United States Patent: 3587082

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United States Patent	3,587,082
Muehter , et al.	June 22, 1971

HOLDUP ALARM SYSTEM

Abstract

A holdup alarm system in which the premises to be protected are coupled to a central station via a transmission line, the central station having a source of direct current potential, a break indicator and a ground indicator connected in series with the line. The protected premises is provided with a normally closed loop circuit connected to the transmission line. The loop circuit includes a manually operable holdup switch having break contacts normally included in the loop circuit and connected to the high side of the transmission line. The holdup switch also has make contacts arranged, upon operation of the holdup switch, to be connected to the low side of the transmission line. An electronic switch is coupled between the high and low sides of the transmission line when the holdup switch is operated. An astable multivibrator is energized when the holdup switch is operated and operates the electronic switch to successively open and ground the transmission line at a repetition rate controlled by the multivibrator. Means also are provided to prevent a holdup alarm from being given upon an accidental break or ground in the wiring. Means also are provided to supervise the line and associated wiring against open and ground faults.

Inventors:	Muehter; Manfred W. (Livingston, NJ), Hill; Frederick G. (Yonkers, NY), LaMartina, Jr.; Anthony C. (Ridgewood, NY)
Assignee:	*American District Telegraph* Company (Jersey City, NJ)
Appl. No.:	04/719,145
Filed:	April 5, 1968

Current U.S. Class: 340/286.04 ; 340/532; 340/533; 340/650; 340/693.1

Current International Class: G08B 21/00 (20060101); G08B 21/02 (20060101); G08b 029/00 (); G08b 019/00 ()

Field of Search: 340/409,276

References Cited [Referenced By]

U.S. Patent Documents


3010100	November 1961	Muehter
3174143	March 1965	Akin
3456251	July 1969	Smith et al.

Primary Examiner: Habecker; Thomas B.

Claims

What we claim is:

1. Holdup alarm apparatus for use in an electrical protection system at a protected premises, said system having a local protection circuit and being coupled to a central station over a transmission line having a high side and a low side, the central station having a current sensitive line break indicator and a source of direct current potential coupled in series between the high and low sides of the transmission line, the holdup alarm apparatus comprising:

a. a manually operable switch having break contacts interconnecting the high side of said transmission line and one end of said local protection circuit and having make contacts;

b. means interconnecting the other side of said local protection circuit and the low side of said transmission line;

c. an electronic switching device;

d. means including said make contacts coupling said electronic switching device between the high and low sides of said transmission line when said manually operable switch is operated; and

e. an astable multivibrator operatively coupled to said electronic switch alternately to open and close said electronic switch thereby alternately to intercouple and uncouple the high and low sides of said transmission line at a rate determined by the repetition rate of said multivibrator alternately to cause current to flow in said transmission line and to break said transmission line thereby to subject said break indicator at said central station alternately to periods of current flow and periods of reduced current flow as a signal indication of operation of said manually operable switch.

2. Holdup alarm apparatus for use in an electrical protection system at a protected premises, said system having a local protection circuit and being coupled to a central station over a transmission line having a high side and a low side, the central station having a current sensitive break indicator and a source of direct current potential coupled in series between the high and low sides of the transmission line, the holdup alarm apparatus comprising:

a. a manually operable switch having break contacts interconnecting the high side of said transmission line and one end of said local protection circuit and having make contacts;

b. means interconnecting the other side of said local protection circuit and the low side of said transmission line;

c. an electronic switching device;

d. means including said make contacts coupling said electronic switching device between the high and low sides of said transmission line when said manually operable switch is operated;

e. an astable multivibrator operatively coupled to said electronic switch alternately to open and close said electronic switch thereby alternately to intercouple and uncouple the high and low sides of said transmission line at a rate determined by the repetition rate of said multivibrator;

f. means including said break contacts, when closed, to render said multivibrator unoperated; and

g. means including said make contacts, when closed, to render said multivibrator operative whereby said electronic switch is operated, alternately to cause current to flow in said transmission line and to break said transmission line thereby to subject said break indicator at said central station alternately to periods of current flow and periods of reduced current flow as a signal indication of operation of said manually operable switch.

3. Holdup alarm apparatus as set forth in claim 2 in which said electronic switch comprises a transistor having a first electrode coupled to the high side of said transmission line, a second electrode coupled through said make contacts to the low side of said transmission line and a third electrode coupled to said multivibrator whereby a change in condition of said multivibrator changes the conductive condition of the circuit including said first and second electrodes.

4. Holdup alarm apparatus as set forth in claim 3 in which a Zener diode is intercoupled between said second electrode and said make contacts, said Zener diode being poled to break down when said transistor is conductive between said first and second electrodes.

5. Holdup alarm apparatus as set forth in claim 4 in which a capacitor is coupled in parallel with said Zener diode and to said multivibrator to provide operating potential for the latter when said transistor is not conductive.

6. Holdup alarm apparatus as set forth in claim 5 in which said multivibrator comprises second and third transistors operatively coupled between said second electrode of said first transistor and said make contacts, so as to become alternately conductive and nonconductive in sequence, one of said second and third transistors being arranged, when conductive, to bias said first transistor to conduction and, when not conductive, to bias said first transistor to nonconduction.

7. Holdup alarm apparatus as set forth in claim 2 in which said electronic switch comprises a first transistor and said multivibrator comprises second and third transistors, said transistors being operatively coupled to said transmission line to receive operating potential therefrom, said break contacts being arranged, when closed, to shunt said operating potential.

8. Holdup alarm apparatus as set forth in claim 7 comprising biasing means arranged to prevent operation of said multivibrator upon removal of said shunt, said biasing means being arranged to be shunted by said make contacts when the latter are closed.

9. Holdup alarm apparatus for use in an electrical protection system at a protected premises, said system having a local protection circuit and being coupled to a central station over a transmission line having a high side and a low side, the central station having a current sensitive break indicator, a tone responsive indicator and a source of direct current potential coupled in series between the high and low sides of the transmission line, the holdup alarm apparatus comprising:

a. a manually operable switch having break contacts interconnecting the high side of said transmission line and one end of said local protection circuit and having make contacts;

b. means interconnecting the other side of said local protection circuit and the low side of said transmission line;

c. an electronic switching device;

d. means including said make contacts coupling said electronic switching device between the high and low sides of said transmission line when said manually operable switch is operated;

e. an astable multivibrator operatively coupled to said electronic switch alternately to open and close said electronic switch thereby alternately to intercouple and uncouple the high and low sides of said transmission line at a rate determined by the repetition rate of said multivibrator;

f. means including said break contacts, when closed, to render said multivibrator unoperated;

g. means including said make contacts, when closed, to render said multivibrator operative whereby said electronic switch is operated alternately to cause current to flow in said transmission line and to break said transmission line thereby to subject said break indicator at said central station alternately to periods of current flow and periods of reduced current flow as a signal indication of operation of said manually operable switch; and

h. a tone generator at said protected premises and coupled in series with said transmission line to superimpose an alternating tone signal on the direct current flowing in said line, absence of said tone being detected by said tone responsive indicator as an indication of a fault in said transmission line.

Description

BACKGROUND

The present invention relates to electrical protection systems and more particularly to such systems in which manually operable means are provided at a protected premises to signal to a central station the occurrence of a holdup.

As used in the electrical protection field, the term "central station" usually refers to a particular type of central signal receiving station. As used herein, however, the term is intended to have a somewhat broader meaning so as to include also other distant places where signals can be received and acted upon, e.g., guard stations and police stations. The term "holdup" usually refers to an armed robbery and presents a protection situation which must be responded to with extreme speed, as distinguished, for example, from the usual burglary which normally will require more time to consummate and which, while requiring prompt action, is not usually as urgent as a holdup. However, the term "holdup" is intended to include generally those emergency situations requiring extremely rapid response.

It is common for mercantile and industrial establishments to be provided with protection devices which will be actuated upon entry of an intruder automatically to signal the fact of such entry to a central station where an operator will take the necessary action, e.g., dispatching armed guards or police to the premises so attacked. When the premises are open for business, some or all of the protection devices usually will be disabled since it is not desired that innocent entry or exit be signalled. However, the protected premises are nevertheless connected to the central station over the transmission line and, even if no entry protection devices are operative, supervisory current will flow over the line to protect against opens on the line.

Where there is a danger of armed robber, i.e., holdup, of a protected premises, it is customary to have manually operable switches, usually concealed, which can be operated to signal the occurrence of a holdup so that prompt corrective action can be taken by police or other appropriate agencies. Because of the extreme emergency represented by a holdup situation, it has been customary to make the holdup alarm signal which is transmitted to the central station distinctive, i.e., readily distinguishable from the usual break and ground signals.

Many thousands of holdup alarm systems have been installed as part of electrical protection systems. A particularly useful example used for many years has involved the use of a manually operable switch with both break and make contacts and a relay with a weighted armature and break contacts connected in series with the relay coil and the switch make contacts. The supervisory current flows through the switch break contacts until interrupted by operation of the switch. The consequent cutting off of supervisory current actuates the central station break drop. Closing of the switch make contacts energizes the relay and also restores the flow of supervisory current, but through the relay coil rather than the switch break contacts. Energization of the relay opens its break contacts, again deenergizing the relay and cutting off supervisory current. The relay will then continue to be successively energized and deenergized at a rate dependent on the armature mechanical constants, which typically might involve 3 operating cycles per second. A special relay or special contacts on the break drop at the central station respond to the interrupted current flow to actuate a holdup signalling device at the central station.

While holdup alarm systems of the character described have afforded highly satisfactory service, they lack the extreme reliability and trouble free operation which can be obtained with properly designed solid state circuitry.

The principal object of the present invention has been the provision of a novel and improved holdup alarm system.

More particularly, it has been an object of the invention to provide such a system which is extremely reliable and trouble free and which requires a minimum of maintenance.

Still another object of the invention has been the provision of such a system which will not transmit a spurious holdup alarm signal in the event of an accidental break or ground.

Another object of the invention has been the provision of such a system which affords positive supervision against accidental opens or grounds on the transmission line.

Other and further objects, features and advantages of the invention will appear more fully from the following description of the invention.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a holdup alarm apparatus for use in an electrical protection system at a protected premises, the system having a local protection circuit and being coupled to a central station over a transmission line, the central station having a current sensitive break indicator, a current sensitive ground indicator and a source of direct current potential coupled in series between the high and low sides of the transmission line. The holdup alarm apparatus at the protected premises comprises a manually operable switch having break contacts interconnecting the high side of the transmission line and one end of the local protection circuit, the manually operable switch also having make contacts. The other side of the local protection circuit is connected to the low side of the transmission line. An electronic switching device is arranged to be coupled between the high and low sides of the transmission lines through the make contacts when the latter are closed. An astable multivibrator is operatively coupled to the electronic switch alternately to open and close the electronic switch thereby alternately to intercouple and uncouple the high and low sides of the transmission line at a rate determined by the repetition rate of the multivibrator. The break contacts of the manually operable switch are arranged normally to render the multivibrator unoperated. Means including the make contacts of the manually operable switch, when closed, render the multivibrator operative whereby the electronic switch is operated alternately to cause current to flow in the transmission line and to break the transmission line thereby to subject the break indicator at the central station alternately to periods of current flow and periods of no current flow as a signal indication of operation of the manually operable switch.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in connection with the appended drawing which illustrates an electrical protection system embodying the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawing, there are shown a central station 10, a protected premises 11 and a transmission line 12 intercoupling the central station and protected premises. The transmission line is shown as a two conductor line which might typically be a leased telephone line. However, a single conductor line with ground return can be used in place of a two conductor line, as is well known in the art.

The central station 10 is provided with a break drop 13, a ground drop 14 and a central station battery 15, all connected in series between the high and low sides of the transmission line. The term high side is intended to refer to the high potential side while the term low side is intended to refer to the low potential or ground side of the line. The system shown is intended to operate with a positive potential on the high side and a grounded negative, but as will be explained below, a positive grounded source can be used with appropriate changes.

The break drop 13 is a current sensitive device which responds to a drop in line current below a predetermined value to give a break signal. The ground drop 14 is a current sensitive device which responds to an increase in line current above another predetermined value to give a ground signal. Such devices are well known in the art in the form of relays and in electronic form. In addition to responding to a decrease in line current, the break drop should respond also to alternate periods of current flow above the break level and below the break level, commonly called a "police call" signal, to actuate an alarm device at the central station. Typically the repetition rate or frequency used for a "police call" signal is of the order of 3 cycles per second. If galvanometer-type drops are used at the central station, as is common, it will be desirable to use a separate break drop relay for the "police call" signal.

In accordance with a further aspect of the invention, a tone responsive device 16 may be included in series with the central station drop for a purpose to be described below.

At the protected premises the high side of the transmission line 12 is connected through a current limiting resistor 17 and a path designated A-B-C-D-E to a conventional burglar alarm circuit 18, the other end of the burglar alarm circuit 18 being connected by a conductor 19 to the low side of the transmission line. A tone generator 20 is shown connected in series between point E and circuit 18. The purpose of tone generator 20 will be described below. The tone generator may be omitted and point E connected directly to circuit 18.

The burglar alarm circuit 18 may be of any type adapted to be connected in a series circuit between the high side and the low side of a transmission line. Typically the circuit 18 might include a control set, door and window contacts, foil, photoelectric beams, sound responsive systems, and capacitance responsive systems. The control set will usually provide for a "day" protection and a "night" protection condition. In the "day" protection condition some or all of the protective devices will be shunted and a normal day supervisory current will flow through line 12. In the "night" protection condition all of the protective devices will be connected in the circuit and a normal "night" current will flow through line 12. Adjustable resistors are provided in the control set to permit adjustment of the "day" and "night" values of current.

As is well known in the art, an alarm may be transmitted by opening the current path, thereby decreasing the current flow through line 12, by grounding the current path, i.e., shunting all following elements and thereby increasing the current flow through line 12, or by opening and then grounding the current path. Distributed constants of the line may provide a current flow even with an open circuit at the protected premises.

For the purpose of understanding the operation of the invention, the burglar alarm circuit 18 could be considered as a resistor selected to provide a normal value of current flow in line 12.

The path A-B-C-D-E comprises a conductor 21, break contacts H1-B and H2-B of manually operable holdup switches H1 and H2, respectively, a conductor 22 and diodes D6, D7 and D8. The return path includes tone generator 20 (if provided), burglar alarm circuit 18 and conductor 19 to the low side of the central station line. The conductor 19 is connected to the fixed terminals of make contacts H1-M and H2-M of holdup switches H1 and H2, respectively.

The holdup alarm switches H1 and H2 are of a type commonly used in holdup alarm systems and comprise an armature normally made with a fixed contact and together forming the "break" contacts, the armature being arranged when the switch is manually operated to leave the fixed contact of the "break" contacts and make with another fixed contact which, together with the armature, forms the "make" contacts. The holdup alarm switches usually will be hidden and will be placed where they can be operated surreptitiously by a hand, arm, foot or other part of a body. After operation the switches preferably lock into their make positions. Any number of holdup alarm switches may be provided, e.g., at each teller's position in a bank. The break contacts of the switches are all connected in series. The fixed terminal of each of the make contacts is connected to the conductor 19.

When one of the holdup alarm switches is operated, it is desired alternately to increase and decrease the transmission line current at a fixed repetition rate, e.g., 3 cycles per second, which will actuate the break drop at the central station to produce a holdup alarm at the central station. For this purpose there are provided an electronic switching device, comprising a transistor Q1, and an astable multivibrator 23, comprising transistors Q2 and Q3.

The transistor Q1, which might be of the 2N4036 type, has its emitter connected to point A and its collector connected through a conductor 24 to one terminal of a Zener diode D9, which might be of the 1N4736 (6.8 v.) type and to the positive side of a capacitor C5. The other terminal of diode D9 and the other side of capacitor C5 are connected to conductor 22. A resistor R1 intercouples the base and emitter of transistor Q1. A resistor R2, a capacitor C4 and a resistor R3, in series circuit, intercouple the base and collector of transistor Q1.

Transistors Q2 and Q3 might be of the 2N2405 and 2N3391 types, respectively. The emitters of transistors Q2 and Q3 are connected to conductor 22. The collector of transistor Q2 is connected to the junction of resistor R2 and capacitor C4 and is coupled to conductor 24 through diode D1 and resistor R4. Diode D1, and also diodes D2--D8, might be of the 1N2070 type. The collector of transistor Q2 is also coupled to the emitter thereof through a capacitor C3. A capacitor C1, poled as shown, intercouples the junction of diode D1 and resistor R4 to the junction of resistor R6 and diode D3. Resistor R6, diode D3 and a resistor R9 are coupled in series between conductors 24 and 22.

A resistor R5, a diode D2 and a resistor R8 are connected in series between conductors 24 and 22. The base of transistor Q2 is connected to the junction of diode D2 and resistor R8 and, through series connected diodes D4 and D5, to point E.

The collector of transistor Q3 is coupled to conductor 24 through a resistor R7 and, through a capacitor C2, to the junction of resistor R5 and diode D2. The base of transistor Q3 is connected to the junction of diode D3 and resistor R9.

A Zener diode D10, which might be of the 1N4762 (82 v.) type is coupled between points A and E to limit the voltage which may appear across the electronic components, e.g., to 82 volts, thus affording protection against surges in line current.

With the circuit in the condition shown, the metallic link A-B-C-D creates a shunt path between points A and D. Hence there is no operating potential for the transistors and all will be nonconductive. Similarly all capacitors will be discharged.

When occurrence of a holdup is to be signalled, the link A-B-C-D will be broken by operation of switch H1 or switch H2 (or both) and the consequent opening of break contacts H1-B or H2-B, as the case may be.

With the shunt link A-B-C-D broken, current will flow from point A to point E through the parallel combination of the emitter-base junction of transistor Q1 and resistor R1 and the following three circuits:

a. resistor R2, capacitor C3, conductor 22 and diodes D6, D7 and D8;

b. resistor R2, capacitor C4, resistor R3, resistor R5, diode D2, and diodes D4 and D5;

c. resistor R2, capacitor C4, resistor R3, capacitor C5, and diodes D6, D7 and D8.

The voltage which appears across diodes D6, D7, D8 reverse biases the base of transistor Q2 through diodes D4 and D5 and prevents it from becoming conductive. This feature prevents the multivibrator Q2, Q3 from starting and producing a false holdup alarm signal in the event of an accidental break in the line A-B-C-D which connects the various holdup switches. An accidental ground on line A-B-C-D cannot cause a false holdup signal because the electronic circuit would still be shunted by line A-B-C-D. The holdup alarm signal can be initiated only by the proper sequence of a break followed by a ground on the line A-B-C-D, as will be described.

The initial current flow through the three paths identified above is small and will result in only a minor charging of capacitors C3 and C4 and a very minor charging of capacitor C5.

When the armature of the operated holdup switch makes with the opposite contact, i.e., when contacts H1-M or H2-M are closed, ground potential is applied to point D. A larger current now flows through the three paths enumerated above, charging the capacitors C3, C4 and, by increasing the base current through transistor Q2, causing the emitter-collector junction of that transistor to become partially conductive and increasing the base current and consequently the collector current of Q1 to allow capacitor C5 to start charging at an appreciable rate. Simultaneously, the application of ground potential at point D shunts diodes D6, D7, D8 thereby removing the reverse bias from the emitter of transistor Q2. Since the shunt provided by diodes D4, D5 and the burglar alarm circuit 18 across the base-emitter circuit of transistor Q2 is not sufficient to prevent transistor Q2 from becoming conductive, the multivibrator will be ready to start when capacitor C5 is sufficiently charged.

The multivibrator, as indicated on the drawing, comprises transistors Q2, Q3 and their associated capacitors and resistors. As is the case with any astable multivibrator, either transistor may become conductive first depending on such indeterminate factors as transient circuit conditions, component tolerances, etc. Assume therefore that transistor Q2 becomes conductive first when capacitor C5 is sufficiently charged to permit base current to flow to transistor Q2 via the path resistor R5 and diode D2 and via the path resistor R7, capacitor C2 and diode D2 and capacitors C1 and C2 are charged in the polarities shown in the drawing.

With transistor Q2 conductive, the collector-emitter circuit thereof allows sufficient current to flow through the emitter-base junction of transistor Q1 to cause it to become fully conductive. The increased current flow through the emitter-collector circuit of transistor Q1 completes the charging of capacitor C5. Current also flows through the path consisting of resistor R6, discharging capacitor C1, diode D1 and the collector-emitter junction of transistor Q2 to ground at point D. The voltage across capacitor C1 reverse biases the base of transistor Q3 keeping that transistor nonconductive until capacitor C1 discharges to the point where transistor Q3 is no longer reverse biased. Transistor Q3 then begins to conduct, drawing base current via resistor R6 and diode D3, and current flows through the path resistor R5, discharging capacitor C2 and the collector-emitter junction of transistor Q3 to ground at point D. The voltage drop across capacitor C2 reverse biases transistor Q2 causing it to become nonconductive for the moment.

When transistor Q2 becomes nonconductive, the base current path for transistor Q1 is removed (capacitors C3 and C4 are charged by now) and transistor Q1 becomes nonconductive. With transistor Q1 nonconductive and the line A-B-C-D open at the operated contacts H1-B or H2-B, the transmission line to the central station is open and this condition represents the beginning of a break period in the pulsing holdup alarm signal.

Transistor Q2 remains reverse biased until capacitor C2 becomes sufficiently discharged to overcome the bias and transistor Q2 conducts again. Because transistor Q1 is now not conducting, base current for transistor Q2 is supplied from capacitor C5 (whose function is to provide current for the multivibrator when transistor Q1 is nonconductive) via the path resistor R7, capacitor C2 and diode D2. With transistor Q2 conductive, transistor Q1 is switched on again and current flows from point A to recharge capacitor C5 and to ground via the breakdown of Zener diode D9. The Zener diode D9 also performs the functions of establishing a constant voltage for the multivibrator and limiting the charge on capacitor C5. Diode D9 might be of the 1N4736 (6.8 v.) type.

The switching on of transistor Q1 marks the end of the break period and the beginning of the ground period in the signalling sequence. The transistor Q3 becomes conductive again when the charge on capacitor C1 overcomes the reverse bias voltage across resistor R6 and the multivibrator continues with transistors Q2 and Q3 turning each other on and off. Whenever transistor Q2 is conductive, transistor Q1 is conductive and a ground is applied to the transmission line 12. When transistor Q2 is nonconductive, transistor Q1 is nonconductive and a break appears in the transmission line 12.

Had transistor Q3 become conductive first when the multivibrator started, the sequence of events would have been the same, but shifted half a cycle in time. The system will continue to operate at a nominal repetition rate as determined by the time constant of the multivibrator until the actuated holdup switch is reset thus restoring the shunt A-B-C-D. The capacitor charges will then bleed off through their associated resistors and the system will return to the normal condition. It is desirable to make the repetition rate about 3 cycles per second so as to conform to the usual practice and to permit operation with existing central station equipment.

The Zener diode D10 provides a bypass from point A to point E to permit high voltage reverse polarity testing of the regular protection circuit, as is customary, and to dispose of any high voltage transients which may appear on the transmission line.

Local annunciation of a holdup alarm may be provided if desired, e.g., by connecting a diode, a local battery and an alarm bell or lamp in series between conductors 19 and 22, the diode being poled to prevent line current from flowing through the alarm device when none of the holdup switches are operated.

In a typical system the central station battery may supply a voltage in the range of 52 to 78 volts. Typical values for the various resistors and capacitors are set forth in the table below. It should be understood, however, that these values as well as the various voltages, transistor types and diode types set forth herein are given only by way of example and should not be taken as limiting the scope of the invention.

Resistor Ohms

R1 100,000

r2 2,700

r3 1,000,000

r4 18,000

r5 12,000

r6 56,000

r7 3,000

r8 120,000

r8 120,000

capacitor Microfarads

C1 5.6 (35 v.)

C2 22.0 (15 v.)

C3 0.1 (100 v.)

C4 0.5 (200 v.)

C5 200.0 (12 v.)

Should it be desired to operate the system with a positive grounded central station battery, PNP transistor Q1 would be replaced with an NPN transistor, e.g., of the 2N2405 type. NPN transistors Q2 and Q3 would be replaced with PNP transistors, e.g., of the 2N4036 and 2N3906 types, respectively. The polarities of the diodes and capacitors would be reversed and the values of resistors R2, R4, R5, R6 and R7 would be changed to 1,800 ohms, 22,000 ohms, 18,000 ohms, 68,000 ohms and 5,600 ohms, respectively.

In order to supervise the system against opens and grounds on the transmission line and on the shunt link A-B-C-D, a tone generator 20 is shown with its output coil 25 connected in series between point E and burglar alarm circuit 18. The tone generator superimposes an alternating signal, e.g., 1,000 cycles per second, on the normal DC current flow. An open or ground on the transmission line or on the link A-B-C-D will prevent the tone from reaching the central station. Presence or absence of the tone is detected by tone responsive device 16 at the central station, and absence of the tone is used to signal a fault condition. If it should be desired to supervise only the transmission line, the tone generator 20 may be connected between the high side and the transmission line and resistor 17 or between resistor 17 and point A.

While the invention has been described in connection with specific embodiments thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.

* * * * *

Current U.S. Class:	340/286.04 ; 340/532; 340/533; 340/650; 340/693.1
Current International Class:	G08B 21/00 (20060101); G08B 21/02 (20060101); G08b 029/00 (); G08b 019/00 ()
Field of Search:	340/409,276