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Publication numberUS3707708 A
Publication typeGrant
Publication dateDec 26, 1972
Filing dateDec 16, 1970
Priority dateDec 16, 1970
Publication numberUS 3707708 A, US 3707708A, US-A-3707708, US3707708 A, US3707708A
InventorsDan Carlo
Original AssigneeMultra Guard Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Muting circuit for a security alarm system providing a sonic alert
US 3707708 A
Abstract
An electronic circuit enabling an operator at a central station to mute a sonic alarm device energized in the event of an emergency condition occurring at a remote station under surveillance at any time from the moment the alarm is heard and identified at the central station to the time when a normal condition is restored. The circuit additionally includes means which reenergizes the sonic device automatically as soon as the normal condition is restored to remind the operator that muting had previously occurred and further requiring the operator to manually reset the circuit to deactivate the reenergized sonic alarm device.
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Description  (OCR text may contain errors)

United States Patent Dan [451 Dec. 26, 1972 [54] MUTING CIRCUIT FOR A SECURITY ALARM SYSTEM PROVIDING A SONIC ALERT [72] Inventor: Carlo Dan, Bethesda, Md.

[73] Assignee: Multra-Guard, Inc., Rockville, Md.

6/1964 Sargent ..340/326X l2/l970. Porter ..340/213.l X

Primary Examiner-David L. Trafton Attorney-Brady, OBoyle & Gates [5 7] ABSTRACT An electronic circuit enabling an operator at a central station to mute a sonic alarm device energized in the event of an emergency condition occurring at a remote station under surveillance at any time from the moment the alarm is heard and identified'at the central station to the time when a normal condition is restored. The circuit additionally includes means which reenergizes the sonic device automatically as soon as the normal condition is restored to remind the operator that muting had previously occurred and further requiring the operator to manually reset the circuit to deactivate the reenergized sonic alarm device.

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PATENTED um: 26 1912 SHEET 5 OF 6 MUTING CIRCUIT FOR A SECURITY ALARM SYSTEM PROVIDING A SONIC ALERT CROSS REFERENCE TO RELATED APPLICATION The present application is related to U.S. Patent Ap- BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to supervised alarm systems and more particularly to a system wherein one or more remote stations under surveillance can respectively communicate with a central monitor station via a two wire telephone line to provide an emergency warning or other type signal theretocomprising, inter alia, a sonic alarm to indicate at the central station that an alarm condition is present. This invention is particularly adapted to be utilized as a burglary or unwanted intrusion alarm, a robbery or hold-up alarm, and even a fire alarm system.

2. Description of the Prior Art Security alarm systems typical of the prior art presently in use are simple in construction and normally comprise a single closed series loop circuit, including a perimeter protection circuit, extending between the central monitoring-station and the protected premises. The perimeter circuit is adapted to include frangible resistive tapes, trap wires, etc. A break in the perimeter circuit thus breaks the closed loop circuit to the central station and initiates an alarm.

The power supply for such systems consists of a battery or other constant voltage power supply located either in the central monitoring station or at the protected premises. With either arrangement, the system is easily compromised or defeated because the specifications for such systems, as dictated by the industry and consumer testing facilities such as the Underwriters Laboratories, presently require that the systems merely give an indication if the resistance in the total circuit loop in the, system varies i 50 percent. Where telephone lines are utilized to interconnect the protected premises to the central station, the resistance of the lines themselves is in the range of 3000 ohms. Where a 100 ohm perimeter circuit is connected in series to the 3000 ohms of the telephone lines, an alarm will not be given until the resistance in the entire loop varies by approximately i 1500 ohms. It can be seen, therefore, that the resistance in the perimeter circuit can be tampered with over a considerably wide margin and in some cases entirely removed without interrupting the tolerance built into the system suff cient to sound an alarm.

In order to defeat such a series system where the battery or power supply providing energy for the system is located in the protected premises, it is merely necessary for one attempting to defeat the system to take a voltage measurement across the phone line entering the protected premises that constitutes the circuit to the central station. This will provide information on the size of the battery that must be substituted across the lines for the battery inside of the protected premises and by connecting the battery of the correct potential SIRO across the line and then cutting the line into the premises, the premises is removed from the circuit to the central station without interrupting the loop which then can be entered at will without an alarm being indicated at the central station. I

Where the battery or power supply is located at the central station as opposed to being in the protected premises, one attempting to compromise the system must first determine the proper resistance to connect across the line going into the premisesthat connects the protected premises with the central station, but does not have to determine the supply potential. In the first example, one must also determine the substitute resistance for the protected premises and this is done in thesame manner as will now be outlined.

A burglar or intruder, for example, must first place a ammeter in one of the lines entering the premises, then cut the line so that the circuit is completed through the ammeter and obtain information as to the current in the line. The line is then spliced together'and then the ammeter removed. A voltage measurement is then taken across the two lines into the premises with a volt meter and with this information and using a simple Ohms law calculation, the resistance in the perimeter circuit of the premises can be determined knowing that a substantial portion of the resistance in the loop circuit is in the phone line itself. A resistance of the value calculated is shunted across the two lines entering the premises and the two lines into the premises are then out since it is removed from the surveillance loop to the central station without interferring with the loop or initiating an alarm at the central station.

The burglar or intruder does not have to be accurate in determining the store resistance and normally if a resistance in the amount of 200 ohms is shunted across the line, this will defeat the system without sounding an alarm because it is well within the tolerance range of the permissible resistance change in the entire loop before the alarm is sounded. In the system where the power supply is in the protected premises, the substitute resistance is placed in series with the substituted battery. In cases where the protected premises is extremely far from the supervising central station the lines into the premises can be jumpered together without any substitute resistance whatsoever without sounding an alarm because the resistance in the store is less than the resistance range tolerated by the system before initiating an alarm.

Realizing these deficiencies, the above crossreference related application is directed to a system wherein a constant current DC generator powers a communication link including a pair of telephone lines coupled between a protected premise and a central monitor station. The communications link includes means for reversing the DC polarity of the constant current from a predetermined polarity indicative of a normal condition to an opposite polarity in response to a detected alarm condition which occurs in response to a change in the operating parameters of a separate supervisory circuit located at the protected premises. The change in polarity is sensed at the central station and an indication of the alarm condition is presented by means of indicator lights and a sonic alarm. DC signalling of the alarm condition as well as the normal condition is thus provided by means of constant current control and DC polarity sensing. Signalling back to the protected premises from the central station is achievable by interrupting the continuity of the communication link and causing the constant current DC generator to sense the open circuit condition.

In addition tosecurity alarm systems per se, the prior art also includes teachings of alarm monitoring systems of which U.S. Pat. No. 3,452,345 issued to G. E. Kinsey is an illustrative example as well as automatic alarm enunciator circuits of which the following are illustrative examples: U.S. Pat. No. 3,525,988, K. C. Linder; US. Pat. No. 3,480,938, M. E. Martin; and US. Pat. No. 3,381,286, R. R. Walsh. The Kinsey patent is generally illustrative. of a system wherein the monitoring units contains an indicating unit which when placed in the alarm condition remains in that condition until it is reset, even through the protection circuit it monitored is restored to its normal condition. The other patents are generally illustrative of circuits which require an acknowledgement of a supervisor having noted an alarm condition. Also a typical muting type switch arrangement for an audible aLarm is disclosed in U.S. Pat. No. 2,971,186, issued to T. Ripepi.

SUMMARY The subject invention is directed to an improvement in alarm monitoring systems and more particularly to a muting circuit for a sonic alarm device utilized in connection with a security alarm system and comprises: first circuit means coupled to the sonic alarm for activating said sonic alarm .device in response to a first or steady state and a second or pulsatingtype of alarm signal and being operable to maintain said sonic alarm activated in absence of said alarm signals once having been applied; second circuit means coupled to said sonic alarm system, being normally inoperative but becoming operative upon the application of a control signal fed-thereto which then provides a shunt across said sonic alarm and thereby render it non-operative; third circuit means, including a muting switch, coupled to said second circuit means becoming operable by the closing of said muting switch to couple said control signal to said second circuit means only during the presence of either of said two alarm signals; fourth circuit means coupled to said third circuit means supplying power thereto only in response to the first alarm signal; and fifth circuit means coupled to said third circuit means supplying power thereto only in response to said second alarm signal, said second circuit means under the control of said third circuit means being adapted to deactivate the sonic alarm device when the muting switch is closed; but upon the cessation of the first or second alarm signal the shunt provided by said second circuit means is removed due to the removal of power supplied to said third circuit means whereupon said first circuit means again reactivates the sonic alarm device. The first circuit means additionally includes a reset switch coupling power thereto which when manually opened is adapted to render said first device circuit means inoperable and thereby deactivates the sonic alarm once more. This circuit thus enables an operator to mute the sonic alarm any time from the moment an alarm signal is detected to the time when the normal condition is restored; however, as soon as the normal condition is restored, the sonic alarm device will again sound and will remain thus until the operator depresses the reset switch.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the security alarm system contemplated for use in combination with the subject invention; 7

FIG. 2 is an electrical schematic diagram of a constant current'DC generator and current sensing circuit utilized by the system shown in FIG. 1;

FIG. 3 is an electrical schematic diagram illustrating the current polarity reversing circuit, the perimeter-intrusion circuit, the hold-up circuit, and audio'surveillance circuit located at the protected premises for the system shown in FIG. 1; I

FIG. 4 is a schematic diagram illustrating an audio receiver amplifier and squelch circuit located at the central monitor station for the system shown in FIG. 1; 5 FIG. 5 is an electrical schematic diagram of the supervisory logic circuit located at the central station for providing supervisory indications of the condition of the protected premises for the system shown in FIG. 1; and

FIG. 6 is an electrical schematic illustrative of the preferred embodiment of the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and more particularly to FIG 1, there is disclosed in block diagrammatic form an overall system diagram wherein a plurality of protected premises 10 10, and 10,, are monitored at a remote central station 12 including a respective plurality of supervisory circuits 14 14 and 14,, which are coupled back to the protected premises by means of respective pairs of telephone lines 16 16 and 16,,. Each protected premises, for example, is provided with signalling circuitry for providing an indication, both visual and sonic, at the central station 12 of various alarm conditions such as burglary and/or unwanted intrusion, robbery or hold--up as well as audio surveillance of the premises under specified conditions, e.g. night" operation or during a hold-up etc.

More particularly, each protected premises contains a constant current DC generator 18 which is adapted to transmit a relatively small DC current (1.5 ma. or 3.0 ma.) over an ungrounded or floating two wire telephone line 16 to a supervisory logic circuit 20 form- 'ing part of the supervisory circuit 14. The DC current is made to pass through a current polarity reverse circuit 22 which is inserted in the current path to the telephone line 16 to provide a polarity reversal in the event of an attempt to compromise a perimeter-intrusion circuit 24 or a hold-up circuit 26 is manually actuated. The current polarity is sensed bythe supervisory logic circuit 20 providing an indication thereof by indicating lights generally shown by reference numeral 28. As will be described in greater detail in the subsequent description, the perimeter-intrusion circuit 24 provides a steady state current reversal in the polarity reverse circuit 22 whereas the hold-up circuit 26 is adapted to provide a continuous alternating reversal whereupon the indicator lights 28 provide a coded indication of which condition is occurring. A sonic alert device 27 and a muting circuit 29 ,therefor which comprises the subject invention is also coupled to the supervisory logic circuit 20 to provide a controlled sonic alarm indication as will be explained subsequently in detail.

Additionally, the perimeter-intrusion circuit 24 when compromised, either intentionally or otherwise, activates an indicator light 30 at the protected premises 10. Audio communication or surveillance is provided by an audio transformer 32 coupled into the DC current communication link including the two wire telephone line 16. An audio amplifier 34 coupled to one or more microphones 36 located in the protected premises forming an audio surveillance circuit 37 is connected to the audio transformer 32 so that audio signals picked up by the microphone 36 are transmitted to the respective supervisory circuit 14 whichalso includesa corresponding audio transformer 38 coupled to the telephone lines at thatend. The outputof the audio transformer 38 is then coupled into an audio amplifier 40 which also has a squelch circuit 41 coupled thereto so that an output to a loud speaker 42 will only be provided when the audio level back at the protected premises exceeds a predetermined value. The squelch circuit 41 also contains a reset switch not shown which is adapted, inter alia, to control the operation of the muting circuit 29.

A current sensor circuit 44 at the protected premises is coupled to the constant current DC generator 18 to sense a current interruption of the DC current flowing between the DC generator 18 and the supervisory logic circuit located at the central station 12. An indicator light 46 is coupled to the current sensor circuit 44 to provide an indication of the DC current failure which may be caused for example by: (1) an accidental break in the telephone line 16,-(2) an attempt to insert a device such as a dummy load into the system for defeating the alarm circuitry, (3) or an intentional momentary opening of the circuit to provide a signalling back to the protected premises from the central monitor station 12. A push button switch 48 or the like contained in the supervisory circuit 14 at the central station 12 can provide the third operating mode. Additionally, the supervisory circuit 20 also contains means, as will be subsequently illustrated, to also sense an open circuit condition or a failure of the constant current DC generator 18 and provide an indication of such a condition by means of the indicator lights 28 as well as by the sonic alert device 27.

The use of the constant current DC generator .18 located at the protected premises 10 as disclosed in FIG. 1 is capable of employing currents at a relatively low value considering prior art relay type signalling circuits which necessarily employ currents as high as or milliamps. These higher currents produce circuit problems with extremely small gauge telephone line pairs presently available for leasing. As wire gauges get smaller, practical signalling distances become even shorter. As noted above, with a constant voltage variable current system a foreign potential of the right voltage and polarity can accidentally or intentionally be substituted for the signalling source without giving a false alarm. The latter condition occurs when a sophisticated or knowledgeable burglar for example defeats the system. The use of a constant current DC generator on the other hand makes it considerably more difficult to compromise the system because in order to do so a constant current generator must be substituted into the communication link externally of the protected premises while at the same time disconnecting the constant current generator 18 without interrupting the current flow in the telephone lines 16 or varying its predetermined magnitude either of which would be sensed by the supervisory logic circuit 20 whereupon an alann condition would be indicated.

Considering now the subject system in greater detail, reference now is made to FIG. 2 wherein reference numeral 50 designates a regulated DC power supply of conventional design for providing a +12 volt supply voltage. A 1 l7 volt/60Hz AC line potential is applied to the terminals 52 and 54 which is then coupled to the power supply 50 by means of a power transformer 56. The 12 volt supply voltage appears across the positive supply buss 58 and the negative supply buss 60. FIG. 2 additionally discloses a schematic representation of the preferred embodiments of the constant current DC generator 18 referred to in FIG. 1 as well as the.current sensor circuit 44. The constant current DC generator basically comprises a feedback controlled DC to DC voltage converter capable of providing two magnitudes (1.5 milliamps and 3.0 milliamps) of constant DC current. The DC T0 DC converter comprises a well known DC to AC chopper circuit including transistor switches 62 and 64 coupled to transformer 66 and driven by means of a free running multivibrator 68 including transistors 70 and 72. The action of the multivibrator 68 causes mutually opposite conductive states to alternately occur in transistors 70 and 72 for equal durations of time so that a square wave output signal appears at their respective collector electrodes. These square wave outputs are respectively coupled to the base electrodes of transistors 62 and 64 by means of resistors 74 and 76 so that transistors 62 and 64 are driven on and off accordingly. For example where transistor 62 is in an on state, transistor 64 is in an off state, and vice versa. Moreover when either of transistors 62 or 64 is on, the +12 volt supply potential is applied to one end of the primary winding 78 of the transformer 66. Depending upon the magnitude of the voltage appearing at the center tap 80 of the primary winding 78, a corresponding AC voltage will appear across the secondary winding 82 for the combined switching of transistors 62 and 64. Where for example the frequency of operation of the multivibrator 68 is in p the order of 2500 to 3000 cycles per second, a corresponding frequency AC output will appear across the secondary winding 82. A smoothing capacitor 84 is connected directly across the winding 82. The AC output voltage appearing across the secondary winding 82 is applied across opposite terminals 86 and 88 of a full wave rectifier bridge 90 so that a DC voltage appears across terminals 92 and 94.

A current sensing feedback control circuit for maintaining a predetermined constant current value at terminals 92 and 94 of the output of the rectifier bridge 90 is included which maintains the voltage at the center tap 80 at a predetermined voltage level so that either a 1.5 or 3.0 ma output is constantly provided. This is effected by means of a current sensing transformer 96 the primary winding 98 of which is coupled in series between one end of the secondary winding 82 and the input terminal 86'of the rectifier bridge 90. The magnitude of the AC current flowing through the primary winding 98' is sensed across the secondary winding 100 which is then rectified by means of the semiconductor diodes 102 and 104 and applied across one side 106 of capacitor 108 which has its opposite side 110 coupled to the center tap 112 of the secondary winding 100. The magnitude of the voltage across capacitor 108 then is proportional to the AC- current flowing in the output circuit of the transformer 66. A voltage divider circuit including fixed resistors 114, 116, 118 and thermistor 120 connected in parallel with resistor 116 is connected across capacitor 108. The voltage appearing at the junction between resistor 116and 118 is applied as one input to a differential amplifier including transistors 122 and 124. It should also be noted that the side 106 of capacitor 108 is also coupled to the +l2 volt supply buss. 58 through resistorv 126 thereby providing a substantially constant voltage at circuit junction 127. The input signal applied to transistor 122 which is proportional to the DC to. DC converter output current is compared against a fixed voltage reference provided by the diodes 128 and 130 as well as resistor 132 which is commonly connected to chassis ground in combination with the base of transistor 124.

Chassis ground is adapted to be at a +6 volt level provided by the perimeter-intrusion circuit 24 which will be subsequently considered. This +6 volt voltage is applied at terminal 134. One section A of a three pole two position Day-Night switch 136 is connected by meansof its normally closed contacts between chassis ground and the common connection between diodes 128 and 130. The normally closed position of switch section l-36-A referred to as the day position D, provides a fixed voltage reference to the other input of the differential amplifier including transistors 122 and 124 to produce a constant DC current of 1.5 ma.'out of the rectifier bridge at terminals 92 and 94. The open position of switch section 136-A is referred to as the night position N and provides a voltage reference which is adapted to cause a constant DC current of 3.0 ma. from the rectifier bridge 90 output.

The differential amplifier output provided at the collector of transistor 122 is directly coupled to the base of transistor 138 which in turn is coupled by means of its collector electrode and resistor 140 to the base of transistor 142. Transistor 142 is shunted across capacitor 144 which is directly connected between the center tap 80 of primary winding 78 and the negative side of the 12 volt supply appearing at the supply buss 60. Transistors 138 and 142 are controlled by the differential output of transistors 122 and 124 to cause transistors 142 to act as a variable resistive impedance across capacitor 144 to control the electrical charge on capacitor 144. If for example transistor 142 is fully conductive, it acts as a virtual short circuit across capacitor 144 whereupon it fully discharges and the center tap 80 is directly connected to the supply buss 60. When this condition occurs, a full 12 volt square wave is altemately impressed across each half of the primary winding 78. In the condition where transistor 142 is only partially conductive, capacitor 144 accumulates a charge whereupon the potential at the center tap rises to a value so that the amplitude of the square wave impressed across theprimary of the transformer 66 is decreased. Thus, the negative feedback provided by the current transformer 96, the differential amplifier including transistors 122 and 124 as well as the transistors 138 and 142 in combination with capacitor 144 will change the voltage at center tap 8 0 to-effectively control the magnitude of the DC current flow out of terminals 92 and 94 of the rectifier bridge and maintain it at a value as determined by the position of the day-night switch section 136- A.

Completing the consideration of the constant current generator 18 one or more stages of RC filter circuitry 146 is coupled across the output terminals 92 and 94 of the rectifier bridge in order to remove any undesired ripple or other noise present from being transmitted to the current polarity reverse circuit shown in FIG. 3.

In the event that circuit interruption for current flow from the secondary winding 82 of the transformer 66 occurs, the current transformer 96 will sense the current interruption and the. voltage appearing across capacitor 108 will fall whereupon transistors'148 and 150 in the current sensor circuit 44 will become con-. ductive since junction is coupled to the base of N- P-N transistor 148 by means of resistor 152. Since the emitter of transistor 148 is coupled to the +12 volt supply buss 58 by means of resistor 126', it will become conductive as the voltage at junction '110 falls. The

conduction of transistor 148 causes transistor 150 to become conductive, due to the coupling of the resistors 154 and 156. When transistor 148 becomes conductive, a signal is coupled to the base of transistor 150 by means of resistor 154 which renders it conductive. The conductive transistor 150 acts like closed switch and applies the +12 volts appearing on supply buss 58 across the green indicator lamp 46 which is connected between the collector of transistor 150 and the supply buss 60. Thus for any discontinuity of the predetermined DC constant current, the indicator light 46 will be turned on and an indication of this condition will be displayed at the protected premises 10.

Now referring to FIG. 3 attention is directed to the constant current polarity reverse circuit 22 which includes a pair of circuit leads and 162 which are coupled to the filter circuit 146 connected across the output terminals 92 and 94 of the rectifier bridge 90 shown in FIG. 2. Leads 160 and 162 are" respectively connected to sets of relay contacts 164 and 166 of a double pole electromagnetically actuated relay 168 which constitutes a current reversing relay energizable by means of the relay coil 170. The circuit leads 160 which has its primary winding coupled to the audio amplifier 34. The circuit lead 160 is connected to the output lead 174 through contacts 164 and the secondary winding 178 when relay 168 is in its normally deenergized position while circuit lead 162 is connected to the output lead 172 through contacts 166 and the secondary winding 176. When the current reversing relay 168 is energized, however, relay contacts 164 and 166 will operate to reverse the connections.

The purpose of the audio transformer 32 is to couple the microphone 36 and amplifier 34 when energized to the output leads 172 and 174. A capacitor 184 is coupled between opposite ends of the secondary windings 176 and 178 so that relay operation does not interfere with the operation of the audio amplifier 34 when energized. In this respect, the audio amplifier 34 is energized by the application of the +l2 volt supply voltage thereto either by means of resistor 186 which is connected to the night terminal N of the second section B of the day-night operation switch 136 or circuit lead 188 coupled back to the hold-up circuit 26. In the present system, therefore, the audio amplifier is energized when the switch 136-8 is manually switched to night operation or whenever the hold-up circuit is energized.

The perimeter-intrusion circuit 24 and the hold-up circuit 26 operate to energize the relaycoil 170 in the following manner. Considering first the perimeter-intrusion circuit 24, a frangibleresistive tape circuit of well known construction is generally illustrated by the fixed resistance 190. The value of this resistance is in the order of 25-100 ohms, depending upon the kind and length of tape and wiring utilized. It is applied to the perimeter of glass windows and doors etc. and is coupled in series between chassis ground and the +12 volt supply potential through a perimeter interlock switch 192, the third section 'C of the day-night operation switch 136, a variable resistor 194 and a fixed resistor 196. The +12 volts is coupled to the circuit by means of terminal 197 and supply buss 199 which is adapted to be connected to supply buss 58 shown in H6. 2. The resistive tape 190 is coupled in series with perimeter interlock switches 192 to the night terminal N of the switch l36-C while a variable resistance 198 is connected between the day terminal D and resistive tape 190, to chassis ground so that during day opera tion a resistance may be substituted for the perimeter interlock switch circuit. The variable resistor 194 is adapted to adjust the value of the potential applied across both the perimeter or protection circuit resistance 190 and the substitute resistance 198.

The voltage divider potential appearing across resistance 190 and the rheostat 198 is coupled to one input of a differential amplifier circuit 200 comprised of transistors 202 and 204. This input is applied through resistor 206 to the base of transistor 202. The other input to the differential amplifier 200 consists of a reference voltage applied to the base of transistor 204 by means of the fixed resistors 208 and 210 in combination with the adjustable resistance 212 all coupled in series between circuit junction 213 and the +12 volt supply voltage appearing in supply buss 199. The variable resistor-212 provides a means of adjusting the value of the reference potential applied to the base of transistor 204 to a predetermined value. Junction 213 is connected to chassis ground which in the embodiment of all the circuitry included at the protected premises 10 is made to be at a value of +6 volts. This is provided by the action of P-N-P transistor 214 which has its emitter directly connected to junction 213. The base of transistor 214 is connected to a voltage divider consisting of fixed resistors 218, 220 and 222 which are connected in series across the 12 volt supply potential. The negative pole f the supply potential is provided by supply buss 223 which is connected to terminal 216 which in turn is adapted to be connected to the buss 60 shown in FIG. 1. The common connection between resistors 220 and 222 is made to have a value of +6 volts so that the connection of the collector of transistor 214 to the negative buss 223 causes transistor 214 to become conductive and apply the +6 volts directly to chassis ground through junction 213. This +6 volt potential is also connected to terminal 221 which is common to terminal 134 shown in FIG. 2. The reason for this condition to exist is in order to allow the output signal voltages of the differential amplifier 200 appearing at circuit junctions 224 and 226 respectively to swing both upwardly and downwardly from the preset operating point. This is established by making the value of the variable resistance 198 substantially identical to the perimeter wiring and interlock switch 192 resistance and then with the switch section 136-C in the night position N, the variable resistance 194 is adjusted to provide' an input in the order of 60 millivolts to the base of transistor 202, at which value the differential amplifier 200 will respond to i 30 percent change in the perimeter tape resistance 190. Any deviation of the resistance 190, e.g. by breaking the tape or attempting to defeat system, from the i 30 percent value as established by the variable resistance 194 will cause one or the other of transistors 228 or 230 to become conductive.

The collectors of both P-N-P transistors 228 and 230 are coupled to the base of N-P-N transistor switch 234 by means of respective collector load resistors 236 and 238 such that when either transistor becomes conductive transistor 234 receives a signal which will render it conductive also. Since the emitter of transistor 234 is directly connected to the negative side of the supply potential appearing on buss 223 and whereas the collector is coupled to one side of the relay coil through diode 240 the conduction of transistor 234 causes the relay coil 170 to energize because the other side of relay coil 170 is directly connected to the +12 volt supply buss 199. Relay contacts 164 and 166 switch upon energization of coil 170 and reverse the polarity of the current flow of the constant current in output leads 172 and 174.

A tunnel diode 242 is coupled across the base emitter junction of transistor 234 in order to provide a more positive actuation of the relay coil 170 since the tunnel diode 242 will effect a much faster turn-on of the transistor 234 whenever either of the transistors 228 or 230 is rendered conductive. The turn-on of transistor 234 also energizes a red indicator lamp 30 which has one side connected to +12 volt supply providing an indication at the protected premises 10 of a compromise of theperimeter-intrusion circuit 24.

Whereas prior art apparatus in some cases utilizes differential relay circuitry which is directly actuated by a change in either the perimeter circuit resistance alone or the combination of the perimeter circuit resistance plus the resistance in the transmission line, the disclosed system is adapted to provide a greater sensitivity in the protection circuit due to the fact that the current reverse relay 168 is not directly controlled by the perimeter resistive tape circuit resistance but acts in response to a change only in the perimeter circuit resistance which is sensed by the differential amplifier 200. It is the highly sensitive differential amplifier 200 including the transistors 202 and 204 which control the relay coil 170 through the tum-on of transistor 234 which is in series with the relay coil across the +12 volt supply potential. In other words, most resistive sensing circuits employ sensitive relays. These relays necessari ly have wide tolerance between pull-in and drop-out current. Furthermore, in direct wire" alarm systems utilizing telephone lines, the line resistance is a part of the overall circuit and compromise protection'is not what it should be over relatively long high resistance lines. Also most conventional circuits will not measure minute changes in current.

In the present embodiment, however, the alarm condition threshold of the perimeter-intrusion circuit 24 is adjusted by means of rheostat 194 until the current flow through the perimeter resistance 190 equals a voltage input to the differential amplifier 200 which will not-trigger the turn-on of transistor 234. When the voltage drop across the perimeter resistance which is applied to the base of transistor 202 drops below or increases above the reference voltage appearing -at transistor 204 by a predetermined percentage-indicative of an alarm condition, the developed error signal operates to turn-on transistor 234 causing the current reverse relay 168 tooperate and maintain its new state until the alarm condition is corrected and provide an alarm signal back to the central station. While not specifically shown, the perimeter resistance 190 may when desirable include a positive or a negative temperature coefficient thermistor to compensate for environmental resistance changes.

In addition to the perimeter intrusion circuit 24 being operable to operate the current reverse relay 168 for a sensed change in the perimeter resistance 190, the hold-up circuit 26 is also adapted to activate the current reverse relay 168 but in a slightly different mode of operation. Whereas a detected change in the perimeter. resistance 190 will cause a steady state actuation of the current reverse relay 168, the hold-up circuit 26 will cause a continuousalternating reversal of the current relay 168 to constantly cause the current to reverse back and forth and provide a flashing type of visual indication at the central station 12 as well as a pulsating sonic beep;

Considering now the hold-up circuit 26, it is com prised of a normally deenergized free running multivibrator circuit 244 including transistors 246 and 248 which receive the +12 volt supply potential on supply buss 199 through a silicon controlled rectifier (SCR) 250 coupled to the +12 volt supply buss 199 over circuit lead 252. The actuation of a normally open pushbutton switch 254 causes a positive trigger voltage to be coupled to the gate of SCR 250 by means of resistors 256 and 257 and the diode 258 to turn the SCR 250 on". When SCR 250 lS on", i.e. in its conductive state, the +12 volt supply potential appears at circuit junction 260 which is at the cathode electrode of SCR 250, and which then applies the +12 volt supplypotential to the multivibrator 244 by means of resistor 262. The application, of the +l2 volt supply potential to the multivibrator 244 causes a square wave voltage output signal to be applied to the base of transistor 264 which in turn is coupled to transistor 266. Transistor 266 is alternately rendered conductive and non-conductive thereby which momentarily returns one side of the relay coil 170 to the negative supply buss 268. In this manner the current reverse relay 168 is operated in a flip-flop fashion having a frequency of operation deter mined by the operating frequency of the multivibrator I 244 which may be, for example, .one pulse per second.

In addition to energizing the multivibrator 244, the turn-on of SCR 250 by means of the hold-up switch 254 also couples the +12 volt supply potential to the audio amplifier 34 in the audio surveillance circuit by means of the diode 268 and resistor 186 in order to provide a .listening-in capability .of the protected premises at the central station 12. In this manner, this capability provides an advantage in that not only can the sounds and voices surrounding the hold-up situation be heard, but a description of the robbers canimmediately be transmittedto the central station for relay to the police department, etc. providing greater speed in apprehen- SlOl'l. I p A silicon controlled rectifier characteristically operates such that when its gate electrode is selectively triggered to turn the device on, it will remain in an on condition until it is deactivated'Thisis provided in the subject circuit by means of a normally open reset push-button switch 270 which is coupled across the device through capacitor 272 which charges up when the reset switch 270 is closed to render the SCR 250 non-conductive. When SCR 250 is turned I off the hold-up circuit 26 again reverts to its normal stand by state and the +12 volt supply voltage is removed both from the multivibrator 244 and the audio amplifier 34.

Completing the description of the hold-up circuit 26, the operation of the multivibrator 244 does not'activate or turn-on the red indicator light 30 in the perimeterintrusion circuit 24 due to the polarity coupling of diode 240, between the lamp 30 and one side of the relay coil which is common to the collector of transistor 266 The present embodiment of the circuitry thus described also has for its objective an override capability over the perimeter-intrusion circuit 24 in the event of the hold-up circuit 26 is simultaneously activated. This priority is provided by transistor 274 coupled across the base and emitter of transistor 234 in the perimeter-intrusion circuit 24. When the +12 volt supply' voltage is applied-to-the multivibrator circuit 244 the +12 volts supply potential is also applied to the base of the transistor 274 by means of circuit lead 276 and resistors 277 and 279 which will render transistor 274 conductive and apply a virtual short circuit across transistor 234 so that it cannot be activated and cause the red indicator lamp 30 to be energized notwithstanding the fact that the perimeter circuit resistance 190 has been compromised and a controlsignal indicative thereof has been coupled to either transistor 228 or 230 from the differential amplifier 200.

What has been described thus far is the circuitry contained at the protected premises where in a normal stand by state the constant current DC generator 18 shown in FIG. 2 applies a relatively minute DC current (1.5 or 3.0 ma. in a predetermined polarity direction over a two wire telephone line 16 which is preferably ungrounded, i.e. floating, to a supervisory logic circuit 20 at the central station 12. In the event of the compromise of the perimeter circuit resistance 190 only, the current reversal relay 168 is actuated in response to a control signal provided by a highly sensitive electronic differential amplifier 200 to cause a steady state polarity reversal of the constant current DC current in the telephone line 16. If a hold-up for example occurs, the hold-up circuit 26 is actuated whereupon the current reversal relay is caused to alternate continuously at a rapid rate causing the polarity of the DC current to switch back and forth as long as the hold-up circuit is initiated. Additionally, the hold-up circuit also causes the audio surveillance circuit including the microphone 36 and audio amplifier 34 to be actuated so that audio signals can additionally be transmitted from the protected premises 10 to the central station 12. Under certain conditions the audio surveillance circuit can be actuated independently such as for night operation by switch 136-8 (FIG. 3) whereupon a continuous audio surveillance can be provided over the two wire telephone lines.

Bearing the foregoing in mind, it becomes necessary that the supervisory circuitry 14 located at the central station 12 respond to the coded signalling respectively provided by the audio surveillance circuit 37, the holdup circuit 26 and the perimeterintrusion circuit 24. Referring now to FIGS. 4 and 5, the other end of the two wire telephone line 16 atthe central station 12 is coupled into a pair of input circuit leads 278 and 280 as shown in FIG. 4 which are connected to the audio transformer 38 shown in FIG. 1 which is identical to the transformer 32 located in the polarity reverse circuit 22 shown in FiG. 3. Circuit leads 278 and 280 are connected to one end of the windings 284 and 286 respectively which at this end of the communication link act as primary windings. The opposite ends of windings 284 and 286 are directly connected to the supervisory logic circuit 20 shown in FiG. 5 by means of circuit leads 288 and 290. In the event that an audio signal is sent over the two wire telephone line 16, it will appear across winding 292 of transformer 38 which is then coupled to a conventional transistor audio amplifier including transistors 294, 296, and 298. The audio signal is capacitively coupled from the transformer winding 292 by means of the capacitor 300 which is connected to a volume adjust potentiometer 302 which is adapted to couple only a selected amount of the signal amplitude appearing across transformer 292 to the amplifier. Ordinarily, any audio signal coupled to the transformer 282 from the protected premises would be amplified and heard at the central station 12 by means of a loudspeaker 304 coupled to the common emitter connection between transistors 296 and 298 by means of the capacitor 306. t

In the embodiment shown in FIG. 4, a squelch circuit 41 is employed to normally render the audio amplifier 40 deenergized until a certain predetermined signal level appears across the winding 292. The squelch circuit 41 is adapted to eliminate unwanted background noise from being constantly monitored. This circuit consists of a sensitivity potentiometer 310 coupled between one end of transformer winding 292 and chassis ground. The slider portion of the potentiometer 310 is capacitively coupled to a two stage transistor amplifier including transistors 312 and 314 which has for its objective the coupling of a triggering signal to the gate of SCR 316 which is coupled across a +12 volt power supply potential appearing on supply buss 317 and chassis ground, across terminals 319 and 321, by means of the series circuit combination of a normally closed push-button reset switch 318, a semiconductor diode 320 and a yellow indicator lamp 322. A normally unenergized supply buss 324 is coupled between the SCR 316 and the collector of transistor 296 in the audio amplifier 40. When the audio signal amplitude exceeds the predetermined level as set by the potentiometer 310, SCR 316 is rendered conductive at which time the +12 volt supply potential supplied through the reset switch 318 is applied simultaneously to the supply buss 324 and through the diode 320 to the yellow indicator lamp 22. Once activated, the squelch circuit 41 will remain in continuous operation with the yellow indicator lamp 22 remaining on until such time that the reset switch 318 is manually opened at the central station 12 whereuponSCR 316 will be turned of and the entire audio amplifier 40 and squelch circuit 41 will revert back to a stand by state.

Referring now to the supervisory logic .circuit 20 shown in FIG. 5, it is adapted to sense: (1) the instantaneous polarity, both steady state and alternating, of the constant DC current transmitted from the constant current DC generator 18 shown in FIG. 2, (2) the value of the constant DC current, and (3) the intermittent or continuous loss of constant DC current, as well as providing indications of these three conditions by means of indicator lights and an audible alarm. The audible or sonic alarm is provided during alarm condition as well as for a loss of constant DC current. The constant current circuit loop between the constant current generator 18 in the protected premises 10 and the "supervisory circuit 14 is provided by means of circuit leads 288 and 290 being coupled together in the logic circuit 20 through a series connection of a signal back push-button switch 323 and resistors 324 and 326. DC current flow in one polarity direction produces a voltage across resistor 324 which is adapted to cause transistors 328 and 330 to become conductive or turn on. A current in the opposite polarity direction, however, will not turn transistors 328 and 330 on due to the way in which the base and emitter electrodes of the transistor 328 is coupled across resistor 324 by means of resistor 332. A current in the opposite direction, however, causes a voltage drop to occur across resistor 326 which is of a proper polarity toturn transistors 334 and 336 on.' It is seen then that the base and emitter electrodes of transistor 334 is coupled across resistor 326 by means of resistor 338 in an opposite sense to that shown with respect to transistor 328.

These transistors are powered from a 12 volt power supply potential produced by the full wave DC rectifier bridge 340 which has 1 17 volt, l-Iz AC line potential applied across terminals 342 and 344 by means of the transformer 346. Terminals 348 and 350 of the rectifier bridge constitute the +12 volt and l2 volt terminals respectively to which supply buss leads 352 and 354 are connected. A capacitor 356 is coupled across terminals 348 and 350 for providing a filter action of the +12 volt DC supply voltage.

Thus when a constant DC current of a first polarity direction flows through circuit leads 288 and 290, transistor 328 and 330 will be rendered conductive. Transistor 330 acts substantially as a closed switch and the 12 volt DC power supply potential appearing across busses 52 and 354 is applied across a green indicator .15 light 358 providing an indication of this first current polarity which by definition is made to indicate'the normal steady state condition of the alarm system. Upon energization of the current reverse relay 168 (FIG. 3) as described'above, being indicative of an alarm condition, the DC constant current polarity will cause the transistor 336 to act as a substantially closed switch and apply the 12 volt supply voltage across a red indicator lamp 360 through the diode 362 providing an indication of an alarm condition. The green indicator lamp 358 meanwhile is of due to the fact that transistor 330 has become non-conductive. If for example the perimeter-intrusion circuit 24 actuates the current reverse relay 168, a continuous red indication will be provided. On the other hand, however, if the holdup circuit 26 actuates the current reverse relay 168 in the alternately mode previously described both the green and red indicator lights 358 and 360 respectively will alternately turn on providing a flashing indication. I

In addition to operating the red indicator lamp 360, transistor 336 is coupled to SCR 364 and relay coil 366 through the diode 368. The purpose of this circuit is to energize a sonic alarm device 27 which is energizable through the embodiment of the subject invention shown in FIG. 6 through the relay contacts 372. Thus both a visual and sonic alarm is provided during an alarm'condition. The purpose of the SCR 364 is to additionally provide a memory voltage signal to the red indicator lamp 360 after the alarm condition has subsided. As noted before, once a silicon controlled rectifier is triggered conductive, it will remain conductive until the circuit is interrupted. Accordingly, SCR 364 will provide a current path for the red indicator lamp 360 from the +12 volt supply buss 352 through a normally closed reset push button switch 374, a diode 376 and a resistor 378. The resistor 378 limits the voltage applied to the indicator lamp 360 so that it will light relatively dimly as compared to the brightness provided for the previously describedalarm condition. The dim red indicator lamp thus designates that an alarm condition has occurred but which has subsided. Pressing the reset switch 374, however, will turn SCR 364 off I and extinguish the dim red light 360. The circuitry surrounding SCR 364 then merely acts as a memory circuit.

in the event that the circuit loop completing the constant DC current flow between the protected premises 10 and the central station 12 is interrupted for any reason, transistor 380 in combination with diodes 382 and 384 act as a logic NAND circuit such that transistor 380 only becomes conductive when both transistors 330 and 336 are nonconductive. When this occurs a blue indicator lamp 386 is turned on by means of +12 volt being applied thereto through diode 388 and circuit lead 390 coupled thereto, since when transistor 380 is made conductive it acts as a closed switch. Additionally, the collector of transistor 380 is coupled to the relay coil 366 by means ofthe diode 392 so that when current interruption occurs, the normally open relay contacts 372 close energizing the sonic alarm device 27 as will be subsequently explained.

' A second memory circuit including SCR 394 is coupled between the NAND circuit'transistor 380 and the reset switch 374 to provide an indication of current interruption after this condition no longer exists. The blue indicator lamp 360 thenis lighted whenever an open line condition-occurs such as when a signal back is desired from the central station 12 to the protected premises merely by momentarily opening the push-button switch 323 which would then be sensed by the current relay 96 (FIG. 2) in the constant current generator 20. A loss of current such as would occur for a malfunction of the-constant current generator 20 would also cause the 'blue indicator lamp 386 to light in the supervisory circuit 14. a shorted telephone line 16 or open lines 172, 174 and 288 and 290 for example would by be indicated b6 the blue lamp 386. SCR 294, however, is triggered on by the NAND circuit transistor 380 by means of the diode 395. After the current interruption condition subsides, however, SCR 394 still conducts and providesa current path to the blue indicator lamp 386 throughthe resistor 39.6 and diode 398. The resistor 396 limits the current through the blue indicator lamp 386 so that it shines dimly in the same manner as previously described with respect to the red indicator lamp 360. A subsequent actuation of the normally closed"reset" switch 374, however, will cause SCR 394 to become non-conductive whereupon the dimly-lit blue indicator lamp is turned off. 5

Finally, a DC constant current level measuring circuit is provided which includes transistors 400, 402 and zener diode 404 in combination with the diodes 406 and 408 which is operative to cause transistors ,400 and 402 to become conductive whenever a predetermined level, for example, 2.5-3.0 ma. is being transmitted from'the constant current DC generator 18 at protected premises 10. When this condition occurs relay coil 410 becomes energized closing the relay contacts 412 whereupon the +12 volt supply potential on supply buss 354 is applied across a yellow indicator lamp 414.

Section A of the day-night switch 136 controls the DC current level so as to provide a 1.5 ma. current flow during day operation and 3.0 ma. during night operation as previously explained. Therefore, the yellow lamp 414 at the central station 12 provides indication at the supervisory circuit 14 when lit that the system is in the night operating mode. 4

The supervisory circuit 14 then provides a coded indication both visually and audibly of the condition of the respective perimeter-intrusion circuit 24 and the hold-up circuit '26 located at the protected premises to which it is connected as well as the condition of the communication link between the protected premises and the central station. The protected premises on the other hand is adapted to provide a visual indication of the condition of the perimeter-intrusion circuit 24 as well as a loss or substantial reduction of the constant DC current flowing in the communication link including the telephone transmission lines.

Turning attention now to the subject invention and more particularly to the preferred embodiment thereof which is shown schematically in FiG. 6, there is disclosed a circuit which enables a monitoring operator at the central monitor station 12 to mute the sonic alarm device 27 any time from the moment that an alarm condition as previously described is heard and identified by the suitable indicator lights 28 to the time when the normal condition is again restored. As soon as the normal operating condition is restored, however, the circuit shown in FIG. 6 will automatically cause the sonic alarm 27 to be sounded again to remind the operator that he has previously muted the sonic alarm and subsequently. requiring him to manually reset the circuit in order to deactivate the reenergized sonic alarm device 27.

Prior to considering FIG. 6 in detail, it should be observed that the relay contacts 372 shown in FIG. are adapted to apply the +12 volt supply potential appearing on buss lead 352 to terminal 373 when closed. Thus when, for example the perimeterintrusion circuit 24 is compromised, relay coil 366 will close relay contacts 372 in a steady state mode as long as this condition exists and therefore a continuous +l2 volt signal will appear at terminal 373. If on the other hand the hold-up .circuit 26 is compromised, relay contacts 372 will be opened and closed at a relatively rapid rate (once per second) as determined by the multivibrator 244 (FIG. 3) at which time a pulsating +12 volt signal will appear at terminals 373; Both the steady state and pulsating +1 2 volt signals constitute an input signal to the muting circuit shown in FIG. 6 for initiating the energization of the sonic alarm device 27. This input signal is coupled to terminal 420 shown in FIG. 6. Additionally, a +12 volt power supply potential is applied to terminal 422- from the squelch circuit 41 shown in FIG. 4. More par ticularly, the +12 volt supply potential applied to terminal 319 in FIG. 41s coupled to terminal 325 through the manual reset switch 318. The reset switch 318 is adapted to provide the manual reset for the muting circuit shown in FIG. 6 as well as resetting the squelch circuit 41 as previously explained.

The point of common reference potential illustrated as ground in FiG. 6 is returned to circuit terminal 424 and is coupled .to either terminal 321 shown in FIG. 4 or 355 shown in FiG. 5 so that a current return path is provided back to the negative terminal 350 of the diode bridge 340 shown in FIG. 5.

Considering now the circuitry shown in FiG. 6, first circuit means including SCR 426 has its anode electrode directly connected to the +12 volt supply potential applied to terminal 422 from the reset switch 318 (FIG. 4). SCR 426 has its gate electrode coupled to input terminal 420 by means of the diode 428 and resistor 430. The cathode electrode of SCR 426 is returned to ground through resistor 432. Circuit junction 434 which is common to the cathode of SCR 426 and resistor 432 is coupled to the red indicator lamp 436 through the diode 438 as well as the sonic alarm device 27 through thediode 440 in combination with the common base transistor configuration including transistor 442. Upon the appearance of either the steady state or pulsing input signal (+l2 volts) at terminal 4 20, indicative of an alarm condition, this signal is coupled to the gate of SCR 426 by means of diode 428 and resistor 430 which immediately becomes conductive i.e. turns on and remains conductive until such time that the +12 volt power supply potential is momentarily interrupted by means of the manual reset switch 318 shown in FIG. 4. When SCR 426 turns on, the +12 volt supply potential appears at circuit junction 434 which then turns the red indicator lamp 436 on as well as activating the sonic alarm device 27. Transistor 442 merely acts as a current limiter for the sonic alarm device 27.

If muting of the sonic alarm device 27 is desired upon the translation of an alarm signal (either steady state or pulsating +l2 volts) to input terminal 420 second circuit means including transistor 446 and third circuit means including a normally opened mute switch 444 and SCR 448 is employed. In the event that a steady state +12 input signal appears at terminal 420 it is coupled to circuit junction 450 which is common to the cathode of SCR 446 by means of fourth circuit means including diode 452. If on the other hand a pulsating +12 input signal appears at terminal 420 it is coupled to fifth circuit means including, inter alia, capacitor 454 by means of capacitor 456, resistor 458 and diode 460. The voltage built up across capacitor 454 by the pulsating input signal is applied to the base of transistor462 which is one element of a Schmitt trigger circuit including transistor 464. The Schmitt trigger circuit is a voltage amplitude sensitive circuit such that it is normally in a first operating state but when a voltage of a predetermined amplitude appears at the base of transistor 462 it switches to a second state whereupon a +l2 volt output signal appears at the collector of transistor 462. This output signal is then coupled to the base of switch transistor 466 by means of coupling resistor 468 which immediately becomes conductive to apply the +12 volt supply potential at terminal 422 to circuit junction 450 by means of the. diode 470. The fourth and fifth circuit means then act to supply power to SCR 448 when a steady state and pulsating input signal, respectively, appears at input terminal 420.

Upon depression of the mute switch 444, +12 volts applied to terminal 445 is coupled to the gate of SCR 448 which turns on and remains conductive as long as any +l2 input signal indicative of an alarm signal appears at terminal input 420. When SCR 448 turns on, +1 2 volts appears at circuit junction 472 which is common to the cathode electrode of SCR 448 and resistor 474. The appearance of the +12 volt potential at circuit junction 472 is coupled to the base of transistor 446 by means of resistors 476 and 478. This turns transistor 446 on which shunts the current heretofore energizing the sonic alarm device 27 to ground due to the fact that the collector of transistor 446 is coupled to the collector of current limiting transistor 442 while the emitter of transistor 448 is connected directly to ground. Thus the sonic alarm 27 is selectively muted during an alarm condition.

Upon the cessation or removal of the alarm condition and the system returns to its normal operating state, relay contacts 372 shown in FIG. 5 open, removing any +l2 volt input signal at terminal 420. The +l2 volt potential heretofore appearing at circuit junction 450 as provided by either the fourth or fifth circuit means is removed whereupon SCR 448 turns off" due to the removal of power. This also results in the turn-off of transistor 446. The non-conduction of transistor 446 effectively removes the shunt from the sonic alarm device 27. However, since SCR 426 is still in a conductive or on state, the turn-off of transistor 448 will result in the reapplication of the energizing current to the sonic alarm device 27 through the current limiting transistor 442 and the diode 440. The sonic alarm 27 will then remain activated untilsuch time that SCR 426 is subsequently turned off by the manual resetting of the reset switch 318.

Thus the present invention operates to sound the sonic alarm device 27 in the event of an'alarm condition and provides means for muting thesonic alarm device 27 in spite of the continuance of the alarm condition; however the circuit will automatically operate to reactivate the sonic alarm device 27 when the alarm condition is removed'and will remain activated until SCR 426 is turned off by means of manually depressing the reset switch 318 in the squelch circuit 41.

What has been shown and described, therefore, isa reliable fully supervised bidirectional signalling system for use in alarm, telemetry, remote control and other applications which is operable in combination. with relatively log wire circuits such as telephone lines while utilizing extremely low currents and which will atthe same time indicate trouble. and/or alarm conditions both visibly and sonically at the. introduction of foreign potential, loss of line continuity or other detrimental circuit faults. The sonic alarm is operated through a muting circuit which enables an operator'to mute the.

sonic alarm at any time from the moment the alarm is noticed to the time when the normal condition is restored; however, assoon as the normal condition is restored, the sonic alarm will sound again to remind the operator to depress the reset button which will then deactivate the sonic alarm until a new alarm condition exists.

Having thus described the present invention with what is at present considered to be the preferred embodiment thereof, 7

I claim as my invention:

1. A muting circuit for'a sonic alarm'device in a security alarm system and being powered by an electrical supply voltage applied thereto, comprising in combination: input signal means;

first circuit means coupled to said input signal means and said sonic alarm device, being responsive to an input signal of a first and a second type coupled to said input means to become operative for energizing said sonic alarm device and remaining operative to supply electrical voltage to said sonic alarm device until reset;

reset means coupling said electrical supply voltage to said first circuit means and selectively rendering said first circuit means inoperative after being rendered operative by said input signal by interrupting said electrical supply voltage coupled thereto;

second circuit means coupled across said sonic alarm device, being normally inoperative but being rendered selectively operative in accordance with a first control'voltage applied thereto for short circuiting said sonic alarm device and thereby muting said alarm device;

third circuit means including normally open electrically controlled switch means coupled to said second circuit means, being rendered closed to generate said first control voltage upon the application of a second control voltage fed thereto as well as a power voltage,-and muting switch means coupled to said controlled switch means for selectively applying said second control voltage thereto and rendering said controlled means closed whereupon said controlled switch means remains closed as long as said power voltage is applied;

fourth circuitmeans coupled from said input means 1 to said third circuit means, being responsive to said first type of input signal for applying saidpower voltagethereto for the duration of said first type of input signal; an

fifth circuit means coupled from said input means to said third circuit means being responsive to said second type of input signal forsupplying said power voltage thereto for the duration of said second input signal, said second and said third circuit means however becoming inoperative upon the termination of said first and second input signal due to the absence of said power voltage whereupon said sonic alarm device again is rendered operative by. means of said first circuit means, remaining energized until said first circuit means is rendered inoperative by said reset means, 2. The invention asdefined by claim 1 and additionally including visual indicator means coupled to and energized. by said first circuit means providing a visual indication of the presence of said first or second type of input signal at said input means.

3. The invention as defined by claim 1 wherein said first circuit means comprises:

a controlled rectifier having a first and a second current conducting electrode and a control electrode, and additionally including circuit means coupling said reset means to one current conducting electrode, the sonic alarm device to the other current conducting electrode, and said control electrode to said input means.

4. The invention as defined by claim 1 wherein said first circuit means comprises:

a semiconductor controlled rectifier having an anode, a cathode and a gate electrode including circuit means coupling saidanode electrode to said reset means, said cathode electrode tosaid sonic alarm device and said gate electrode to said input means.

5. The invention as defined by claim 1 and additionally including current limiting means coupled between said first circuit means and said sonic alarm device.

6. The invention as defined by claim 1 wherein said second circuit means comprises a semiconductor switch and said electrically controlled switch means comprises a controlled rectifier having a first and a second current conducting electrode and a control electrode and including circuit means coupling one of said current conducting electrodes to a circuit terminal for the application of said power voltage, the other current conducting electrode to said semiconductor switch, said control electrode to said muting switch means, and said fourth and fifth circuit means to said circuit terminal whereupon said muting switch means is adapted to render said controlled rectifier conductive upon the application of said power voltage to said circuit terminal upon the appearance of either said first or second input signal at said input means and whereupon said other current conducting electrode couples said first control signal to said semiconductor switch.

7. The invention as defined by claim 1 wherein said fourth circuit means comprises diode means coupling said first type of input signal to said electrically con- 8. The invention as defined by claim 1 wherein said fifth circuit means comprises:

capacitive means coupled to said input means and being responsive to said second type of input signal to be charged thereby, and amplitude sensitive circuit means, coupled to said capacitance means, being responsive to the amplitude of the voltage across said capacitive means and coupling said supply voltage to said electrically controlled switch when the voltage across said capacitor exceeds a predetermined amplitude, said supply voltage thereby comprising said power voltage.

9. The invention as defined by claim 8 wherein said amplitude sensitive circuit means comprises a Schmitt trigger circuit.

10. A muting circuit for a sonic alarm device adapted to be powered by a supply potential coupled from a power source comprising, in combination:

a first controlled rectifier coupled to said sonic alarm system and including circuit means for being rendered conductive upon the application of a steady state and a pulsating alarm signal coupled thereto to couple said supply potential therethrough to said sonic alarm;

reset means coupling said power source to said first controlled rectifier for supplying power thereto and for selectively rendering said first control rectifier non-conductive when actuated;

normally open switch means coupled to said sonic alarm device and being adapted to be rendered closed to short circuit said sonic alarm;

second controlled rectifier means coupled to said normally open switch means and additionally including circuit means for applying a control signal to said switch means when either said steady state or said pulsating alarm signal is coupled to said first controlled rectifier, and being adapted to be selectively rendered conductive only during the presence of said steady state or said pulsating alarm signal to close said switch means; and

muting switch means coupled to said second controlled rectifier for selectively rendering said second control rectifier conductive only during the presence-of said alarm signals to cause said normally open switch means to short circuit said sonic alarm device, said second controlled rectifier automatically becoming non-conductive upon the disappearance of said steady state or said pulsating alarm signal and removing said control signal from said switch means causing said switch means to become open whereupon said, first controlled rectifier reactivates said sonic alarm until said first controlled rectifier means is rendered non-conductive by said reset means.

11. The invention as defined by claim 10 wherein said first and second controlled rectifier means comprises a semiconductor controlled rectifier and said normally open switch means comprises a transistor.

12. The invention as defined by claim 11 wherein each said semiconductor controlled rectifier has a first and second current conducting electrode and a gate electrode and additionally including input circuit means coupling said steady state and said pulsating alarm signal to the gate electrode of said first semiconductor controlled rectifier, and circuit means coupling said mutin switch means to the gate electrode of said second con rolled rectifier where y said muting switch means couples a trigger signal thereto when actuated.

13. The invention as defined by claim 12 wherein said reset means is coupled to one current conducting electrode of said first controlled rectifier and additionally including diode means coupled between said second current conducting electrode of said first controlled rectifier and said sonic alarm device; and

circuit means coupling a voltage to one current conducting electrode of said second controlled rectifier during the presence of either said steady state or pulsating alarm signals and the other current conducting electrode to said transistor.

14. The invention as defined by claim 13 wherein said last recited circuit means includes:

a capacitor responsive to said pulsating alarm signal and being charged thereby, a normally non-conductive transistor coupled between said power source and said one current conducting electrode of said second controlled rectifier, and an amplitude discriminator circuit coupled between said capacitor and said transistor, said transistor being rendered conductive by said amplitude discriminator when the charge across said capacitor exceeds a predetermined amplitude level to couple said supply potential to said one current conducting electrode of said second controlled rectifier; and

a semiconductor diode responsive to said steady state alarm signal, coupled to said one current conducting electrode of said second controlled rectifier and being selectively poled to apply said steady state alarm signal to said one current conducting electrode of said second controlled rectifier.

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Classifications
U.S. Classification340/503, 340/328, 379/44, 340/550, 340/521, 340/509, 340/533, 340/574, 340/327
International ClassificationH04M11/04
Cooperative ClassificationH04M11/04
European ClassificationH04M11/04