GB1561392A - Current detector circuit for a telephone line - Google Patents

Description

(54) A CURRENT DETECTOR CIRCUIT FOR A TELEPHONE LINE (71) We, THE PLESSEY COMPANY, LIMITED, a British Company of 2/60 Vicarage Lane, Ilford, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a current detector circuit for a telephone line.

According to the present invention a cur rent detector circuit for a telephone line includes two light emitting diodes connected in parallel with each other and with one side of the line in a manner whereby one of the light emitting diodes is caused to emit light irrespective of the direction of current flow in the line, each light emitting diode being situated adjacent to a separ ate photo-transistor included in a current integration circuit, the current integration circuit having a time constant longer than the breaks that occur in the line due to dialling, current limiting means for limiting the amount of line current flowing through each light emitting diode; and con trol means for controlling the current sensitivity of the detector circuit.

With this arrangement of the current detector circuit, the current limiting means and control means are adapted to cater for current flowing in both directions.

In a preferred arrangement, the current integration circuit is a Miller integratic;!l circuit.

The foregoing and other features according to the invention will be better understood from the following description of a preferred embodiment of the invention given by way of example only with reference to the single figure of the drawings accompanying the Provisional Specification which illustrates a circuit diagram of a current detector circuit for a telephone line according to the invention.

Referring to the single figure of the drawings, the detector circuit illustrated therein includes two series connected diodes 1)1 and D2 which are poled in the same direction and connected in series with one side of a telephone line 1, and two series connected diodes D3 and D4 which are poled in the same direction and connected in parallel with the diodes D1 and D2. The diodes D1 and D2 are poled in the opposite direction to the diodes D3 and D4.

The diodes D3 and D4 are shunted by a resistance R1 connected in series with two parallel connected light emitting diodes D5 and D6 which are poled in opposite directions. The light emitting diode D5 is poled in the same direction as the diodes D1 and D2, and the light emitting diode D6 is poled in the same direction as the diodes D3 and D4.

A resistance R2 is connected in parallel with the resistance Rl/diodes D5, D6 combination.

The diodes D5 and D6 are respectively adjacent to, but electrically isolated from, photo-transistors T1 and To w'ilich form part of a Miller integradon circuit.

The gates of the photo-transistors T1 and T2 are connected together and to a negative (-Ve) electrical supply lead via a resistance R4. The emitters of the phototransistors T1 and T2 are connected directly to the Ve electrical supply lead whilst the collectors thereof are connected together and to a positive !(+Ve) electrical supply lead via a resistance R3. The col Sectors of the photo-transistors T1 and T2 are also connected to the gates thereof via a capacitance C1 and to an output terminal 2.

When a telephone instrument (not illustrated) associated with the telephone line 1 goes 'off-hook' i.e. when the handset of the telephone instrument is removed - in order to make a call, a current is caused to flow in the line 1 and this current will flow through the resistance R1 and either the light emitting diode D5 or the light emitting diode D6 depending on the direction of current flow in the line 1.

The current that can be carried by the light emitting diodes D5 and D6 for a long life must be much less than the current flowing in the line 1, therefore, the current flowing in the diodes D5 and D6 is limited respectively by the diodes D1, D2 and the diodes D3, D4. The diode arrangement is such that relatively small line currents flow through the resistance fR1 and either the diode D5 or the diode D6 depending on the direction of current flow, the voltage drop across this combination being insufficient to cause current to flow through, depending on the direction of current flow in the line 1, either the diodes D, D2 or the diodes D3, D4.

However, relatively large line currents cause a higher voltage drop across the resistance Rl/diode D5, 1)6 combination and thereby cause the majority of the line current to flow through either the diodes Do, do or the diodes D3, 1)4 depending on the direction of current flow.

The sharing of the current between the diodes Di, 1)2 and D5 'or the diodes D3, D4 and D6 is controlled by the resistance RI i.e. the ohmic value of the resistance R1 determines how much of the line current is carried by each of the diodes.

The current sensitivity of the detector circuit is controlled by the resistance R2.

One of the light emitting diodes D5 or D6 is, therefore, caused to emit light when a subscriber connected to the line 1 goes 'off-hook', and the emitted light is detected by the associated one of the phototransistors T1, T2. The detection of the emitted light by the associated one of the photo-transistors T1, T2 causes this transistor to conduct and thereby the Miller integration circuit to operate in a known manner to produce a signal at the output terminal 2.

The time constant of the Miller integration circuit is longer than the relatively short breaks that occur in the telephone line l due for example, to dialling, and the signal at the output terminal 2 is, therefore, maintained whilst the subscriber associated with the line 1 is setting up a call to say another subscriber. The breaks that occur in the telephone line 1 due for example, to dialling are detected by equipment not illustrated in the single figure of the drawings for utilisation in the setting up of a call to say another subscriber.

Any breaks that occur in the line 1 that are of a duration longer than the time con stant of the Miller integration circuit will of course cause a break in the output of the integration circuit.

The light emitting diodes 1)5 and 1)6 are respectively optically coupled to but electrically isolated from the photo-transistors T1 and T2, and irrespective of the direction of current flow in the telephone line 1, an output signal representative of an 'off-hook' condition of the telephone instrument associated wth the line 1 will be produced at the output terminal 2, and will be maintained during dialling.

It should be noted that the detector circuit outlined in preceding paragraphs is adapted to cater for current flow in both directions in the line 1. If, however, the 'operating conditions are such that line currents will flow only in one direction then, depending on the direction of current flow either the diodes D1, D2 and D5 or the diodes D3, D4 and D6 couId be eliminated together with the associated one of the photo-transistors T1 and T2.

It should also be noted that whilst the Miller integration circuit is preferred, any current integration circuit could be utilised in the detector circuit according to the invention provided that its time constant is of the right order.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation in the scope of the invention which is defined by the appended claims.

WHAT WE CLAIM IS: - 1. A current detector circuit for a telephone line including two light emitting diodes connected in parallel with each other and with one side of the line in a manner whereby one of the light emitting diodes is caused to emit light irrespective of the direction of current flow in the line, each light emitting diode being situated adjacent to a separate photo-transistor included in a current integration circuit, the current integration circuit having a time constant longer than the breaks that occur in the line due to dialling, current limiting means for limiting the amount of line current flowing through each light emitting diode, and control means for controlling the current sensitivity of the detector circuit.

2. A detector circuit as claimed in claim 1 in which the current integration circuit is a Miller integration circuit.

3. A detector circuit substantially as described with reference to the drawing ac companving the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. life must be much less than the current flowing in the line 1, therefore, the current flowing in the diodes D5 and D6 is limited respectively by the diodes D1, D2 and the diodes D3, D4. The diode arrangement is such that relatively small line currents flow through the resistance fR1 and either the diode D5 or the diode D6 depending on the direction of current flow, the voltage drop across this combination being insufficient to cause current to flow through, depending on the direction of current flow in the line 1, either the diodes D, D2 or the diodes D3, D4. However, relatively large line currents cause a higher voltage drop across the resistance Rl/diode D5, 1)6 combination and thereby cause the majority of the line current to flow through either the diodes Do, do or the diodes D3, 1)4 depending on the direction of current flow. The sharing of the current between the diodes Di, 1)2 and D5 'or the diodes D3, D4 and D6 is controlled by the resistance RI i.e. the ohmic value of the resistance R1 determines how much of the line current is carried by each of the diodes. The current sensitivity of the detector circuit is controlled by the resistance R2. One of the light emitting diodes D5 or D6 is, therefore, caused to emit light when a subscriber connected to the line 1 goes 'off-hook', and the emitted light is detected by the associated one of the phototransistors T1, T2. The detection of the emitted light by the associated one of the photo-transistors T1, T2 causes this transistor to conduct and thereby the Miller integration circuit to operate in a known manner to produce a signal at the output terminal 2. The time constant of the Miller integration circuit is longer than the relatively short breaks that occur in the telephone line l due for example, to dialling, and the signal at the output terminal 2 is, therefore, maintained whilst the subscriber associated with the line 1 is setting up a call to say another subscriber. The breaks that occur in the telephone line 1 due for example, to dialling are detected by equipment not illustrated in the single figure of the drawings for utilisation in the setting up of a call to say another subscriber. Any breaks that occur in the line 1 that are of a duration longer than the time con stant of the Miller integration circuit will of course cause a break in the output of the integration circuit. The light emitting diodes 1)5 and 1)6 are respectively optically coupled to but electrically isolated from the photo-transistors T1 and T2, and irrespective of the direction of current flow in the telephone line 1, an output signal representative of an 'off-hook' condition of the telephone instrument associated wth the line 1 will be produced at the output terminal 2, and will be maintained during dialling. It should be noted that the detector circuit outlined in preceding paragraphs is adapted to cater for current flow in both directions in the line 1. If, however, the 'operating conditions are such that line currents will flow only in one direction then, depending on the direction of current flow either the diodes D1, D2 and D5 or the diodes D3, D4 and D6 couId be eliminated together with the associated one of the photo-transistors T1 and T2. It should also be noted that whilst the Miller integration circuit is preferred, any current integration circuit could be utilised in the detector circuit according to the invention provided that its time constant is of the right order. It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation in the scope of the invention which is defined by the appended claims. WHAT WE CLAIM IS: -

1. A current detector circuit for a telephone line including two light emitting diodes connected in parallel with each other and with one side of the line in a manner whereby one of the light emitting diodes is caused to emit light irrespective of the direction of current flow in the line, each light emitting diode being situated adjacent to a separate photo-transistor included in a current integration circuit, the current integration circuit having a time constant longer than the breaks that occur in the line due to dialling, current limiting means for limiting the amount of line current flowing through each light emitting diode, and control means for controlling the current sensitivity of the detector circuit.

2. A detector circuit as claimed in claim 1 in which the current integration circuit is a Miller integration circuit.

3. A detector circuit substantially as described with reference to the drawing ac companving the Provisional Specification.