US2928081A

US2928081A - Tuned intruder alarm system

Info

Publication number: US2928081A
Application number: US632863A
Authority: US
Inventors: Anthony J Grosso
Original assignee: American District Telegraph Co
Current assignee: American District Telegraph Co
Priority date: 1957-01-07
Filing date: 1957-01-07
Publication date: 1960-03-08
Anticipated expiration: 1977-03-08

Description

March 8, 1960 A. J. GRosso TUNED INTRUDER ALARM SYSTEM Filed Jah. 7, 195'? a 2,928,081 TUNED nuTRUDER ALARM srsrrnivrl o v Anthony J. Grosso, Long Island City, N.Y., assigner to @American District Telegraph Company, Jersey City, i NJ., a corporation of N ew Jersey The present invention relates to intruderV alarm systems and more particularly to such systems which are. intended to provide an alarm indicationk at a remote point.

In a direct current intruder alarm system where alarm signals are transmitted from protected premises to a remote station through a transmission line (usually a telephone line), it is Sometimes possible to defeat the protection b y substituting, at an intermediate point along the transmission line, a circuit which is the equivalent of that provided at the protectedapremises. Such defeat is most often feasible Where the rernote station is a police lacadquarters or the like rather than a @cnil-al station since the equipment and personnel osnally available at a `police headquarters are not as prepared to oope'with snel1 asili1` ation as are the equipment. and personnel at a central station. a

A principal object oi the invention has been the provision of a novel and improved circuit .for interconnecting protected. preprisosand a rernote stat-ion More particularly, it has been an object of the invention to provide a novel and improved circuit of the above type which will provide an alarm indication upon any effort to compromise the integrity of the system.

A feature of the invention has been the provision of a circuit in which current variations aredilferent at the protected premises and at the reporting station, and in which substitution of equipment at the protected premises cannot be effected with full knowledge of circuit conditions without access to both ends of the system.

The invention will now be described in greater detail with reference to the appended drawing in which there is illustrated one embodiment of the system in accordance with the invention.

Referring now to the drawing, there is shown a protected premises and a remote station 11 connected by a transmission line 12. At the subscribers premises there is provided a complexl impedance network'formed by a choke coil 13, a capacitor 14 and a resistor 15. The transmission line has distributed constants which may be represented by the resistors 16 and 17-and the capacitors 18. At the remote station there is provided a -complex impedance network formed by a choke coil 19,

a resistor 20, a transformer 21, a resistor 22 in parallel with a transformer 23, all connected in series, the combination being in parallel with a variable capacitor 24. The transformer 21 is preferably of the voltage stabilizing type.

The primary winding of transformer 21 is connected United States Patent fice sion line and the reaetancenetworls. attire remote station,

The loop is. brought to resonance, i-e-l a unity power, facs tor by tuning the reniole station reactance against the reactance oi the protected premises and the transmission line distributed constants Toning may most conveniently be effected byadjustrnent of the capacitora24.

A portion of thee-Cf current iles/ins in the loop is used to operate the. Il@- sigrlallins network- This ourz rent, which is delivered to the,l signalling network by' the. secondary Winding of transformer 2,3. is rectified by reclir` tier 2.5 and ilows throughalarm indicating deviceZo and resistor 27- The device A2,6 may be of any suitable. type but preferably is basically a salvanonieter type. relayvvhich can sive indications of both. increases and decreases in currente Such in 'cations maybe used to illu` Lts lights, ring bells or in any convenient way attract;'appro` priate attention when. the. currentr ow goes up or down..

soV

fier 25, an alarm indicating device 26 and a resistor 27.

`suppressed bythe capacitor 28..

Momentary sarees vA normal allarmis' initiated at the protected premises.

by shorting resistor 15- This causes an increase in AC- cdrrentow in the loop. An increase in A.C- current is reflected by an increase in current flow through the device 26, producing an alarrn signal Shorring of the resistor. 1,5 may be eiocted in anyconvenient way as by closing of norrnsllyopen contacts 219 of a relay 30 upon deenergization of relay 30. Relay 30 in turn will. be deenersized upon-operation of any of the protective cil'- cuits at the protectedpremises. for example. torturing Y of a window foilbpon anv intruders breaking a window.,

A break inthe transmission line at any point will resaltfin decreased corr-ent dow. which causes tbe device.

2.o to respond by giving al trobbie signal.-

Ap attempt, to compromise the reactance network. at the protected premises witha resistance network, and by coupling such a network to the transmission line, detunes the loop from unity power factor thereby initiating an alarm or trouble signal at the remote station depending upon whether the loop current increases or decreases. The coils 13 and 19 may be chosen so that shorting of the transmission line at or near the protected premises will cause the supervisory current at the remote station to decrease (giving a trouble signal) while the current at the protected premises would remain constant or increase slightly. Such aconstruction gives an added advantage since anyone attempting to defeat the system through substitution of the protected premises reactance network will not, at any time, have complete knowledge of actual circuit current variations. For this purpose, the inductor 19 should be operated in a region of its hysteresis curve so that it is, in effect, a swinging choke, i.e., one in which the induction increases and decreases with increases and decreases, respectively, in current.

Use of the transformer 23 isolates the D.C. network from the A.C. loop, thereby eliminating the possibility of circuit compromise through D.C. voltage substitution.

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

What is claimed is:

1. An intruder alarm system for providing protection to protected premises from a remote station coupled thereto by a transmission line, comprising a first reactance network at the protected premises, a second reactance network at the remote station, said reactance networks being intercoupled by said transmission line to form a loop, a source of alternating current for -said loop, and means at said remote station responsive to increases and decreases in the current flow in said loop to provide an alarm indication, said reactance networks being adjusted to provide unity power factor in said loop.

' 2. An intruder alarm Asystem for providing protection to protected premises from a remote station coupled thereto by a transmission line, comprising a iirst reactance network at the protected premises,` a second reactance network at the 'remote station, said reactance networks being intercoupled by said transmission line to form a loop, a source of alternating current for lsaid loop, means at said remote station responsive to increases and decreases in the current flow in `said loop to provide alarm and trouble'indications upon increase or decrease in the loop current, said reactance networks being acljusted to provide unity power factor in said loop, and means at said protected premises to increase the current flow Vin said loop upon detection of an intrusion.

3. An intruder alarm system for providing protection to protected premises from a remote -station coupled thereto by a transmission line, comprising a first reactance network at the protected premises, a second reactancc network at the vremote station, said reactan-ce networks being intercoupled by said transmission line to form a loop, a source of alternating current for said loop, including a rectiiier and a device responsive to increases and decreases in current flow at said remote station to provide an alarm indication transformer vfor coupling said last mentioned means to said loop, said reactance networks being adjusted to lprovide unity power factor in said loop, at the -frequency of said alternating current, and means at-said protected premises to increase the current ow in said loop upon detection of an intrusion.

4. An intruder alarm system for providing protection to protected premises from a remotetcentral station coupled thereto by a transmission line, comprising a iirst impedance network coupled across said transmission line at the protected premises end thereof and including, in

prising a second inductor and a second resistor in series across said line and a second capacitor in parallel therewith across said line, said impedance networks and said transmission line forming a loop, a iirst transformer having a secondary winding included in said loop to supply alternating current thereto, a second transformer having a primary winding included in said loop at said remote station, a rectier and a current sensitive indicating device coupled to the secondary winding oftsaid second transformer to provide a local D.C. signalling circuit, and means responsive to detection of an intrusion into said protected premises to change the Value of said first resistor, the reactance of said second impedance network being adjusted, at the frequency of said alternating current, so as to be in resonance with the reactance of said rst impedance network and said line whereby said loop is normally at unity power factor.

5. An intruder alarm system as set forth in claim 4 in which said first and second inductors are adjusted so that a short on said line adjacent said protected premises will decrease the current in said loop at said remote station.

6. An intruder alarm system as set forth in claim 4 -in which said current sensitive indicating device provides different indications for increases and decreases in loop current.

References Cited in the le of this patent UNITED STATES PATENTS 967,113 Dean Aug. 9, 1910 1,992,255 Swadener Feb. 26, 1935 2,064,980 Kelly Dec. 22, 1936 '2,067,443 Gewerty Jan. 12, 1937 2,386,942 Edelman Oct. 16, 1945 2,455,376 Lindsay Dec. 7, 1948 2,720,627 Llewellyn Oct. 11, 1955 2,768,355 Nebel Oct. 23, 1956 2,771,598 Gerber Nov. 20, 1957