CA1263776A - Telephone test set - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An improved telephone test set is disclosed which may be operated with low voltage line conditions.
Transmitting and receiving voice communication circuits are connected in parallel with leads which are adapted to be connected to the tip and ring lines of a subscriber's telephone line to isolate the voice transmitting and receiving circuits from the voltage drop of a polarity guarding bridge which is connected to the signalling circuits of the telephone test set. A polarity indicating circuit to indicate the direction of current flow between the tip and ring lines of a subscriber's telephone line is connected in series with the voice transmitting and receiving circuits.

Description

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03 Field of the Invention 04 The present invention relates to telephone test 05 sets.

07 Brief Description of the Drawings 08 Fig. 1 illustrates a prior art telephone test set.
09 Fig. 2 illustrates a ~lock diagram of a telephone test set in accordance with the present invention.
11 Fig. 3 is a circuit schematic of the switchir.g 12 circuit of Fig. 2.
13 Fig. 4 is a circuit schematic of the polarity 14 indicating circuit of Fig. 2.
Fig. S i5 a circuit schematic of the high voltage 16 protection circuit of Fig. 2.
17 Fig. 6 illustrates an electrical schematic of the 18 preferred embodiment of a test set in accordance with the 19 present invention.
21 Description of the Prior Art 22 Fig. 1 illustrates a prior art telephone test set i377~

01 which has been manufactured by t'he assignee of the present 02 invention as the DRACON~ Model TS 21-080 test set~
03 A telephone test set is commonly used in two 04 distinct modes of operation. The first mode of operation 05 involves the operation on normal telephone lines which 06 involve the establishment of talking circuits between the 07 test set and the telephone central office which typically have 08 5 volts or more voltage drop within the test set. The second 09 mode of operation involves the so-called "dry loop" wherein a pair of test sets are connected to the opposite ends of a pair 11 of wires to enable the line persons to identify the ends of a 12 single pair of wires. This dry loop mode of operation does 13 not receive battery voltage from the central office and 14 functions with the pair of test sets sharing 3 volts or less provided by a small battery minus line losses. There is 16 considerable resistance by the various telephone operating 17 companies to the purchase of test sets which can only operate 12Ei3776 in a single one of the two modes of operation described, 6upra.
Prior art telephone test ~ets which can perform in both of the above-described operating mode~ typically include a two-position ~witch which i8 commonly known as a "keypad-in/out" 6witch. The 6witch permits choice between the mode of operation requiring signaling capability and the mode which does not require ~ignaling capability. The prior art test 6et of Fig. l has a three pole two po~ition switch, having contacts SlA, SlB and SlC, ueed to determine the mode of operation under which the test set operates. In the po6ition labelled "out", the speech network iB connected directly to the tip and ring lines. The contact SlC prevents the lineper60n from being subjected to l~ud bur~ts caused by inadvertent contact with the keypad of the dial pulse 6witch.
The contact S2 i~ used in it6 closed position for conducting a polarity test of the tip and ring lines to check the direction of current flow between the tip and ring lines by the activation of the oppositely poled light emitting diodes DSl or DS2 which respectively emit light of a different color when current flows through each diode to inform the lineperson of the direction of current flow.
With the prior art ctrcu~t of Fig. 1 being operated in the "keypad in" mode, it i6 necessary to connect the 6peech mute 6witch in series with the speech network. In thi6 configuration, the mute 6witch mu6t function regardle6s of the 12~;377f~

direction of current passing through the test set and 60 polarity guard BR i6 necessary. Unfortunately, the non-linear voltage drop of the polarity guard BR can cause the ~peech network to become inoperative at extremely low tip to ring voltages which are present when the telephone test 6et i6 used for conducting line tests with dry loops which are not powered by a central office battery. Moreover, voltage drops in the dial pul6e switch can al~o æeverely affect the operation of the speech network in low voltage conditions.
Finally, with low voltage condition~ aggravated by the series connection of the dial pulse switch, ~peech mute 6witch and 6peech network with the polarlty guard bridge, the total voltage drop within the tesk Bet i5 such that the insertion of any additional 6eries connected elements would result in excessive total voltage drop; and so a continuously indicating series connected polarity indicating mean6 is not feasible in this prior art. Thus, the polarity indicating circuit i6 6slectively coupled across the tip and ring lines by the closure of switch S2 to permit only intermittent polarity ~testing.
The three pole two position ~witches Sl and S2 are Realed 6witches which are expensive and difficult to obtain, are a leading cau~e of test ~et in the field failures, maXe the operation of the test set cumbersome to the lineper60ns and make it nece66ary for the decign of the hou6ing for the ~2~;377~5 -te6t set to have a number of openings where moisture ~ay enter.
United States Patent 3,708,634 disclo~es telephone test ~et in which a voice receiver unit ~5 partially coupled across the tip and ring lines. The voice transmitter unit is connected in 6eries with a polarity guarding bridge. A SCR
is connected in 6erie6 with the polarity guarding bridge and the voice transmitting unit. The SCR i6 biased into conduction by the manual closing of a momentary contact switch which then provides a conduction path for the operation of the voice transmitter unit. The SC~ is turned on when the voice transmitter unit and voice receiving unit are operational.
The system disclo6ed in United States Patent 3,708,634 is not well ~uited for operation on dry loops which have low operating potential because of the fact that the voice transmitting unit i6 connected in 6eries with the polarity guard bridge.
The u6e of a thermi6tor in eeries with the lead of a telephone teet set which i8 ~dapted to be connected to the tip or ring lines of a 6ub~criber'6 telephone line i6 known to protect the telephone test against high current conditions on the 6ubscriber'6 telephone line. Moreover, the u6e of a ~ilicon controlled rectifier ae a protective mechanism to a hold circuit in a telephone te6t Bet iB known.
A telephone test oet in accordance with the present invention hae advantages which are not ~chieved by the prior ~2~i377~

art telephone test sete. The telephone test 6et of the pre6ent invention may be u6ed to activate the voice transmitting and receiving circuits when connected to a dry loop which does not receive operating potential from the central telephone office. The tran6mitting snd receiving voice un1t6 are connected directly acro6s the lead6 which are adapted to be connected to the tip and ring lines which removee the effect of the voltage drop in the polarity guard from influencing the operation of the transmitting and receiving units. The elimination of the voltage drop caused by the polarity guard makes it possible to connect an indicator for the direction of current flow between the tip and ring line~ directly in serie6 with the voice transmitting and receiving unite to permit a continuous indication of the direction of current flow which i6 u6eful to the lineper~on.
A telephone test 6et in accordance with the pre6ent invention includes fir6t and second leads which are respectively adapted to be connected to a tip line and a ring line of a telephone line; a full wave rectifier for providing a rectified output of an input 6ignal and having a-pair of input terminals which are re6pectively connected to the ~ir6t lead and the second lead and a pair of ou~put terminal6 which re6pectively output a voltage of a first polarity and ~ ~econd polarity; a ~hunt circuit coupled between the pair of input terminals of the full wave rectifier which i~ comprised of a serie6 connection of a tran6mitter and 12~fi377~

receiver of voice communication6 and ~ ~witch having an open circuit condition when a control ~ignal of a fir~t magnitude iE applied to a control terminal ~nd having a closed circuit condition when the control 6ignal i6 applied to the control terminal of a second magnitude to permit current to flow within the shunt circuit; a signaling circuit for placing a telephone call over the tip and ring lines, coupled between the pair of bridge rectifier output termlnals, for producing ~ignals to place a telephone call over the tip and ring lines to a central office when in an activated ~tate and for not 1'^ j D producing signals when in an inactive ~tate;and a control circuit re~ponsive to the signaling circuit for producing the control signal having the first magnitude when the signaling circuit for placing a telephone call is in the active state and for producing the control ~ignal of the second magnitude when the signaling circuit for placing a telephone call i6 in the inactive 6tate.
The shunt circuit may include an indicator for continuously indicating the direct~on of current flow between the tip and ring lines of the sub~criber's line which i6 ; connected in ~eries with the transmitter and receiver of voice communications and the switch. The switch may be compri6ed of a pair of field effect tran6istor6 each having connected fiource6 located within the path of current flow ln the 6hunt circuit with ~he gate6 of each of the field effect tran6i6tors coupled to the signaling circuit. The control of the field lZ~377~

effect transi6tors may be made by ~n optical i601ator which is activated by the signaling circuit which causes the field effect tran~i6tor~ to be turned off when the signaling circuit iB activated.
The indicator for continuously indicating the direction of current flow between the tip and ring line6 compri6es a pair of oppositely poled light emitting diodes which respectively emit light of a different color. An audio frequency bypass, which 6hunts the oppositely poled light emitting diodes, permits audio ~requsncy co~munications ~o be coupled to the sub6cribers' telephone lines without ~ttenuation by the diodes. A resistance is provided, which 6hunt~ the oppo~itely poled light emitting diodes, to permit conduction o~ current in the ~hunt circuit when a Yoltage across the light emitting diode6 is insufficient to bias one of the diodes sub6tantially into conduction. A voltage limiter is provided which shunts the pair of light emitting diodes to limit the voltage which is applied to the diodes to a level which will not cau6e their failure. The voltage limiter may be a plurality of ~erially connected diodes located within a bridge.
The ~ignaling circuits are protected by ~ voltage limiter to prevent exce6sive voltages being applied which i6 comprised of a ~econd shunt c$rcuit coupled between the output terminal8 which Bhunt6 the output terminals of the full wave rectifier and the signaling circuit6. The ~econd shunt ~ 2fi377~

circuit includes a variable impedance which ghunt6 the output terminal6 with an impedance which decrease6 nonlinearly as an applied potential increa6es in magnitude. Preferably the variable impedance i6 a metal oxide varistor. Furthermore, a ~witching circuit iB coupled between the output terminals which is biased into conduction by the ~pplication of a control signal of a first magnitude to cause the output terminal6 to be ~hort circuited together and to have an open circuit condition when the c~ntrol ~ignal i~ below the fir~t magnitude. The control signal i8 produced by a RC circuit co~pled to the variable impedance and to one of the output terminals. Preferably, the switching circuit i6 a SCR having an anode and a cathode coupled respectively to the output terminal6 and a gate coupled to the RC circuit. A voltage limiting circuit may be coupled to the variable impedance and one of the output terminal6 with the gate of the SCR being coupled to $he voltage limiting circuit to protect the gate against excessive voltage.
An indicating circuit within a telephone fiet for continuously si~naling the direction of current flow between the tip and ring lines of a telephone line in which the indicating circuit i6 connected ln 6erie6 with a tran~mitter and receiver of voice communications within the test set include6 a pair of oppo6itely poled light emitting diodes for re6pectively indicating different direction6 of current flow between the tip and ring line6 by emitting light of a 12~;377~5 different color, the pair of oppositely poled diodes being coupled between one of the lines of the telephone line and the transmitter and receiver of voice communication6; an audio frequency bypass which shunt6 the oppositely poled light emitting dlodes to permit audio frequencies to bypass the light emitting diodes; and a resistance which 6hunts the oppositely poled light emitting diodes to psrmit conduction of current ln the shunt circuit when a voltage ~cro~ the oppositely poled light emitting diodes i5 insufficient to bias one of the light emitting diodes substantially into conduction. A voltage limiter may be provided which 6huntR
the pair of light emitting d$odes to limit the voltage which is applied to the diode~. Pre~erably, the voltage limiter is a plurality of series connected diodes which are configured in a bridge.

377~S

Description of the Preferred Embodiment Fig. 2 illustrates a block diagram of a test set in accordance with the present invention. A telephone test 6et 10 in accordance with the present invention connects the speech circuit~ for tran~mitting And receiving voice communi-cations in H ~eries circuit which i6 connected to leads which are adapted to be connected to the tip and ring l~nes of a BUbSCriber t 8 telephone line to remove the speech circuit~ ~rom being deleteriously influenced by low voltage conditions existing when the ~ubscriber's telephone line extends a long distance from the telephone company' B central office or when a pair of telephone test sets are being used at either end of a pair of telephone wires which ~re not coupled to the telephone oentral office battery. In the6e conditions, the presence of a polarity guarding circuit, which iB connected acros6 the leads that are adapted to be connected to the tip ~nd ring l~ne5 of a 8ubscriber's telephone line for protecting ~ignalling c1rcuits for placing a telephone call to the telephone company central office, does not disable the transmitter ~nd receiver for voice communications a6 in the lOa ., , ~., .

1~3776 prior art of Fig. l. Furthermore, because the effect of the voltage drop across the polarity guard on the operation of the tran6mitter and receiver for voice communications has been removed, a polarity indicating circuit may be connected in series with the transmitter and receiver for voice communication6 to provide a continuous polarity indication without di6abling the transmitter and receiver for voice communications because of the voltage drop required to operate the polarity i~dicating circuit. A ~peech mute 6witch i6 coupled in ~eries with the transmitter and receiver for voice communications which ~8 normally closed, that iB ~utomatically opened in response to the lineperson activating the signaling circuits for placing a call over the sub6criber' E telephone line to the telephone company central office.
Fig. 2 illu~trate6 a block diagram of a telephone test ~et lO in accordance with the present invention. A pair of leads 12 and 14, which are re6pectively labelled ~tip" and "ring", are adapted to be coupled to a line pair of a telephone ~ubscriber~ B telephone line. A thermi6tor 16 i8 connected in 6erie6 with the lead 12 which protects the telephone te~t set ~gain6t sustained high current conditions by having a high 6erie6 resistance. The thermistor functions ns a fixed value resi6tor under normal line current condition6, but when prolonged overcurrent conditions exist, it heats to a temperature wherein the resistance lncreases po6itively with increa6ing temperature. The resl6t~nce of the .. . ..

...

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thermistor 16 increa~es rapidly to a thermal equilibrium producing a high resistance when a ~ustained overcurrent condition exists. A rectifying bridge 18 having four diodes 20 has a first input terminal 22 connected to line 12 and a ~econd input terminal 24 to line 14. A fir~t output terminal 26 i6 coupled to ground. A 6econd output terminal 28 i~ coupled to a high voltage protection circuit 30. The preferred form of the high voltage protection circuit 30 1 discussed, infra, with regard to Fig. 5. The high voltage protection circuit 30 16 coupled to the signaling circuits 32 of conventional design which produce signals for making a call over the leads 12 ~nd 14 through a ~ubscriber' 8 line to a central telephone of~ice. The signaling circuits will produce DTMF tones or loop di6connect pul6es for placing a call. Thelinput terminals 22 and 24 are coupled to a shunt circuit 34 which is compri6ed of a 6eries combination of a polarity indicating circuit 36, which i~ de~cribed in detail, Lne~, in conjunction with Fig. 4, a speech mute 6witch 38, which i6 de6cribed in detail, lnfra, in con~unction with Fig. 3, ~nd ~peech circuits 40 which are comprised of a conventional transmitting and receiving unit. With the configuration illustrated in Fig. 2, the speech circuits 40 ~re not rendered inoperatlve by low voltage line condition~ ~s a con~equence of a voltage drop across the rectlfying bridge 18 a~ with the prior art in Fig. 1. Moreover, the polarity indicating circuit 36 may be connected in ~eries with ., 1~377~

the spQech circuits to produce a continuous indicat~on of the direction of current flow between the tip and ring line6 as a consequence of the 6peech circuits not being rendered inoperative by the voltage drop acro6s the full wave rectifying bridge 18 as with the prior art of Fig. 1 during low voltage condition6.
Fig. 3 illustrate6 the preferred form of 6peech mute ~witch 38 in ~ccordance with the pre6ent invention. An optical ~601ator 40 having a light e~itting diode 42, which is activated by the operation of the signaling circuits 32, is optically coupled to a phototransi6tor 44. The phototransistor 44 shunt6 a serie6 combination of a resistor 46 ~nd a battery 48. A series combination of two field effect tran6istor6 50 and 52, with their source6 coupled together, functions as a high impedance when the signaling circuits 32 are in the active 6tate and as a low impedance when the signaling circuits are in the inactive 6tate. The activation of the field effect tra~sistors 50 and 52 into the low impedance condition is produced when the signaling circuits are inactive which provides the 6hunt circuit 34 with a path for coupling the received and transmitted voice communications directly with the lines 12 and 14. The re~istance 46 provlde6 a high impedance which limits the current drawn by the phototran~istor 44 when it i8 conductive. The connection of the source6 together of the ~ield effect tr~nsistor~ 50 and 52 provide6 ~n extremely high ~Zfi3775 impedance against the application of signals of either polarity to the le~d6 12 and 14.
Fig. 4 illustrate~ the preferred embodiment of the polarity indicating circuit 36 o~ Fig. 2. The polarity indicating circuit 36 ha6 a pair o~ oppositely poled light emitting diodes 60 ~nd 62, which respectively emit green and red light continuously, to ~ignal the direction of current flow between the leads 12 and 14 when forward bi~ed. A
re~istance 64 i6 coupled in ~erie6 with the oppositely poled parallel combination o~ light emitting diodes 60 ~nd 62. ~he re61stance 64 limits the ~mount of current which may be drawn by the light emitting diodes. A capacitance 66 ohunt~ the oppo~itely poled light emitting diodes 60 and 62 to provide an audio frequency bypa6s of low impedance for communications tran6mitted and received by the speech circuits 40. A
resi~tance 68 shunts the oppositely poled light emitting diodes 60 and 62 which permit6 the speech circuit~ 40 to continue to operate even when the line potential is too low to 6upport substantial conduction of either the light emitting diode 60 or 62. A voltage limiter 70, which is compri~ed of a plurality of diodes 72 connected in ~eries in ~ bridge, l~mits the amount o~ voltage which can be applied to the diodes 60 and 62.
Fig. 5 lllu~trates the high voltage protection circuit 30 o~ Fig. 2. Metal oxide varistor 80 iB connected betwe~n terminal 28 ~nd the cathode of ~ Zener diode 82 having 12?i377fi 01 an anode connected to terminal 24. The function of the metal 02 oxide varistor is to provide a nonlinear resistance 03 characteristic which rapidly decreases with an increase in 04 applied potential to limit the voltage drop applied across the 05 terminals 26 and 28 to safeguard the signaling circuits 32 and 06 circuits within the shunt circuit 34. An RC circuit 84 07 consisting of a resistor 86, capacitor 88 and resistor 90 is 08 connected between the cathode of Zener diode 82 and the gate of 09 the voltage limiting SCR 92. The RC circuit 84 has a time constant whch integrates the voltage applied across resistor 86 11 by conduction of the metal oxide varistor 80 to cause a short 12 circuit between the terminals 26 and 28 when the high voltage 1 condition has existed for a time sufficient to bias the gate of 14 the SCR 92 sufficiently to turn the SCR on. Once the SCR 92 is turned on, the potential applied to the signaling circuts 32 16 and to the shunt circuit 34 is limited to the forward drop 17 across the SCR. When the terminal 28 is effectively grounded 18 by conduction of SCR 92, the input terminals 22 and 24, which 19 are respectively coupled to the tip and ring lines, appear virtually shorted together (two diode drops) which limits the 21 potential applied to shunt circuit 34 to a safe level.
22 Resistance 94 is connected between the RC circuit 84 and the 23 gate of the SCR 92, to isolate the gate of SCR 92 from the 24 transient loading effect of capacitor 88.
Fig. 6 illustrates a preferred embodiment of a 26 telephone test set in accordance with the present invention.
27 Idential parts are identified with the same reference numerals 28 as in the preceding figures. Semiconductor chips are 29 identified by their manufacturer's part number.

-~2fi3776 While the invention has been de6cribed in terms of its preferred e~bodiment, it 6hould be understood that numerous modification6 may be made thereto without departing from the ~pirit and scope of the invention 8S defined in the appended claims. It i6 intended that ~11 6uch modifications fall within the ~cope of the ~ppended claim6.

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Claims (25)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A telephone test set comprising:
(a) a first lead and a second lead which are respectively adapted to be connected to a tip line and a ring line of a telephone line;
(b) a full wave rectifying means for providing a rectified output of an input signal and having a pair of input terminals which are respectively connected to the first lead and the second lead and a pair of output terminals which respectively output a voltage of a first polarity and a second polarity;
(c) a shunt circuit coupled between the pair of input terminals of the full wave rectifying means which is comprised of a series connection of a means for transmitting and receiving voice communications and a switching means having an open circuit condition when a control signal of a first magnitude is applied to a control terminal and having a closed circuit condition when the control signal of a second magnitude is applied to the control terminal to permit current to flow within the shunt circuit;
(d) a signaling means coupled between the pair of output terminals for producing signals to place a telephone call over the tip and ring lines to a central office when in an activated state and for not producing signals when in an inactive state; and (e) control means responsive to the signaling means for producing the control signal having the first magnitude when the signaling means for placing a telephone call is in the active state and having the second magnitude when the signaling means for placing a telephone call is in the inactive state.

2. A telephone test set in accordance with claim 1 wherein the shunt circuit further comprises means for continuously indicating the direction of current flow between the tip and ring lines of the telephone line which is connected in series with the means for transmitting and receiving voice communications and the switching means.

3. A telephone test set in acordance with claim 2 further comprising means, coupled between output terminals of the full wave rectifying means and the signaling means for placing a telephone call, for protecting the signaling means for placing a call and the shunt circuit against the application of high voltages which would damage the signaling means for placing a telephone call or the shunt circuit.

4. A telephone test set in accordance with claim 2 wherein the switching means comprises a pair of field effect transistors having source electrodes connected together and located within the path of current flow in the shunt circuit with the gates of each field effect transistor coupled to the control means.

5. A telephone test set in accordance with claim 4 further comprising:
(a) an optical isolator having a light emitting device which is caused to emit light when the signaling means for placing a telephone call is activated:
and (b) a switching means which is biased into conduction when the light emitting device is activated, the switching means being coupled to the gates of the field effect transistors to cause the field effect transistors to turn off when the switching means is turned on.

6. A telephone test set in accordance with claim 5 further comprising:
(a) a battery having terminals with respectively first and second polarities;
(b) a resistance coupled between one of the terminals and the gates of the field effect transistors; and wherein (c) the switching means is a phototransistor with the emitter and collector electrodes coupled to series with the resistance and the battery and the light emitting device is a light emitting diode.

7. A telephone test set in accordance with claim 2 wherein the means for continuously indicating the direction of current flow between the tip and ring lines comprises:
a pair of oppositely poled light emitting diodes which respectively emit light of a different color when forward biased.

8. A telephone test set in accordance with claim 7 wherein the means for continuously indicating the direction of current flow between the tip and ring lines further comprises:
an audio frequency bypass means which shunts the oppositely poled light emitting diodes to permit audio frequencies to bypass the light emitting diodes.

9. A telephone test set in accordance with claim 7 wherein the means for continuously indicating the direction of current flow between the tip and ring lines further comprises:
a resistance which shunts the oppositely poled light emitting diodes to permit conduction of current in the shunt circuit when a voltage across the light emitting diodes is insufficient to bias one of the diodes substantially into conduction.

10. A telephone test set in accordance with claim 7 wherein the means for continuously indicating the direction of current flow between the tip and ring lines further comprises:
(a) an audio frequency bypass means which shunts the oppositely poled light emitting diodes to permit audio frequencies to bypass the light emitting diodes: and (b) a resistance which shunts the oppositely poled light emitting diodes to permit conduction of current in the shunt circuit when a voltage across the light emitting diodes is insufficient to bias one of the diodes substantially into conduction.

11. A telephone test set in accordance with claim 7 wherein the means for continuously indicating the direction of current flow between the tip and ring lines further comprises:
a voltage limiting means which shunts the pair of light emitting diodes to limit the voltage which is applied to the diodes.

12. A telephone test set in accordance with claim 11 wherein the voltage limiting means is comprised of a plurality of diodes connected in series.

13. A telephone test set in accordance with claim 10 wherein the means for continuously indicating the direction of current flow between the tip and ring lines further comprises:

a voltage limiting means which shunts the pair of light emitting diodes to limit the voltage which is applied to the diodes.

14. A telephone test set in accordance with claim 13 wherein the voltage limiting means is comprised of a plurality of diodes connected in series.

15. A telephone test set in accordance with claim 3 wherein the means to protect the signaling . means for placing a telephone call against the application of voltages which would damage the signaling means for placing a call comprises:
a second shunt circuit coupled between the output terminals which shunts the output terminals and the signaling means for placing a telephone call.

16. A telephone test set in accordance with claim 15 wherein the second shunt circuit comprises a variable impedance which shunts the output terminals with an impedance that decreases nonlinearly as an applied potential increases in magnitude.

17. A telephone test set in accordance with claim 16 wherein the variable impedance comprises a metal oxide varistor.

18. A telephone test set in accordance with claim 16 further comprising a switching means coupled between the output terminals which has a closed condition caused by the application of a control signal of a first magnitude to cause the output terminals to be short circuited together and has an open circuit condition when the control signal is less than the first magnitude, the control signal being produced by an RC circuit coupled to the variable impedance and to one of the output terminals.

19. A telephone test set in accordance with claim 18 wherein the switching means is a SCR having an anode and cathode coupled respectively to the output terminals and a gate coupled to the RC circuit.

20. A telephone test set in accordance with claim 19 further comprising:
(a) a voltage limiting circuit coupled to the variable impedance and one of the output terminals; and (b) the gate is coupled to the voltage limiting circuit to protect the gate against excessive voltage.

21. An indicating circuit within a telephone test set for continuously signaling the direction of current flow between tip and ring lines of a telephone line in which the indicating circuit is connected in series with a means for transmitting and receiving voice communications within the test set comprising:
(a) a pair of oppositely poled light emitting diodes for respectively indicating different directions of current flow between the tip and ring lines by emitting light of a different color, the pair of oppositely poled diodes being coupled between one of the lines of the telephone line and the means for transmitting and receiving;
(b) an audio frequency bypass means which shunts the oppositely poled light emitting diodes to permit audio frequencies to bypass the light emitting diodes; and (c) a resistance which shunts the oppositely poled light emitting diodes to permit conduction of current in the shunt circuit when a voltage across the oppositely poled light emitting diodes is insufficient to bias one of the light emitting diodes substantially into conduction.

22. An indicating circuit in accordance with claim 21 further comprising:
a voltage limiting means which shunts the pair of light emitting diodes to limit the voltage which is applied to the diodes.

23. An indicating circuit in accordance with claim 21 wherein the voltage limiting means comprises a plurality of series connected diodes which shunt the oppositely poled light emitting diodes.

24. A telephone test set comprising:
(a) a first lead and a second lead which are respectively adapted to be connected to a tip line and a ring line of a telephone line;
(b) a shunt circuit coupled between the first lead and the second lead which is comprised of a series connection of a means for transmitting and receiving voice communications and a switching means having an open circuit condition when a control signal of a first magnitude is applied to a control terminal and having a closed circuit condition when the control signal of a second magnitude is applied to the control terminal to permit current to flow within the shunt circuit;
(c) a signaling means coupled to the first and second leads for producing signals to place a telephone call over the tip and ring lines to a central office when in an activated state and for not producing signals when in an inactive state, the signaling means not being coupled in series with the shunt circuit;
(d) means for providing electrical power to the signaling means; and (e) control means responsive to the signaling means for producing the control signal having the first magnitude when the signaling means for placing a telephone call is in the active state and having the second magnitude when the signaling means for placing a telephone call is in the inactive state.

25. A telephone test set in accordance with claim 24 wherein the shunt circuit further comprises means for continuously indicating the direction of current flow between the tip and ring lines of the telephone line, and which indicating means is connected in series with the means for transmitting and receiving voice communications and the switching means.