GB2121648A - Ring detector and telephone line monitoring system for telephone answering instrument - Google Patents

Abstract

A circuit and system is provided for coupling a telephone answering instrument to a telephone line and which monitors direct current voltage levels on the telephone line to distinguish between valid ring signals and dial pulses so that the telephone answering instrument will not attempt to answer when a local telephone is being dialed for an out-going call. The circuit and system of the invention also responds to direct current voltage differentials on the telephone line, independent of the actual direct current voltage level, to cause the telephone answering instrument to return immediately to its automatic answer position in the event the calling party hangs up, or in the event the called party picks up, during the normal announcement interval (T1), or during the message recording interval (T2). This latter feature permits the instrument immediately to return to its original condition in the presence of either of the two foregoing events, without wasting magnetic tape on blank messages.

Description

SPECIFICATION Ring detector and telephone monitoring system for telephone answering instrument BACKGROUND OF THE PRIOR ART There are several requirement for telephone answering systems if the systems are to operate properly and efficiently. A first requirement is that the system must distinguish between an incoming ring signal and outgoing dialing pulses from the telephone to which the system is connected. This discrimination must be such that the system will answer in the presence of an actual ring signal, but will not attempt to answer in the presence of outgoing dialing pulses.

Another requirement, if the telephone answering instrument is to operate efficiently, is that it should immediately stop operating and return to its original automatic answer position should the calling party hang up before the end of the normal operation cycles of the instrument, or should the called party pick up before the end of the normal operational cycles.

The circuit and system of the present invention provides a simple and economical means which fulfills all the foregoing requirements.

The circuit and system of the invention is of the same general type as is described in U.S.

Patent 4,074,081. However, the circuit and system of the present invention achieves the results achieved by the aforementioned U.S.

patent in a different manner and by the use of simpler circuitry.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic representation of the presently preferred embodiment of the circuit and system of the invention.

DETAILED DESCRIPTION OF AN ILLUSTRA TIVE EMBODIMENT The circuit illustrated in Fig. 1 includes a series of amplifiers Z1 A, Z1 B, Z1 C and Z1 D, which may all be contained in an integrated circuit of the type designated LM324. Amplifier ZiA is connected as an input attenuator amplifier with a gain of less than unity. For example, the gain of amplifier Z1A may be of the order of .025. Amplifiers tlB and ZiC are connected as absolute value amplifiers, and may have a gain, for example, of the order of 3.7. Amplifier Z1 D is connected as a comparator.

A pair of resistors R1 and R3, each of which may have a value of 22 megohms, are used to connect the inverting input terminal and non-inverting input terminal of amplifier Z1A to the telephone line. These resistors provide the high impedance coupling of the telephone answering instrument to the telephone line which is acceptable to the Federal Communications Commission in the United States, as well as to the British Post Office in the United Kingdom.

The output of amplifier Z1 is connected back to the input through a 560 kilo-ohm resistor R2, and the non-inverting input terminal is connected to a potential lead designated Vre, through a 560 kilo-ohm resistor R4. Lead Vre, is connected to the common junction of a pair of 1 kilo-ohm resistors R14 and R15 which are connected between the positive terminal of a 1 2 volt source and ground.

Therefore, the lead Vre, provides a reference voltage of 6 volts.

The amplifier ZiA is connected as an input attenuator amplifier with a gain, as stated, of 0.025. Therefore, variations in the voltage of the telephone line between, for example, + 40 volts causes the amplifier ZiA to provide an output of + 1 volt. This amplifier permits telephone line voltages up to 240 volts, for example, to be measured without distortion, and without damage to the system.

The output of amplifier Z1A is connected through a 10 kilo-ohm resistor R6 to the inverting input terminal of amplifier Z1 B, and through a 10 kilo-ohm resistor R5 to the inverting inut terminal of amplifier Z1C. The non-inverting input terminals of amplifiers ZiB and Z1C are connected to the voltage reference lead Vret The output of amplifier Z1 B is connected to a pair of diodes D1 and D2 which, together with a 10 kilo-ohm resistor R7 are connected to the inverting input terminal of amplifier Z1B, and through a 5.1 kilo-ohm resistor R8 to the inverting input terminal of amplifier Z1C. The output of amplifier Z1C is connected back to the non-inverting input terminal through a 39 kilo-ohm resistor R9.

The amplifiers Z1 B and Zi C are connected as a full-wave rectifier circuit, and function as absolute value amplifiers with a gain, for example, of 3.7. These amplifiers provide independence to the polarity of the voltage appearing on the telephone line, so that, so that for example, an output of + 1 volt from the amplifier Z1A would cause the amplifier circuits Z1 B, Zi C to produce an output of + 3.7 volts.

The output of the attenuator amplifier Z1A is also coupled through a .1 microfarad capacitor C2 to the base of an NPN transistor Q2 whose emitter is connected to ground. The capacitor is connected to the base through a 22 kilo-ohm resistor R15, which, in turn, is connected to a 10 kilo-ohm grounded resistor R14, and to a 47 kilo-ohm grounded resistor R1 6. The junction of capacitor C2 and resistor R15 is also connected to a grounded diode D3.

The circuit of transistor Q2 responds to the output of amplifier Z1A to provide a ring signal for the telephone answering instrument.

The output of aplifier Zi C is connected through a Zener diode Z01 to a grounded 4.7 kilo-ohm resistor R1 2. The junction of the Zener diode and resistor R 1 2 is connected through a 4.7 kilo-ohm resistor R13 to the base of a grounded emitter NPN transistor Q1. The transistor Q1 is normally conductive.

However, if the voltage of the telephone line drops, for example, to less than 20 volts, the transistor Q1 becomes non-conductive, and its output voltage rises abruptly.

The nominal line voltage of the telephone line is normally above 20 volts, for example, in the neighborhood of 50 volts, for most telephone systems. Therefore, under normal conditions, transistor Q1 is conductive. Then, should a ring signal occur, the ring signal is detected by the circuit of transistor Q2, and the ring signal from the transistor Q2 is used to activate the telephone answering system.

However, should the local telephone be in an off-hook condition, the telephone line voltage accordingly would drop to less than 20 volts causing transistor Q1 to be non-conductive. Under such conditions, any signals on the telephone line could be dialing pulses from the local telephone, and, in any event, the telephone answering instrument should not answer, because the local telephone is offhook. An appropriate circuit in the telephone answering system looks at the output voltage from transistor Q1 directly after the occurrence of what appears to be a ring signal on the line.This circuit permits the ring signal output from the transistor Q2 to activate the telephone answering system, only if the voltage output from transistor Q1 is at the low value, and it prevents the telephone answering instrument from answering if the voltage output from transistor Q1 is at a high level.

Accordingly, the telephone answering instrument will answer only in the presence of a true ring signal, and it will be unresponsive to outgoing dialing pulses, which occur when the local telephone is in an off-hook condition.

The output of amplifier Z1C is also connected to the non-inverting input of comparator Z1 D, and through a 6.2 kilo-ohm resistor R10 to the inverting input terminal of the comparator. The inverting input terminal is also connected to a grounded 10 microfarad capacitor C1, which is shunted by a 100 kiloohm resistor R1 1. These latter elements provide a self-biasing circuit for the comparator, so that the inverting input is biased, for example, to approximately 94% of the noninverting input. The comparator, accordingly, responds to voltage differentials on the telephone line and is unresponsive to absolute voltage levels.

The comparator Z1 D produces an output, for example, when a voltage differential in excess of a predetermined value occurs. For example, when a genuine ring signal appears on the telephone line, the telephone answering system will answer, and this causes the voltage of the telephone line to drop, for example, to a level of the order of 10 volts.

Now, should the called party pick up during the announcement time (T1) or during the normal message recording time (T2), an additional line voltage drop to a level of the order of 6 volts will occur. Comparator Z1 D will detect the 4 volt voltage differential to produce a monitor signal, which will cause the instrument immediately to "hang up", and not to complete its normal T1 and/or T2 cycles.

Likewise, should the calling party hang up after the instrument has answered, this likewise will cause a line voltage drop, and the comparator Z1 D will respond to the resulting voltage differential again to produce the monitor signal.

Therefore, the system produces a monitor signal should the called party pick up during the announcement and/or message recording interval, or should the calling party hang up during the same intervals, and appropriate circuitry is provided to cause the instrument immediately to "hang up" and return to its initial state.

The invention provides, therefore, an improved and simplified circuit and system which serves to distinguish between ring sig nals and dial pulses on the telephone line, and which also is effective to detect a calling party hang-up or called party pick-up situation.

It will be appreciated that while a particular embodiment of the system of the invention has been shown and described, modifications may be made.

Claims (11)

1. A circuit for coupling a telephone answering instrument to a telephone line, comprising: an input attenuator amplifier circuit having a gain of less than unity for producing an output voltage which is a fraction of the line voltage of the telephone line; an absolute value amplifier circuit having a gain greater than unity connected to the output of said attenuator amplifier circuit for producing a single polarity output in response to outputs from said attenuator amplifier of positive or negative polarity; and an output circuit connected to the output of said absolute value amplifier for providing an output signal when the telephone line voltage drops below a particular level.

2. The circuit as claimed in Claim 1, in which said attenuator amplifier circuit has a gain of the order of .025.

3. The circuit as claimed in Claim 1, in which said absolute value amplifier has a gain of the order of 3.7

4. The circuit as claimed in Claim 1, in which said input attenuator amplifier circuit includes high impedance elements directly connecting the system to the telephone line.

5. The circuit as claimed in Claim 1, in which said output circuit includes a normally conductive transistor which becomes non-conductive when the direct current telephone line voltage drops below a predetermined level.

6. The circuit as claimed in Claim 1, and which includes a second output circuit which responds to direct current voltage differentials on the telephone line independent of the direct current voltage level to produce a second output signal when such voltage differentials exceed a predetermined minimum value.

7. The circuit as claimed in Claim 1, in which said output circuit includes a comparator having an inverting input terminal and a non-inverting input terminal, said non-inverting input terminal being connected to the output of said absolute value amplifier circuit, and self-biasing circuitry connected to the reversing input terminal to bias the reversing input terminal to a predetermined percentage of the voltage applied to the non-inverting input terminal.

8. A circuit for coupling a telephone answering instrument to a telephone line including: input circuitry connected to said telephone line, and output circuitry connected to said input circuitry and responsive to D.C.

voltage differentials on said telephone line independent of the normal D.C. monitor voltage level thereof to produce an output signal when said voltage differentials exceed a predetermined minimum value.

9. The circuit as claimed in Claim 8, in which said output circuitry includes a comparator having an inverting input terminal and a non-inverting input terminal, said non-inverting input terminal being connected to the output of said input circuit, and self-biasing circuitry connected to the inverting input terminal to bias said inverting input terminal to a predetermined percentage of the voltage applied to the non-inverting input terminal.

10. The circuit as claimed in Claim 8, in which said input circuitry comprises an input attenuator amplifier circuit having a gain of less than unity for producing an output voltage which is a fraction of the voltage of the telephone line; and an absolute value amplifier circuit having a gain greater than unity connected to the output of said attenuator amplifier circuit for producing a single polarity output in response to outputs from said attenuator amplifier circuit of positive or negative polarities.

11. The circuit as claimed in Claim 10, in which said input attenuator amplifier circuit includes high impedance elements directly connecting the system to the telephone line.

1 2. A circuit for coupling a telephone answering instrument to a telephone line substantially as herein described with reference to and as illustrated in the accompanying drawings.