD Is applied to the lines 19 and 20 through a pair of impedance elements, namely, resistors R1 and R2, each of 100 Kohms. The resistor R1 Is connected go between the line 19 and ground, while pilot voltage VCD is applied to the line 20 through the resistor R2 in order to cause a current to flow through the inductive coil of the earpiece speaker of a headset when the headset is electrically connected to the Jack socket 7 A montonng circuit 32 of the detecting circuit 14 is connected to a node 33 on the line 19 adjacent the resistor R1 for monitoring the voltage across the resistor R1 between the line 19 and ground. If the earpiece speaker of the headset Is electrically connected to the terminals 10 and 11 of the Jack socket 7, a current of approximately 25 mcroamps flows through the Inductive coil of the earpiece to speaker, thus developing a voltage at the node 33 across the resistor R1 with reference to ground of approximately 2 6 volts.

The voltage at the node 33 Is applied to the inverting input of an operational amplifier

(op-amp) 35, which Is configured as a comparator A reference voltage which Is derived through a voltage divider comprising resistors R3 and R4 from the pilot voltage VCD is applied to the non-'nvertng Input of the op- amp 35. The values of the resistors R3 and R4 are selected so that the reference voltage applied to the 5 non-inverting Input of the op-amp 35 will be less than the voltage at the node 33 when the headset is electrically connected to the jack socket 7. In this ernbodment of the Invention the resistor R3 Is of 33 Kohms, while the resistor R4 is of 10 Kohms, thus giving a reference voltage of approximately 1 5 volts The output of the op-a mp 35 Is applied to the line 15, which Is read by the microprocessor 6. A pull-up resistor R5 of 15 Kohrns coupled to the pilot voltage VCD pulls the output of the op-amp 35, and In turn the line 15 high for as long as the voltage on the Inverting input of the op-amp 35 remains below the reference voltage.

In other words for as long as the voltage on the node 33 remains at about ground, 15 and well below the reference voltage of 1.5 volts, thereby indicating the absence of an earpiece speaker, and In turn the absence of a headset electrically connected to the jack socket 7, the line 15 remains high On the voltage on the Inverting input of the op-amp 35 crossing over, and exceeding the reference voltage, the output of the op- amp 35 goes low, thus pulling the output voltage on the line 15 low. Accordingly, So a low on the line 15 is indicative of the presence of a headset electrically connected to the jack socket 7, whlie a high Is indicative of the absence of a headset. The microprocessor 6 ope rates the switch 5 appropriately in response to the signal on the line 15.

A resistor R6 of 100 Kohms forms a feedback loop across the op-amp 35 to provide hystenses protection in order to prevent spurious outputs. A pair of diodes D1 and D2, which are coupled In series between the pilot voltage VCD and ground, protect the inverting input of the op-amp 35 by shorting any externally Induced static voltages in the line 19.

A pair of b'-polar electrolytic capacitors C1 and C2 are provided in the lines 19 and 20 between the resistors R1 and R2 and the switch 5 fomsolating the DC pilot voltage VCD from the switch 5, and In turn from the telephone circuit 2, so that

speech signals are unaffected by the pilot voltage VCD and the detecting circuit 14 Capacitors C3 and C4 couple the lines 17 and 18 to ground for providing electro-

magnetc compatibility (EMC) protection. A common mode choke L1 coupled to the lines 19 and 20 also provide EMC protection, particularly against RF signals picked up by the headset lead from atmospheric borne RF transmission signals In use, the switch 5 normally switches the Jack socket 7 directly to the telephone circuit 2. The voltage at the node 33 developed across the resistor R1 in response l o to the pilot voltage VCD Is applied to the Inverting input of the op-anap 35. For as long as an earpiece speaker is electrically connected to the jack socket 7, the voltage at the node 33 remains at approximately 2.6 volts. This, thus, holds the output voltage on the inane 15 low. The microprocessor 6 in response to the low on the line 15 holds the switch 5 with the jack socket 7 connected to the telephone 15 circuit 2, and the handset 3 isolated from the telephone circuit 2. In the event of the earpiece speaker being electrically disconnected, the terminals 10 and 11 go open circuit, and thus, the voltage at the node 33 Is pulled approximately to ground. This, thus, causes the output voltage on the line 15 to go high. On the microprocessor 6 detecting the high on the line 15, the microprocessor 6 operates the switch 5 for ho connecting the handset 3 to the telephone circuit 2, and isolating the jack socket 7 from the telephone circuit 2. For as long as the earpiece speaker remains electrically disconnected, the voltage on the node 33 remains at approximately ground, and thus, the handset 3 Is switched through to the telephone circuit 2. On the earpiece speaker being reconnected, the voltage on the node 33 rises to > approximately 2.6 volts, thus causing the output voltage on the line 15 to again go low. On detecting the low on the inane 15, the microprocessor 6 operates the switch 5 for reconnecting the Jack socket 7 to the telephone circuit 2, and Isolating the handset 3 therefrom 30 The advantages of the invention are many, a particularly Important advantage of the invention is that by virtue of the fact that the detecting circuit 14 accurately detects disconnection of a headset from the Jack socket 7, the handset 3 is automatically switched to the telephone circuit 2 In the absence of a headset. This avoids the

need to manually switch the handset 3 to the telephone circuit 2, which otherwise would be required, and would cause considerable inconvenience. A particularly Important advantage of the Invention Is achieved by virtue of the fact that the detecting circuit 14 detects the presence or absence of the headset by detecting flow of current through the earpiece speaker. This has the particularly important advantage that If the headset Is of the type which permits the earpiece speaker and the mouthpiece rncrophone to be separated from the Jack plug of the headset, the handset 3 Is automatically switched to the telephone circuit 2 in the absence of the earpiece speaker being detected, even though the Jack plug of the headset may 10 remain engaged in the jack socket 7. Accordingly, irrespective of whether the headset Is disconnected by disconnecting the Jack plug from the jack socket 7, or by disconnecting the headset from the Jack plug, the handset is still automatically switched directly to the telephone circuit in the absence of the headset. This overcomes the disadvantage of prior art detecting circuits which comprise a micro-

switch in the jack socket for mechanically detecting the presence or absence of a Jack plug In the jack socket.

Referring now to Fig. 3, there Is illustrated a circuit diagram of a portion of a circuit 40 of a telephone according to another embodiment of the invention. The circuit 40 go of Fig. 3 is substantially similar to the circuit of Fig. 2, and similar components are identified by the same reference numerals. A detecting circuit 14 similar to the detecting circuit 14 described already with reference to Fig. 2 is also provided for detecting the presence or absence of a headset. The only difference between the circuit 40 and the circuit of Fig. 2 Is that Instead of providing EMC protection by the : common mode choke, the EMC protection in the lines 19 and 20 is provided by a pair of inductance coils L2 and L3 The inductance coils L2 and L3 are of Impedance which is such as to provide a high Impedance to RF signals, but a low impedance to audio signals between the earpiece and the telephone circuit.

to Otherwise, the telephone of fig. 3 and its operation are similar to the operation of the telephone described with reference to Figs. 1 and 2.

Referring now to Fig 4, there is illustrated a circuit diagram of a portion of a circuit

50 of a telephone according to another embodiment of the invention. The circuit 50 of fig. 4 is substantially similar to the circuit of Fig 2, and similar components are Identified by the same reference numerals. A detecting circuit 14 similar to the detecting circuit 14 described already with reference to fig. 2 is also provided for detecting the presence or absence of a headset. The only difference between the circuit 50 and the circuit of Fig. 2 Is that Instead of providing EMC protection by the common mode choke, the EMC protection in the lines 19 and 20 Is provided by a pair of resistors R7 and R8.

10 Otherwise, the telephone of Fig 4 and its operation are similar to the operation of the telephone described with reference to figs. 1 and 2.

The EMC protection against RF signals picked up by the lead of the headset provided by the common mode choke L1,l lustrated In Fig. provides optimum EMC protection, however, it is a relatively expensive option by comparison to the EMC protection circuit comprising the high Impedance coils L2 and L3 of the circuit of Fig. 3, which does not provide quite as good EMC protection against RF signals, although it is a less expensive option. The EMC protection offered by the resistors R7 and R8 of the circuit of Fig. 4 provides minimum EMC protection against RF 7 signals. While the voltage resulting from the pilot voltage VCD has been described

as being monitored across the resistor connecting cane of the terminals of the Jack socket to ground, it will be readily apparent to those skilled in the art that the voltage being :> monitored could have been the voltage developed across the resistor connecting the other terminal of the Jack socket to the pilot voltage VCD While specific values of resistors, voltages and currents have been described, it will be readily apparent to those skilled in the art that any other suitable values of JO resistors, voltages and currents may be used.

While the comparator has been described as being implemented by an op-amp, any other suitable comparator may be used

While the detecting circuit has been described as detecting current flow through an earpiece speaker of a headset, it is envisaged, in certain cases, that the detecting circuit may detect the presence or absence of a headset by detecting current flow through a mouthpiece microphone of a headset, depending on the type of mouthpiece microphone used.

Claims (58)

1< Claims

1. A detecting circuit for detecting if a headset of the type comprising an earpiece speaker is electrically connected to a telephone, the detecting circuit composing a monitoring circuit for monitoring a charactenstc indicative of current flowing through the headset, a comparing means for comparing the monitored characteristic with a reference value for determining if the headset is electrically connected to the telephone, and for outputting an output signal for indicating at least one of the presence or absence of the headset being electrically connected to the telephone 1()

2. A detecting circuit as claimed in Claim 1 In which the output signal outputted by the comparing means is indicative of the absence of the headset

3. A detecting circuit as claimed in Claim 1 or 2 in which the output signal outputted by the comparing means Is of alternate values fomodicatng alternately the presence and absence of the headset.

4. A detecting circuit as claimed In any preceding claim in which a pilot voltage Is applied across a pair of headset terminals of the telephone into which the headset aft may be electrically connected for applying the plot voltage to the earpiece speaker of the headset when the headset Is electrically connected Into the headset terminals, and the montonng circuit monitors a voltage resulting from the pilot voltage.

5. A detecting circuit as claimed in Claim 4 In which a means for applying the pilot voltage to the headset terminals is provided

6 A detecting circuit as claimed in Claim 4 or 5 in which the pilot voltage Is a DC voltage to

7. A detecting circuit as claimed in any of Claims 4 to 6 In which the pilot voltage is a low level voltage

8. A detecting circuit as claimed in any of Claims 4 to 7 in which the pilot voltage

does not exceed ten volts.

9. A detecting circuit as claimed in Claim 8 In which the pilot voltage does not exceed five volts.

10 A detecting circuit as claimed In any of Claims 4 to 9 In which the pilot voltage Is applied to at least one of the headset terminals through an Impedance element.

11 A detecting circuit as claimed In Claim 10 in which the monitoring means i' monitors the voltage developed across the impedance element resulting from the pilot voltage.

12. A detecting circuit as claimed in Claim 10 or 11 in which the impedance element is connected to ground, so that the voltage developed across the Impedance element resulting from the pilot voltage Is referenced to ground.

13 A detecting circuit as claimed in any of Claims l O to 12 in which the comparing means comprises a comparator which compares the monitored voltage developed across the impedance element resulting from the pilot voltage with the to reference value, which is provided by a reference voltage.

14. A detecting circuit as claimed in Claim 13 in which the comparator outputs the output signal Indicative of one of the presence and absence of the headset when the monitored voltage crosses the reference voltage.

s

15. A detecting circuit as claimed In Claim 13 or 14 In which the comparatoms Implemented by an operational amplifier (op-amp) configured as a comparator

16. A detecting circuit as claimed in Claim 15 in wh ich the monitored voltage Is applied to the inverting Input of the op-amp.

17. A detecting circuit as claimed In Claim 15 or 16 in which the reference voltage is applied to the non-'nvertng input of the op-a Imp.

18 A detecting circuit as clanged in any of Claims 13 to 17 In which the reference voltage Is denved from the pilot voltage through a voltage divider circuit.

19 A detecting circuit as claimed In any of Claims 13 to 18 In which the value of the reference voltage Is below the value of the monitored voltag e which would be Indicative of the presence of the headset

20 A detecting circuit as claimed In any of Claims 13 to 19 i n which the reference lo voltage does not exceed 50% of the value of the monitored voltage which would be indcatve of the presence of the headset.

21 A detecting circuit as claimed In any of Claims 10 to 20 in which a pair of Impedance elements are provided, and the pilot voltage Is appli ed to the headset I s terminals of the telephone through the two impedance elements.

22 A detecting circuit as claimed in Claim 21 in which one of the Impedance elements couples one of the headset terminals of the telephone to ground, and the othemmpedance element couples the other of the headset terminals of the telephone to the pilot voltage.

23 A detecting circuit as cleaned In Claim 21 or 22 in which the Impedance elements are of similar impedance value.

24 A detecting circuit as clowned in any of Claims 10 to 23 In which each rnpedance element is provided by a resistor.

25 A detecting circuit as Planned In any of Claims 4 to 24 In which the detecting circuit Is adapted for detecting the presence or absence of a headset of the type 30 connprisng an earpiece speaker having an inductive coil through which a current flows In response to a voltage applied thereto.

26 A detecting circuit for detecting if a headset of the type comprising an

earpiece speaker Is electrically connected to a telephone, the detecting circuit being substantially as described herein with reference to and as illustrated in the accompanying drawings.

27. A telephone comprising a telephone circuit, a handset and at least two headset terminals for receiving a headset of the type comprising an earpiece speaker, a switch means for selectively and alternately connecting the headset terminals and the handset to the telephone circuit, and a detecting circuit as claimed in any preceding claim for detecting at least one of the presence or absence of a IQ headset electrically connected to the headset terminals, the switch means being responsive to the output signal from the detecting circuit for selectively connecting the handset to the telephone circuit In response to the output signal from the detecting circuit Indicating the absence of the headset being electrically connected to the headset terminals

28. A telephone comprising a telephone circuit, a handset and at least two headset terminals for receiving a headset of the type comprising an earpiece speaker, a detecting circuit for detecting one of the presence or absence of a headset being electrically connected to the headset terminals, the detecting circuit So comprising a monitoring circuit for monitoring a characteristic indicative of current flowing through the headset, a comparing means for comparing the monitored characteristic with a reference value for determining if the headset is electrically connected to the headset terminals, and for outputting an output signal for Indicating at least one of the presence or absence of the headset being electrically connected 2 to the headset terminals, and a switch means for selectively and alternately connecting the headset terminals and the handset to the telephone circuit, the switch means being responsive to the output signal from the comparing means for connecting the handset to the telephone circuit, when the output signal is indicative of the absence of the headset.

29. A telephone as claimed In Claim 28 In which the output signal outputted by the comparing means is indicative of the absence of the headset.

30. A telephone as claimed in Claim 28 or 29 In which the output signal outputted by the comparing means is of alternate values for indicating alternately the presence and absence of the headset

31. A telephone as claimed In any of Claims 28 to 30 In which a pilot voltage is applied across the headset terminals for applying the pilot voltage to an earpiece speaker of the headset when the headset Is electrically connected to the headset terminals, and the monitoring circuit monitors a voltage resulting from the pilot voltage.

32. A telephone as claimed in Claim 31 in which a means for applying the pilot voltage to the headset terminals Is provided

33. A telephone as claimed In Claim 31 or 32 in which the pilot voltage is a DC voltage

34. A telephone as claimed in any of Claims 31 to 33 In which the pilot voltage Is a low level voltage.

JO

35 A telephone as claimed in any of Claims 31 to 34 in which the pilot voltage does not exceed ten volts.

36. A telephone as claimed In Claim 35 In which the pilot voltage does not exceed five volts -

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37. A telephone as claimed In any of Claims 31 to 36 in which the pilot voltage is applied to at least one of the headset terrnnais through an impedance element. -=

38 A telephone as claimed In Claim 37 In which the monttonng circuit monitors 30 the voltage developed across the Impedance element resulting from the pilot voltage.

39 A telephone as claimed In Claim 37 or 38 in which the impedance element is

connected to ground, so that the voltage developed across the impedance element resulting from the pilot voltage Is referenced to ground.

40 A telephone a s claimed in any of Claims 37 to 39 in which the comparing means comprises a comparator which compares the monitored voltage developed across the impedance element resulting from the pilot voltage with the reference value, which Is provided by a reference voltage.

41. A telephone as claimed In Claim 40 in which the comparator outputs the 10 output signal Indicative of one of the presence and absence of the headset when the monitored voltage crosses the reference voltage.

42 A telephone as claimed in Claim 40 or 41 in which the comparator is implemented by an operational amplifier (op-amp) configured as a comparator.

43. A telephone as claimed In Claim 42 in which the monitored voltage is applied to the inverting input of the op-amp.

44. A telephone as claimed In Claim 42 or 43 in which the reference voltage is 0 applied to the non-'nvertng Input of the op-amp

45. A telephone as claimed in any of Claims 40 to 44 in which the reference voltage is derived from the pilot voltage through a voltage divider circuit.

46 A telephone as claimed in any of Claims 40 to 45 in which the value of the reference voltage is below the value of the monitored voltage which would be Indicative of the presence of the headset.

47. A telephone as claimed in any of Claims 40 to 46 in which the reference 0 voltage does not exceed 50% of the value of the monitored voltage which would be indicative of the presence of the headset.

48. A telephone as claimed in any of Claims 37 to 47 In which a pair of

impedance elements are provided. and the pilot voltage Is applied to the headset terminals through the two Impedance elements.

49 A telephone as claimed In Claim 48 In which one of the impedance elements couples one of the headset terminals to ground, and the other impedance element couples the other of the headset terrnnals to the pilot voltage.

50 A telephone as claimed in Claim 48 or 49 In which the impedance elements are of similar impedance value.

51 A telephone as claimed in any of Claims 48 to 50 in which each impedance element is provided by a resistor

52 A telephone as claimed in any of Claims 28 to 51 In which the telephone is 15 adapted for receiving a headset of the type comprising an earpiece speaker having an inductive coil through which a current flows in response to a voltage applied thereto.

53. A telephone substantially as described herein with reference to and as do Illustrated In the accompanying drawings.

54. A method for detecting the presence of a headset of the type composing an earpiece speaker electrically connected to a telephone circuit of a telephone, the method comprising the steps of applying a pilot voltage across a pair of headset 5 terminals to which the headset may be connected for connecting the headset to the telephone circuit, monitoring a characteristic indicative of current flowing through the headset in response to the pilot voltage, and comparing the monitored characteristic with a reference value for determining If a headset is electrically connected to the telephone, and for outputting an output signal Indicative of at least one of the 30 presence and absence of the headset being electrically connected to the telephone circuit.

55. A method as claimed in Claim 53 in which the pilot voltage is applied to one

- - of the headset terminals through an Impedance element, and a voltage developed across the impedance element resulting from the pilot voltage is monitored for determining at least one of the presence and absence of the headset being electrically connected to the telephone circuit -

56 A method as clamed n Clam 55 n whch the impedance element to whch the pilot voltage Is applied Is connected to ground.

57 A method as claimed In Claim 55 or 56 In which the monitored voltage Is 0 compared with the reference value provided by a reference voltage, and on the monitored voltage crossing the reference voltage the output signal indicates one of the presence and absence of the headset.

58. A method for detecting the presence of a headset of the type comprising an I S earpiece speaker electrically connected to a telephone circuit of a telephone, the method being substantially as described herein with reference to and as illustrated in the accompanying drawings.