U.S. patent number 3,678,509 [Application Number 05/059,413] was granted by the patent office on 1972-07-18 for security alarm system.
This patent grant is currently assigned to Multra-Guard, Inc.. Invention is credited to Dan Carlo, James F. Pinkman.
United States Patent |
3,678,509 |
Carlo , et al. |
July 18, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
SECURITY ALARM SYSTEM
Abstract
A constant DC current generator powers a communication link
including a pair of telephone lines coupled between a protected
premises 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. 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. Also when desirable
an audio signal is adapted to be transmitted over the communication
link from the protected premises to the central station.
Inventors: |
Carlo; Dan (Bethesda, MD),
Pinkman; James F. (Kensington, MD) |
Assignee: |
Multra-Guard, Inc. (Rockville,
MD)
|
Family
ID: |
22022797 |
Appl.
No.: |
05/059,413 |
Filed: |
July 30, 1970 |
Current U.S.
Class: |
340/504; 340/521;
340/550; 340/511; 340/533 |
Current CPC
Class: |
H04M
11/04 (20130101) |
Current International
Class: |
H04M
11/04 (20060101); G08b 029/00 () |
Field of
Search: |
;340/409,420,416,216,310,276 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Slobasky; Michael
Claims
Having thus described the present invention with what is at present
considered to be the preferred embodiment thereof, claim as our
invention:
1. A security alarm system for supervising a protected premises
remotely located with respect to a central monitor station,
comprising, in combination:
transmission line means interconnecting said protected premises to
said central monitor station;
a constant current DC generator including current sensing feedback
means for providing a substantially constant amplitude DC output
current of a selected magnitude coupled to said transmission line
means;
a supervisory circuit located at said central monitoring station
coupled to said transmission line means, said supervisory circuit
providing a normally closed current path for said DC output current
from said constant current DC generator and including circuit means
for providing an indication of the direction of flow of said DC
output current and an absence thereof;
protection circuit means located at said protected premises
including control circuit means coupled thereto for providing a
control signal in response to the operating characteristics of said
protection circuit; and
current polarity reversal means coupled to said transmission line
means intermediate said constant current DC generator and said
supervisory circuit, being additionally coupled to said control
circuit means and responsive to said control signal to reverse the
polarity of current flow of said DC output current from one current
direction indicative of normal operation of said protection circuit
to an opposite current direction indicative of an abnormal
operations when a predetermined change in operation of said
protection circuit means occurs.
2. The invention as defined by claim 1 wherein said transmission
line means includes a two wire telephone transmission line.
3. The invention as defined in claim 2 wherein said two wire
telephone transmission line comprises a floating pair of telephone
lines.
4. The invention as defined in claim 2 wherein said constant
current DC generator is coupled to one end of said two wire
telephone transmission line and said supervisory circuit is coupled
to the opposite end of said two wire telephone transmission
line.
5. The invention as defined in claim 4 wherein said constant
current DC generator is located at said protected premises.
6. The invention as defined by claim 5 where said two wire
telephone transmission line comprises an ungrounded pair of
telephone lines.
7. The invention as defined in claim 4 wherein said current
polarity reversal means comprises a relay having relay contact
means and a relay coil being electrically energizable for operating
said relay contact means, and additionally including means coupling
said relay contact means to said transmission line means and said
relay coil to said control circuit means.
8. The invention as defined by claim 7 and wherein said protection
circuit means comprises perimeter resistance means and a potential
coupled thereto for providing a voltage drop thereacross, said
resistance means being adapted to change in resistance value or
become an open circuit when said protection circuit means is
compromised and provide a change in voltage drop thereacross, said
control circuit being responsive to said change in voltage drop to
generate said control signal for energizing said relay coil.
9. The invention as defined by claim 8 wherein said perimeter
resistance means includes a frangible resistive element
strategically located at said protected premises.
10. The invention as defined by claim 9 and additionally including
a variable resistance coupled in series with said frangible
resistive element intermediate said potential and having a common
circuit connection therebetween, said variable resistance providing
a means for controlling the potential applied across said frangible
resistive element.
11. The invention as defined by claim 10 and wherein said control
circuit means comprises:
a differential electronic amplifier circuit having first and a
second input means and at least one output means;
circuit means coupling said common circuit connection to said first
input means; and
means for providing a predetermined reference voltage coupled to
said second input means, said differential amplifier circuit being
responsive to a predetermined change in the voltage across said
frangible resistive element to provide an output signal at said one
output means indicative of the signal difference between said
voltage across said resistive element and said reference voltage,
said output signal thereby constituting said control signal for
operating said relay.
12. The invention as defined in claim 11 wherein said differential
amplifier comprises a first and a second transistor each having an
emitter, a base, and a collector and wherein said first and said
second input means includes said base of said first and said second
transistor, said at least one output means includes the collector
of one transistor, and additionally including a common
emitter-impedance coupled to the emitters of said first and second
transistor.
13. The invention as defined in claim 11 and additionally including
normally open electronic switch means coupled to said relay coil to
energize said relay when closed, and additionally including circuit
means coupling said switch means to said differential amplifier
circuit being operated by said control signal to become closed and
causing said relay to become energized whereby said contact means
causes said constant DC current flow to reverse polarity.
14. The invention as defined in claim 13 and additionally including
indicator means coupled to said electronic switch means, being
adapted to be rendered operative by said switch means said switch
means is closed to provide a visual indication of said
predetermined change in operation of said protection circuit means
at said protected premises.
15. The invention as defined in claim 13 wherein said normally open
electronic switch means comprises a transistor having a base, an
emitter and a collector and wherein said circuit means coupled to
said differential amplifier circuit includes the base of said
transistor, and wherein said collector and emitter are coupled to
one side of said relay coil.
16. The invention as defined by claim 7 and additionally including
second protection circuit means located at said protected premises
and including second control circuit means providing a control
signal in response to the operating characteristics of said second
protection circuit, said second control circuit being additionally
coupled to said relay coil and including circuit means for
preempting the operation of said relay by said first recited
protection circuit means.
17. The invention as defined in claim 16 wherein said second
protection circuit means includes a normally non-conductive
controlled rectifier coupled across a supply potential and switch
means coupled thereto for selectively causing said controlled
rectifier to become conductive in the event of an alarm condition
and wherein said control circuit means comprises an electronic
multivibrator circuit coupled to said controlled rectifier and
receiving a supply potential therefrom when said control rectifier
is rendered conductive, said multivibrator circuit coupling a
substantially square wave control signal to said relay coil for
providing alternating operation of said relay, said operation being
indicative of a second type of alarm condition.
18. The invention as defined by claim 17 and additionally including
normally open electronic switch means coupled between said
multivibrator circuit and said relay coil, said switch means being
rendered operatively opened and closed by said substantially square
wave control signal to control the energization of said relay
coil.
19. The invention as defined in claim 1 wherein said constant
current DC generator comprises a DC to DC converter having an
output circuit and including feedback circuit means coupled to said
output circuit and being responsive to the magnitude of output
current therefrom for providing an error signal for controlling the
operation of said converter to produce said substantially constant
amplitude DC output current and current level circuit means coupled
to said feedback means for additionally controlling said error
signal to produce a predetermined magnitude of said DC output
current.
20. The invention as defined by claim 19 wherein said feedback
circuit means includes a transformer having a primary and a
secondary winding and including means for coupling said primary
winding in series with said output circuit;
a differential amplifier having first and second input means and at
least one output means;
a reference voltage coupled to one input means; and
circuit means coupling a voltage signal from said secondary winding
of said transformer which is a function of said output current to
said second input means of said differential amplifier, said error
signal being provided at said output means thereby.
21. The invention as defined in claim 20 and additionally including
means for varying said reference voltage applied to said first
input means of said differential amplifier for selectively
providing a plurality of magnitudes of constant current DC
output.
22. The invention as defined in claim 20 wherein said DC to DC
converter comprises:
a DC to AC chopper circuit including a free running multivibrator,
a first and a second electronic switch coupled to and driven by
said multivibrator, a second transformer having a primary and
secondary winding, means coupling said first and second electronic
switch respectively to opposite ends of said primary winding of
said second transformer, said primary winding of said second
transformer having a center tap, means coupling said center tap to
said output means of said differential amplifier; and
a full wave rectifier circuit coupled by means of its input circuit
to the secondary winding of said second transformer, said constant
current output DC being provided at the output circuit of said
rectifier circuit.
23. The invention as defined by claim 22 and additionally
including:
capacitor means coupled between said center tap and a point of
reference potential; and
a transistor having a base, emitter and collector, said emitter and
collector being coupled directly across said capacitor means
providing a variable impedance thereacross which is a function of
the conductive state of said transistor, circuit means coupling
said base to said output means of said differential amplifier
whereby conductive state of said transistor is controlled by said
error signal and whereby a charge on said capacitor is varied in
response to the amplitude of said output current.
24. The invention as defined in claim 20 including circuit means
coupled to said secondary winding of said transformer for detecting
a predetermined decrease and absence of said output current and
providing an indication thereof at said protected premises.
25. The invention as defined by claim 2 wherein said supervisory
circuit additionally includes logic circuit means coupled to said
normally closed current path and including first circuit means for
detecting said one current direction, second circuit means
detecting said opposite current direction, and third circuit means
for detecting an open circuit condition of said normally closed
current path, and circuit means coupling said first, second and
third circuit means to said circuit means for providing said
indication.
26. The invention as defined in claim 25 and additionally including
audible alarm circuit means coupled to said second and third
circuit means for providing an audible alarm for current flow in
said opposite direction and for said absence of said constant
amplitude DC output current.
27. The invention as defined in claim 25 wherein said third circuit
means comprises a logic NAND circuit.
28. The invention as defined in claim 27 and additionally including
memory circuit means coupled to said NAND logic circuit and said
second circuit means for providing a predetermined visual
indication of said opposite current direction and absence of said
constant amplitude DC output current after said last mentioned
conditions cease.
29. The invention as defined in claim 26 and additionally including
circuit means coupled to said normally closed current path for
detecting the magnitude of said constant amplitude DC output
current and providing an indication of a predetermined magnitude of
said DC output current.
30. The invention as defined in claim 6 and additionally
including:
transformer means respectively coupled to each end of said pair of
telephone lines;
audio amplifier means coupled to each transformer means, an audio
pick up device located at said protected premises coupled to one of
said audio amplifier means for transmitting audio signals over said
pair of telephone lines, and audio reproducer means coupled to the
other audio amplifier means located at said central station for
receiving said audio signals transmitted from the protected
premises and rendering an audible indication at said central
station.
Description
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 to indicate at the central station the
source and type of trouble. 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 Underwriter's
Laboratories, presently require that the systems merely give an
indication if the resistance in the total circuit loop in the
system varies .+-. 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 3,000 ohms.
Where a 100 ohm perimeter circuit is connected in series to the
3,000 ohms of the telephone lines, an alarm will not be given until
the resistance in the entire loop varies by approximately .+-.
1,500 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 sufficient 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 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.
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 premises that 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 the same manner as will now be outlined.
A burglar or intruder, for example, must first place an 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 cut 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, prior art alarm system have attempted
to overcome the problem. For example, U.S. Pat. No. 584,789 issued
to H. M. Sutton et al. disclose a method for protecting the lines
between a discloses station and a remote station in an alarm system
by rapidly and continuously alternating the polarity of a signal
current and alternately shifting this current from one set of line
wires to another set of line wires, thereby leaving one set of
wires momentarily dead at any given moment and causing an alarm
signal to operate at the central station upon a variation in the
predetermined current flow. Although this patent refers to a
"current of constant strength," it is in reality a constant voltage
system and any interference with the main line or the protected
structure increases or decreases the current through a relay to
close an alarm circuit.
U.S. Pat. No. 3,069,673 issued to E. J. Ward et al. discloses a
security system having a separate perimeter protection loop and a
separate supervisory loop coupled to a central station. A
differential relay circuit operated by a control relay in the
perimeter loop actuates the supervisory loop to send an alarm back
to the central station.
U.S. Pat. No. 3,025,505 issued to A. B. Hube, discloses a two
circuit system for protecting enclosed spaces wherein the failure
of a blower motor circuit in an air pressure supported structure
unbalances a relay to reverse the contacts of a three wire loop to
the central station and thus communicate an alarm thereto.
The concept of DC current polarity reversal for an alarm condition
of an alarm system is also broadly disclosed in U.S. Pat. No.
3,351,934 issued to N. J. Vietz.
While the above recited prior art presumably operates in the manner
set forth in the above cited patents, the general use of
electromechanical relay devices as sensing means provides inherent
limitations in the system not only due to the fact that
electromechanical devices vary in sensitivity from component to
component but also in operating characteristics as a result of
aging and changes in environmental conditions. Also conventional
relay type signalling circuits require relatively high currents in
the order of 30 and 40 milliamperes. Furthermore, the prior art
systems operate with a substantially constant voltage power supply
which by its very nature makes the system relatively easy to
compromise by a sophisticated burglar or intruder.
SUMMARY
The subject invention is directed to an improved security
protection system which utilizes a constant current generator
powering a supervisory loop coupling a protected premises to a
central monitor station by means of a pair of ungrounded or
floating telephone communication lines. The supervisory loop
additionally includes means for reversing the polarity of a
relatively small (1.5 -3.0 ma.) DC constant current wherein said
means is controlled by the output of a differential electronic
amplifier having one input thereof coupled to circuit means
responsive to the change in resistance of a separate perimeter
intrusion circuit to reverse the polarity of the DC current in the
supervisory loop and thereby signal an alarm condition to the
central station. The invention additionally includes means for
causing the constant current DC generator to sense a current
interruption in the supervisory loop for providing an indication at
the protected premises of a system malfunction or as a means for
signalling back to said protected premises from the central
station. The constant current DC generator inherently makes the
system more tamper proof due to the fact that it is capable of
utilizing extremely low currents over relatively long line
telephone circuits and is immediately responsive to the
introduction of any foreign potential, loss of line continuity, and
other detrimental circuit faults.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the security alarm system contemplated
by the subject invention;
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;
FIG. 4 is a schematic diagram illustrating audio receiver amplifier
and squelch circuit located at the central monitor station for the
system shown in FIG. 1; and
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.
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.sub.1,
10.sub.2 and 10.sub.n are monitored at a remote central station 12
including a respective plurality of supervisory circuits 14.sub.1,
14.sub.2, and 14.sub.n which are coupled back to the protected
premises by means of respective pairs of telephone lines 16.sub.1,
16.sub.2 and 16.sub.n. Each protected premises, for example, is
provided with signalling circuitry for providing an indication 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 forming 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 by the
supervisory logic circuit 20 providing an indication thereof by
indicating lights and sonic alarm means 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. 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 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 which also includes a
corresponding audio transformer 38 coupled to the telephone lines
at that end. The output of 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.
A current sensor circuit 44 is coupled to the constant current DC
generator 18 to sense a current interruption to of the DC current
flowing between the DC generator 18 of the protected premises 10
and the supervisory logic circuit 20 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) a break in the telephone line 16
accidentally, (2) in the event that one attempts 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 of the like
contained in the supervisory circuit 14 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.
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 30 or 40 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 of
varying its predetermined magnitude either of which would be sensed
by the supervisory logic circuit 20 whereupon an alarm condition
would be indicated.
Considering now the subject invention 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 117 volt/60 Hz 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 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 to 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 transistors 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 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 116 and 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 + 12 volt supply buss 58 through
resistor 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 means of its normally closed contacts between chassis
ground and the common connection between diodes 128 and 130. The
normally closed position of switch section 136-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 alternately 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 80 rises to a value so that the
amplitude of the square wave impressed across the primary 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 80 to effectively control the magnitude of the DC current flow
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 90 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 conductive
since junction 110 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
conductive 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 160 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 and 162 are selectively
coupled to a pair of output circuit leads 172 and 174 by means of
the current reversing relay 168 and the secondary windings 176 and
178 of the audio transformer 32 shown in FIG. 1 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.
The audio surveillance circuit shown in FIG. 3 can be eliminated
when desirable which in turn removes the requirement for the
transformer 32 whereupon relay contacts 164 and 166 would directly
interconnect circuit leads 160 and 162 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 + 12 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 invention, therefore, the audio amplifier is energized when
the switch 136-B 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 relay coil 170 in the following manner.
Considering first the perimeter-intrusion circuit 24, a frangible
resistive 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 FIG. 2. The resistive tape 190
is coupled in series with perimeter interlock switches 192 to the
"night" terminal N of the switch 136-C while a variable resistance
198 is connected between the "day" terminal D and resistive tape
190, to chassis ground so that during "day" operation 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-D 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 of 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. 2. 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 .+-. 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
the system, from the .+-. 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 170 through diode 240 the
conduction of transistor 234 causes the relay coil 176 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 the perimeter 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 present invention 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 190 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 turn-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 necessarily 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 to operate 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
continuous alternating reversal of the current relay 168 to
constantly cause the current to reverse back and froth and provide
a flashing type of visual indication at the central station 12.
Considering now the "hold-up" circuit 26, it is comprised 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 push-button
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 is "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 supply potential to the
multivibrator 244 by means of resistor 262. The application of the
+ 12 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
determined by the operating frequency of the multivibrator 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 can immediately be transmitted to the central station
for relay to the police department, etc. providing greater speed in
apprehension.
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. This is 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 "off" the hold-up circuit 26 again reverts
to its normal "standby" 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 perimeter-intrusion circuit 24 due to
the polarity coupling of diode 240 between the lamp 30 and one side
of the relay coil 170 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 control signal 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-B (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, the "hold-up" circuit and the perimeter
intrusion circuit. Referring now to FIGS. 4 and 5, the other end of
the two wire telephone line 16 at the 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 282 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 10 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.
In the preferred embodiment of the subject invention squelch
circuit 41 shown in FIG. 4 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 whereupon SCR 316 will be turned "off" 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.
Also an audible 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 transistor 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 to
turn 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 as 117
volt, 60 Hz 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 - 12 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 352 and 354 is applied across a green indicator 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) (FIG. described above, being indicative of an alarm
condition, the DC constant current polarity will cause 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 "off" 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 "hold-up" 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.
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 an audible alarm 370 through
the relay contacts 372 which is actuated by the relay coil 366.
Thus both a visual and audio 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 described alarm 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" 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 non-conductive. 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 of the diode 392 so that
when current interruption occurs, the audio alarm 370 is also
actuated.
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 then is 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 also be indicated by the blue lamp
386. SCR 394, however, is triggered "on" by the NAND circuit
transistor 380 by means of the diode 394. After the current
interruption condition subsides, however, SCR 394 still conducts
and provides a current path to the blue indicator lamp 386 through
the resistor 396 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.
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.
The supervisory logic circuit 20 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.
What has been shown and described therefore is a reliable fully
supervised bi-directional signalling system for use in alarm,
telemetry, remote control and other applications which is operable
in combination with relatively long wire circuits such as telephone
lines while utilizing extremely low currents and which will at the
same time indicate trouble and/or alarm indications at the
introduction of foreign potential, loss of line continuity, or
other detrimental circuit faults. DC signalling is achieved by
means of constant current control and DC polarity sensing over a
communication link which is completely independent of a protection
circuit which senses predetermined operating conditions at a
location remote from a central monitor station. Signalling back to
the remote location is achieved by breaking the line continuity and
causing the constant current generator located at the remote
location to sense the open line.
* * * * *