US3553687A - Alarm system sensitive to cut cables and insensitive to r-f interference - Google Patents

Abstract

IN AN ALARM SYSTEM WITH A BALANCED SHIELDED MICROPHONE CABLE A BARE DRAIN WIRE IS TIED TO THE CABLE SHIELD AT THE END OF THE MICRPHONE CABLE, THE END MICROPHONE BEING TERMINATED WITH RESISTORS SO THAT CABLE CUT BRIDGE CIRCUIT DETECTS AN UNBALANCE WHEN ANY MICROPHONE CABLE IS CUT TO PROVIDE AN ALARM SIGNAL AND REDUCE R-F INTERFERENCE.

Description

SEAQOM u 3,553,687

M. H. REISS ALARM SYSTEM SENSITIVE TO CUT CABLES AND Jan. 5, 1971 INSENSITIVE TO R-F INTERFERENCE Filed June 5 l Int. Cl. GOSf 29/00 U.S. Cl. 340-409 `ABSTRACT OF THE DISCLOSURE In an alarm system with a balanced shielded micro- United sta-'tes Pat-em o 11 Claims 1 phone cable a bare drain wire is tied to the cable shield l at the end of the microphone cable, the end microphone being terminated with resistors so that a cable cut bridge circuit detects an unbalance when any microphone cable is cut to provide an alarmsignal and reduce R-F interference.

BACKGROUND OF THE INVENTION The present/invention relates in general to alarm sys-v tems and more particularly concerns an acoustical alarm system of the type including an input transducer for sensing an alarm condition having means for sensing that a cable from the sensing transducerto the alarm indicating equipment has been cut.

Acoustical alarm systems, such as the Detectalarm syst tem made by Alarmtronics Engineering, Inc. perform well for the purpose of indicating an alarm condition when a microphone receives an unfriendly noise produced by an'intruder. Should Vvan unfriendly intruder somehow cut the cable.` joining the sensing microphone to the alarm indicating unit, such as by gaining access to the'cable during the day when the alarm system is disabled, it is possible that such an intruder could gain en- SUMMARY OF ,THE INVENTION Accordingto the invention, there is transducing means at a remote location for sensing an unfriendly signal source and providing an electrical signal overa transmission cable to the alarm condition monitoring unit.

The transmision cable includes a drain conductor con-` n nected to the end of the cable shield where the most remote transducer is located, this most remote transducer l 3,553,687 y Patented Jan. 5, 1971 'icc sensing transducer have a pair of equal resistors con nected therebetween with the junction of these resistors connected to the shielded conductor. The alarm unit end of the cable is coupled by a pi network to the primary of the input transformer that carries normal microphone sounds to the main channel amplifier. The primary of this transformer is center tapped and coupled to a detecting circuit that senses a D-C imbalance whenever a cable is cut to provide an appropriate signal that could be an -alarm signal. Twisted conductors and a grounded drain wire coupled to a balanced input help reduce R-F interference.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawings.

BRIEFA DESCRIPTION OFTHE DRAWING The single gure showsa schematic circuit diagram of a preferred embodiment .of the invention.

DETAILED DESCRIPTION OF PREFERRED l EMBODIMENTS With reference now to the drawing, there is shown a schematic circuit diagram of an exemplary embodiment of `when an unfriendly noise is sensed. The output of detecting circuitry 22 will also cause alarm indicator 21 to indicate an alarm condition when any of the cables have been cut.

Each of the microphones 14, 15 and 16 are the end microphones of cables 11, 12 and 13. It is to be understood that additional lmicrophones may be connected into the main cables 11, 12 and 13 at intermediate points,

` such as those designated by the letters A, B and C along the line, but these microphones are not terminated with 'resistors as are in microphones 14, 15 and 16. The detailed description which follows shall be only of elements associated with main cable #l because those associated with cables `#2 and #3 are similar.

Each of the end microphones may be of a type having an input winding such as 31 with each end connected to one of the two twisted microphone signal conductors, such as 32 and 33. Equal resistors, such as 34 and 35, terminate the ends of winding 31 as shown with their junction 36 connected to the microphone case 37 and to the bare drain wire 41 shown by broken line for clarity inside microphone cable 11 which in turn is grounded v at 'the input terminal 42 at the alarm monitoring unit.

being terminated in impedance means such that cutting of the cable produces an unbalance that is sensed by a detecting unit. More specifically, the transmission cable'is balanced two-wire shielded cable including a third conductor in side the shield that is connected to the shield at the microphone end and to ground at the unit end. The two twisted signal wires that are connected to the remote This bare drain wire 41 is also vconnected to the cable shield 43 at the end 44 as indicated. Signal conductors 32 and 33 are connected by equal isolating resistors 45 and 46, respectively, to R-F chokes 47 and 48, respectively, which in turn are connected to respective ends of the primary 51 of transformer 52. The secondary 53 of transformer 52 is coupled to the main alarm channel amplifier 17 to transmit sounds received by any of the microphones to alarm indicator 2l for analysis and provision of an alarm indication when the noise is unfriendly.

Primary winding 51 has a center tap 50 that is coupled by R-F choke to the base of transistor Q1 and one end ofbalancing potentiometer 56 of cable cut bridge -circuit 57. The bridge circuit 57 includes only three outside arms, the first beingbalancing potentiometer 56 in series with resistor 61, the second being resistor 62 and the third being resistor 63. When transistor Q1 conducts, the series combination of its base-emitter resistance and resistor 64 is connected across opposed junctions 65 and 66 is normally at` +-3 v. DC when the cables are not cut and the end microphones terminate the cable. However, if the cable becomes cut, center tap 54 is no longer at a +3 v. DC potential. The -base of transistor Q1 then begins to rise toward the +6 supply voltage on terminal 67 at the said drain conductor being connected to said shield at the end thereof near said transducing means. 3. An alarm system in accordance with claim 1 and further comprising,

junctions of resistors 61 and 62. Transistor Q1 then conat ducts, and its collector dropsin potential as current is drawn through collector resistor 71. Since transistor Q2 PNP transistor Q2, previously nonconductive because its emitter is coupled through emitter resistor 72 to the +6 volt potential on terminal 67, conducts. The potential on the collector of transistor Q2 then begins to rise as current flows through its collector resistor 73 connected to the -4 volt potential on terminal 74. This positive going potential rise is coupled by diode D1 to alarm indicator 21 to provide an alarm indication, thereby warning that the cable has been cut.

The invention thus allows virtually any practical number of microphones to be connected to the monitoring unit while providing an indication that even one microphone signal conductor has been cut, for cutting any one signal conductor willdestroy the balance and cause tran sistor Q1 to conduct. This `cable cut monitoring is` ac `complished reliably land with a relatively few inexpensive componentsthat consume negligible power.

In sumary the invention detects cable cut, reduces vR-F interference and utilizes a pair of lines performing the dual functionof bothcarrying D-C cable cut current and audio signal current. a

It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific embodiment describedherein without departing from the inventive concepts. Consequently, the invention is to be construed asembracing each and every novel feature and novelcombination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spiritand scope of the appended claims.

The embodiments of the invention .in which an exclu v vhas its base connected to the collector of transistor Q1,

n 10 Potentiometer 56 is normally adjusted so .that tran-rv vsistor Q1 is nonconductive. The base of transistor Q1 `alarm signal'monitoring means at the remote location having an input balanced with respect to a reference f cable cut signal indica able havin been QL.

4. An alarm system in accordance with claim 3 wherein Asaid means for coupling to .the latter balanced input comprises a pi resistive network having first and second legs of substantially equal resistance in series with respective ones of said firstand second signal conductors.

sive property or privilege is claimed, aredefinedas follows:

-1. An alarm system comprising, i' transducing means for providing a signal representative of an alarm condition, cable means for coupling said transducing means to a ocation remote from said transducing means, said .cable means having first and second signal conductors, and a drain conductor, and first and second substantiallyA equal resistors con- 1 nected in series across the input'to saidtransducing means, the junction of said series-connected resistors being connected to said drain conductor, the other end of each of said resistors being connected 1 vto a respective one of said first andsecond signal 5. `An alarm system in accordance with claim 3 wherein said latter balanced input comprises a transformer having a tapped primary, the tap on said primary comprising said intermediate terminal.

6. An alarm system in accordance with claim 5 wherei in said .detecting means comprises,

' a bridge having first, second and third resistive legs connected in series between a first bridge junction and said reference terminal to define asecond bridge junctionbetween said first and second legs and a third bridge junction between said second and third legs,

means for `D,C coupling said tap to said rst bridge junction,

a normally nonconductive first transistor having its base connected to said first bridge junction, its emitter D-C coupled to` said third bridge junction and its collector D-C coupled to a source of energizing potential that is'connected to said second junction,

:jwhereby cutting of said cable causes said transistor tov become conductive.

7. An alarm system in accordance with claim 6 where 1n said cable comprises said first and second signal con- .ductors forming a twisted pair inside an insulatedly separated conducting-shield also enclosing said drain conductor,

said drain lconductor being connected to said shield at the end thereof near said transducing means. 8. An alarm system in accordance with claim 7 wherem said means for coupling to the latter balanced input y comprises api resistive network'having first and second legs of substantially equal resistance in series with respective ones'of said first and second signal conductors. 9. An'alarm system in accordance with claim 3 and further comprising at least one other of said transducing means,

at least one other of said cables'forlcoupling said at l least one other transducing means to the remote location, f `at least one other pair of first and second substantially equal resistors connected in series Aacross the input to said at least one other transducing means, the junction of the latter series connected resistors being connected to the drain conductor of said at least one other cable,

the other end ofeach of the\latter resistors being conand means for coupling the latter rst and second signal f conductors.. to said alarm signal'monitoring means balanced input.

10. An alarm systern in accordance with claim 9 wherein each of said cables comprises said rst and second signal conductors forming a twisted pair inside an insulatedly separated conducting shield also enclosing said drain conductor,

said drain conductor being connected to said shield at the end thereof near said transducing means.

11. An alarm system in accordance with claim 1 and further comprising at least one other transducing means having first and second input terminals connected respectively to said rst and second signal conductors respectively at a point on said cable spaced from the end there- UNITED STATES PATENTS 1/1944 Cash et al 178-69 1/1965 Lamer 34e- 409 DONALD J. YUSKO, Primary Examiner 10 C. M. MARMELSTEIN, Assistant Examiner U.S. Cl. X.R.

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