U.S. patent number 3,786,501 [Application Number 05/162,401] was granted by the patent office on 1974-01-15 for current monitoring system and method.
Invention is credited to Costas S. Marnerakis.
United States Patent |
3,786,501 |
Marnerakis |
January 15, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
CURRENT MONITORING SYSTEM AND METHOD
Abstract
An electronic current monitoring system and method in which a
current in a transmission line is detected, converted to an output
voltage representative of the value of the current and compared
with reference voltages representative of minimum and maximum
current values in the transmission line. An alarm signal is
generated in the event that the output voltage drops below the
minimum reference voltage or rises above the maximum reference
voltage. Two or more alarm currents can be monitored
simultaneously, each alarm current representing a different
condition at a subscriber station with a separate alarm being given
for each alarm current which represents a different condition at
the subscriber station. An alternating current signal can be
impressed on the transmission line and detected with the frequency
thereof compared to predetermined values to detect tampering with
the alarm system. An alarm is activated in response to an alarm
signal in the event the frequency in the line changes from the
predetermined frequency value. The reference voltages are
adjustable to adjust the tolerance level from the normal current
and, in addition, alternate reference voltages are coupled to the
system by a switch so that different reference voltages can be
switched into the line to quickly change the level of current being
monitored.
Inventors: |
Marnerakis; Costas S. (Wyoming,
MI) |
Family
ID: |
22585453 |
Appl.
No.: |
05/162,401 |
Filed: |
July 14, 1971 |
Current U.S.
Class: |
340/509; 340/511;
340/521; 340/533; 340/541; 340/593; 379/47 |
Current CPC
Class: |
G01R
19/16571 (20130101); G01R 19/17 (20130101); G08B
13/22 (20130101); G08B 29/06 (20130101) |
Current International
Class: |
G08B
29/06 (20060101); G08B 29/00 (20060101); G01R
19/165 (20060101); G01R 19/17 (20060101); G08B
13/22 (20060101); G08b 021/00 () |
Field of
Search: |
;340/409,412,248A,248C,248P,253A,253P,276 ;179/5R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trafton; David L.
Attorney, Agent or Firm: McGarry; John E.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are as follows.
1. A current monitoring apparatus for detecting the level of
current in a given line and for producing an alarm responsive to a
predetermined deviation from a current level in said line, said
apparatus comprising:
means for detecting the level of current flow through said line,
said detecting means including means for measuring the voltage drop
across an impedance in said line and for generating an output
voltage representative of the measured voltage drop across said
impedance in said line, said generating means including an
amplifier for amplifying the voltage drop across said impedance so
that said output voltage is proportional to the level of current in
said line;
first reference voltage generating means for generating a first
reference voltage representative of a minimum current value in said
line;
first comparator means coupled to said detecting means and to said
first reference voltage generating means for comparing said first
reference voltage with said output voltage, said first comparator
means generating an alarm signal when said output voltage drops
below said first reference voltage;
second reference voltage generating means for generating a second
reference voltage representative of a maximum current value for
said line;
said comparator means coupled to said detecting means and to said
second reference voltage generating means for comparing said second
reference voltage with said output voltage, said second comparator
means generating an alarm signal when said output voltage rises
above said second reference voltage; and
alarm means coupled to said first and second comparator means to
produce an alarm when an alarm signal is generated by said first or
second comparator means.
2. A current monitoring apparatus according to claim 1 wherein each
of said first and second reference voltage generating means have
means for adjusting the reference voltages generated therefrom.
3. A current monitoring apparatus according to claim 1 and further
comprising:
a third voltage generating means for generating a voltage
representative of a current value in said given line intermediate
said minimum current value and a normal current value;
third comparator means coupled to said detecting means and to said
third reference voltage means for comparing said third reference
voltage with said output voltage, said third comparator means
generating an alarm signal when said output voltage drops below
said third reference voltage;
said alarm means coupled to said third comparator means to produce
an alarm when said alarm signal is generated by said third
comparator means.
4. A current monitoring apparatus according to claim 3 wherein said
alarm comprises a visual alarm and an audible alarm, and there is a
separate visual alarm coupled to each of said first, second, and
third comparator means.
5. A current monitoring apparatus according to claim 3 wherein said
alarm means comprises a separate alarm coupled to each alarm signal
from said first, second, and third comparator means, each of said
separate alarms representing a different current condition in said
line.
6. A current monitoring apparatus according to claim 5 and further
comprising discriminator means coupled to said first and third
comparator means and to said alarm means for blocking said alarm
signal to said alarm for said third comparator when said output
voltage drops below said first reference voltage, said
discriminator means permitting said alarm signal to pass to said
alarm means when said output voltage is at an intermediate value
between said first and third reference voltages.
7. A current monitoring apparatus according to claim 1 and further
comprising a third reference voltage generating means for
generating a voltage representative of a current value in said line
intermediate said maximum current value and a normal current
value;
third comparator means coupled to said detecting means and to said
third reference voltage means for comparing said third reference
voltage with said output voltage, said third comparator means
generating an alarm signal when said output voltage rises above
said third reference voltage;
said alarm means coupled to said third comparator means to produce
an alarm when said alarm signal is generated by said third
comparator means.
8. A current monitoring apparatus according to claim 1 and further
comprising an AC monitoring means including:
means for detecting an AC signal in said line;
means for sensing the frequency of said AC signal;
means coupled to said frequency sensing means for comparing the
sensed AC signal frequency with predetermined minimum and maximum
AC signal values, and for generating an alarm signal when said
sensed AC signal is above or below said predetermined values;
and
means coupling said alarm means to said alarm signal generating
means of said AC frequency comparing means to produce an alarm when
said AC signal frequency rises above said maximum signal value or
falls below said minimum signal value.
9. A current monitoring apparatus according to claim 8 and further
comprising means for adjusting said minimum and maximum AC signal
values.
10. A current monitoring system comprising:
a subscriber station and a monitoring station;
means electrically coupling said subscriber station with said
monitoring station;
means for generating a current flow through said electrical
coupling means;
said subscriber station including means to alter the current flow
through said electrical coupling means;
means at said monitoring station for detecting the level of current
flow through said electrical coupling means, said detecting means
including means for measuring the voltage drop across an impedance
in said line and for generating an output voltage responsive
thereto, said output voltage being proportional to the current
level in said electrical coupling means, said generating means
including an amplifier for amplifying the voltage drop across said
impedance in said electrical coupling means;
means coupled to said detecting means for comparing said output
voltage with minimum and maximum voltage values representative of
predetermined minimum and maximum current values in said electrical
coupling means;
means for generating an alarm signal when said voltage drops below
said minimum value or rises above said maximum voltage value;
and
alarm means coupled to said alarm signal generating means for
producing an alarm responsive to an alarm signal from said alarm
signal generating means.
11. A current monitoring system according to claim 10 and further
comprising means to adjust said minimum and maximum voltage
values.
12. A current monitoring system according to claim 10 and further
comprising means at said subscriber station to impress a given AC
frequency signal on said current in said electrical coupling
means;
means at said monitoring station for detecting said AC frequency
and comparing said frequency to minimum and maximum frequency
values;
means coupled to said comparing means for generating an alarm
signal when said frequency drops below said minimum frequency value
or rises above said maximum frequency value; and
means coupling said alarm signal generating means for said AC
signal to said alarm means to produce an alarm when the frequency
of said AC signal drops below said minimum value or rises above
said maximum value.
13. A current monitoring system according to claim 12 wherein said
comparing means includes means for converting said AC signal to a
voltage representative of the frequency of said AC signal; means
for generating a reference voltage representative of a minimum
frequency level; and means for generating a reference voltage
representative of a maximum frequency level.
14. A current monitoring system according to claim 13 wherein said
reference voltage generating means are adjustable to vary said
minimum and maximum frequencies for which an alarm is given.
15. A current monitoring system according to claim 12 wherein at
least one of said reference voltage generating means have a
plurality of voltage generating means for generating a plurality of
different reference voltages; and switch means for singularly
coupling each of said voltage generating means to said comparing
means whereby different reference voltages can be selectively
activated by said switch means.
16. A current monitoring system according to claim 12 wherein said
alarm means includes a first visual alarm to indicate the presence
of a frequency below said minimum frequency value, and a second
visual alarm to indicate the presence of a frequency above said
maximum frequency value.
17. A method of monitoring current flowing through a transmission
line so as to detect tampering with said line or the presence of
deviation of current in said line from a predetermined value, said
method comprising:
detecting the level of current in said transmission line by
measuring the voltage drop across an impedance in said transmission
line;
generating an output voltage proportional to the value of detected
current level in said line, said generating step including
amplifying the measured voltage drop across said impedance;
comparing said output voltage to reference voltages representative
of minimum and maximum current values in said transmission
line;
generating an alarm signal when said output voltage falls below
said minimum or rises above said maximum current representative
reference voltages; and
activating an alarm responsive to said alarm signal, said alarm
indicating the presence of a minimum or maximum current value in
said transmission line.
18. A method of monitoring current according to claim 17 wherein
said generating step includes generating a first alarm signal
responsive to said output voltage falling below said minimum
current reference voltage, and generating a second alarm signal
responsive to said output voltage rising above said maximum current
reference voltage; said alarm activating step includes activating a
first alarm responsive only to said first alarm signal and
activating a second alarm responsive only to said second alarm
signal.
19. A method of monitoring current according to claim 17 and
further comprising the step of selectively altering said minimum
and maximum reference voltages responsive to a change in the normal
level of current in said transmission line.
20. A method of monitoring current according to claim 17 and
further comprising impressing an AC signal in said transmission
line;
detecting the frequency of said AC signal;
comparing said frequency of said AC signal to predetermined maximum
or minimum reference frequencies; and generating a second alarm
signal when said detected AC signal drops below said predetermined
frequency or rises above said predetermined maximum frequency; and
activating an alarm responsive to said second alarm signal.
21. A method of monitoring current according to claim 20 wherein
said frequency detecting step includes converting said AC signal to
a second output voltage representative of the frequency of said AC
signal, and said comparing step comprises comparing said second
output voltage with reference voltages representative of minimum
and maximum frequencies of said AC signal in said transmission
line.
22. A method of monitoring current according to claim 20 wherein
said generating step comprises:
generating a first alarm signal responsive to said signal frequency
dropping below said predetermined minimum frequency and generating
a second alarm signal responsive to said AC signal frequency rising
above said predetermined maximum frequency; and
said alarm activating step comprises activating a first alarm
responsive only to said first alarm signal and activating a second
alarm responsive only to said second alarm signal.
23. An alarm system comprising:
a subscriber station and a monitoring station;
means electrically coupling said subscriber station with said
monitoring station;
means generating a current flow through said electrical coupling
means;
means at said monitoring station for detecting current flow through
said electrical coupling means;
means coupled to said detecting means for actuating an alarm at
said monitoring station responsive to a drop in said current below
a predetermined value or a rise in said current in said electrical
coupling means above a predetermined value;
means for impressing an AC signal on said electrical coupling
means;
means at said subscriber station for detecting the frequency of
said AC signal;
means coupled to said frequency detecting means for comparing said
frequency to predetermined minimum and maximum reference
frequencies;
means coupled to said comparing means for generating an alarm
signal when the frequency of said detected AC signal falls below
said minimum reference frequency or rises above said reference
frequency; and
alarm means coupled to said signal generating means for producing
an alarm responsive to said alarm signal.
24. An alarm system according to claim 23 wherein said comparing
means includes means for converting said AC signal to a voltage
representative of the frequency of said AC signal;
means for generating a reference voltage representative of a
minimum frequency level; and
means for generating a reference voltage representative of a
maximum frequency level.
25. An alarm system according to claim 24 wherein said reference
voltage generating means is adjustable to vary said minimum and
maximum frequency values for which an alarm is given.
26. An alarm system according to claim 25 wherein at least one of
said reference voltage generating means has a plurality of voltage
generating means for generating a plurality of different reference
voltages, and switch means for singularly coupling each of said
reference voltage generating means to said comparing means whereby
different reference voltages can be selected by activating said
switch means.
27. A current monitoring apparatus for detecting the level of
current in a given line and for producing an alarm responsive to a
predetermined deviation from a current level in said line, said
apparatus comprising:
means for detecting the level of current flow through said line,
said detecting means including means for measuring the voltage drop
across an impedance in said line and for generating an output
voltage representative of the measured voltage drop across said
impedance in said line, so that said output voltage is proportional
to the level of current in said line;
first reference voltage generating means for generating a first
reference voltage representative of a minimum current value in said
line;
first comparator means coupled to said detecting means and to said
first reference voltage generating means for comparing said first
reference voltage with said output voltage, said first comparator
means generating an alarm signal when said output voltage drops
below said first reference voltage;
second reference voltage generating means for generating a second
reference voltage representative of maximum current value for said
line;
second comparator means coupled to said detecting means and to said
second reference voltage generating means for comparing said second
reference voltage with said output voltage, said second comparator
means generating an alarm signal when said output voltage rises
above said second reference voltage;
each of said first and second reference voltage generating means
having means for generating a plurality of different output
voltages, and switch means for singularly coupling each of said
output voltage generating means to its respective comparator means;
and
alarm means coupled to said first and second comparator means to
produce an alarm when an alarm signal is generated by said first or
second comparator means.
28. A current monitoring apparatus according to claim 27 wherein
each of said plurality of voltage generating means is adjustable to
vary the output voltage from each of said different voltage
generating means.
29. A current monitoring apparatus for detecting the level of
current in a given line and for producing an alarm responsive to a
predetermined deviation from a current level in said line, said
apparatus comprising:
means for detecting the level of current flow through said line,
said detecting means including means for measuring the voltage drop
across an impedance in said line and for generating an output
voltage representative of the measured voltage drop across said
impedance in said line, so that said output voltage is proportional
to the level of current in said line;
first reference voltage generating means for generating a first
reference voltage representative of a minimum current value in said
line;
first comparator means coupled to said detecting means and to said
first reference voltage generating means for comparing said first
reference voltage with said output voltage, said first comparator
means generating an alarm signal when said output voltage drops
below said first reference voltage;
second reference voltage generating means for generating a second
reference voltage representative of maximum current value for said
line;
second comparator means coupled to said detecting means and to said
second reference voltage generating means for comparing said second
reference voltage with said output voltage, said second comparator
means generating an alarm signal when said output voltage rises
above said second reference voltage;
a third voltage generating means for generating a voltage
representative of a current value in said given line intermediate
said minimum current value and a normal current value;
third comparator means coupled to said detecting means and to said
third reference voltage means for comparing said third reference
voltage with said output voltage, said third comparator means
generating an alarm signal when said output voltage drops below
said third reference voltage;
a fourth reference voltage generating means for generating a
voltage representative of a current value in said line intermediate
said maximum current value and said normal current value;
fourth comparator means coupled to said detecting means and to said
fourth reference voltage means for comparing said fourth reference
voltage with said output voltage, said fourth comparator means
generating an alarm signal when said output voltage rises above
said fourth reference voltage; and
alarm means coupled to said first, second, third, and fourth
comparator means to produce an alarm when an alarm signal is
generated by said first, second, third, or fourth comparator
means.
30. A current monitoring apparatus according to claim 29 wherein
said alarm means includes first, second, third, and fourth visual
alarms coupled respectively to said first, second, third, and
fourth comparator means for giving a visual display of the current
level in said line.
31. A current monitoring apparatus according to claim 30 and
further comprising a first discriminator means coupled to said
first and third comparator means and to said alarm means for
blocking said alarm signal to said alarm means for said third
comparator when said output voltage drops below said first
reference voltage, said first discriminator means permitting said
alarm signal to pass to said alarm means for said third comparator
when said output voltage is at an intermediate value between said
first and third reference voltages;
a second discriminator means coupled to said second and fourth
comparator means and to said alarm means for blocking said alarm
signal to said alarm means for said fourth comparator means when
said output voltage rises above said second reference voltage, said
second discriminator means permitting said alarm signal to pass to
said alarm for said fourth comparator when said output voltage is
at an intermediate value between said second and fourth reference
voltages.
32. A current monitoring apparatus according to claim 29 wherein
each of said first, second, third and fourth voltage generating
means have a plurality of voltage generating means for generating a
plurality of reference voltages, and switch means for singularly
coupling each of said voltage generating means to its respective
comparator means.
33. A current monitoring system comprising:
a subscriber station and a monitoring station;
means electrically coupling said subscriber station with said
monitoring station;
means for generating a current flow through said electrical
coupling means;
said subscriber station including means for establishing a
plurality of different current levels in said electrical coupling
means, each current level being representative of a different
condition of said subscriber station;
means at said monitoring station for detecting the level of current
flow through said electrical coupling means, said detecting means
including means for measuring the voltage drop across an impedance
in said line and for generating an output voltage responsive
thereto, said output voltage being proportional to the current
level in said electrical coupling means;
means coupled to said detecting means for comparing said output
voltage with minimum and maximum voltage values representative of
predetermined minimum and maximum current values for each of said
current levels in said electrical coupling means;
means for generating an alarm signal when said voltage drops below
said minimum value or rises above said maximum voltage value for
any of said respective current levels; and
alarm means coupled to said alarm signal generating means for
producing an alarm responsive to an alarm signal from said alarm
signal generating means.
34. A current monitoring system according to claim 33 and further
comprising a separate alarm for each of said current levels, and
means coupling each of said alarms with said comparing means,
including means for actuating a single alarm representative of one
of said different current levels responsive to an alarm signal from
said comparing means such that only one of said alarms will be
activated when one of said different current levels is reached in
said electrical coupling means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to current monitoring. In one of its
aspects, the invention relates to a current monitoring system for
burglar alarms and the like wherein a subscriber station is
electrically connected to a monitoring station and an alarm is
generated at the monitoring station in the event of a change in the
current flow at the subscriber station.
In another of its aspects, the invention relates to a method and
system for monitoring a current in a line for changes in the
current to two or more current levels wherein a different alarm
signal is generated when the line current reaches each
predetermined level.
2. State of the Prior Art
Conventional burglar alarm systems employ a monitoring station
electrically connected through telephone wires to a subscriber. A
low level of current flows between the subscriber and the
monitoring station with the level of current being primarily
determined by an adjustable resistance at the subscriber location.
If a short occurs in the line at the subscriber, the current value
rises and this rise in current is detected at the monitoring
station. Generally an alarm is sounded, indicating trouble at the
subscriber station. An alarm is also sounded if an open circuit
occurs at the subscriber station. This latter condition may result
from the severing of the burglar alarm system at the subscriber.
The open circuit causes a drop in the current level at the
monitoring station. Generally, the drop in current is associated
with someone burglarizing the subscriber or someone tampering with
the alarm system to disconnect the system so that the subscriber
may be deactivated for burglarizing the subscriber at a later
time.
One type of monitoring system presently employed has relays in
series with a DC line. The relays are actuated responsive to a
current drop or current rise in a line. At any given time, the
monitoring system can detect only two alarms. Separate pairs of
these relays must be employed for each different set of alarm
currents. The relay systems do not lend themselves to adjustment so
as to adjust the tolerance levels, i.e., allowable variation for a
given current. Compared to the more modern electronic components,
the relays are relatively insensitive, unreliable, larger and
expensive.
In addition, because of the fact that all relays are slightly
different, it is very difficult to manufacture two relay monitoring
systems with exactly the same tolerance levels.
Frequently, it is desirable to change the tolerance level for a
given level of monitoring current. In some instances, wider
tolerance limits are required because of larger fluctuations from
the normal current level. In other instances, smaller tolerance
levels are required. Heretofore, it has not been possible to change
the relays at different times for different tolerance levels.
With the advent of such burglar alarm systems, modern burglars have
become much more sophisticated and have employed electronic
equipment to deactivate the alarm systems. Depending on the skill
of the person, tampering with the burglar alarm system with
electrical equipment does alter the current flow somewhat, but, in
certain instances, the tampering is not enough to activate the
relays with wide tolerance limits. It is therefore necessary to
have as close tolerance limits as possible at the monitoring
station in order to detect tampering with the alarm system. In
addition, more sophisticated systems are required to stay ahead of
the tampering methods of the modern criminal.
The above described alarm system suffers from another deficiency.
The relay detecting systems permit only two conditions, i.e., under
current and over current, to be detected at a given time. It would
be desirable to monitor more than two alarm currents at the
subscriber station. For example, in addition to the open circuit
condition, it would be desirable to monitor for smoke, water, hold
ups, heat, etc. Heretofore, this has been impossible with the
present systems without separate lines and separate monitoring
systems for each pair of conditions.
Another problem with relays is that the relays have a relatively
wide hysteresis band for the low level of current monitoring. This
hysteresis band adversely affects the accuracy of the system after
an alarm has been triggered.
A still further problem with the relay monitoring system is that
the system is limited by distance. Subscribers located more than a
fixed distance from the monitoring station cannot be protected. The
relays require a certain fixed current level which is a function of
the line resistance and the voltage. The line resistance is
directly proportional to distance. Thus, when the subscriber is
more than a predetermined distance from the monitor, the line
resistance lowers the line current below a point at which it can no
longer be effectively detected by the relays.
Monitoring systems have also been devised wherein semi-conductor
circuits including transistors have been substituted for the
relays. The transistors are biased so as to be off under normal
conditions and are switched on to activate alarms when the current
exceeds certain limits. These systems, although more reliable than
the relay systems, suffer from many of the other disadvantages of
the relay systems. First of all, separate detecting circuits are
required for different current levels, thereby making versatile
monitoring circuits rather expensive. In addition, the tolerance
levels cannot be adjusted and the line resistance limits the
distance between subscriber and monitoring station. Further, the
semiconductor systems require a voltage regulator for the line
current, thereby increasing the cost of the system. Still further,
only one set of conditions can be monitored at a time and the
system is subject to misfiring due to noise, transients, lightning,
etc.
BRIEF STATEMENT OF THE INVENTION
I have now discovered a new and improved electronic current
monitoring system and method. The monitoring system is entirely
electronic, providing a smaller and lighter compact unit at the
monitoring station. In addition, the monitoring system is more
sensitive and reliable than the conventional systems now employed.
The new system is less expensive to manufacture and is easily
adjustable to different current monitoring levels. Still further,
the tolerance levels for any given current can be adjusted very
easily. With the system according to the invention, two or more
conditions at a subscriber station can be simultaneously monitored
with a single subscriber line. The electronic system adapts itself
to more sophisticated monitoring techniques to detect tampering
with the subscriber alarm system.
According to the invention, the current monitoring apparatus has a
means for detecting the current flow through the subscriber line
and for generating an output voltage representative of the current
value in the line. This output voltage is compared to reference
voltages which represent the minimum current and maximum current
values for the subscriber line. When the output voltage drops below
the minimum reference voltage, an alarm signal is generated and an
alarm is activated responsive to the alarm signal. Conversely, when
the output voltage rises above the maximum line voltage, another
alarm signal is generated which activates an alarm. Separate alarms
can be employed for each alarm signal to thereby indicate the
different conditions.
The reference voltages are supplied by adjustable potentiometers
which can be adjusted easily to adjust the minimum and maximum
levels for a given monitoring current in the subscriber line. In
addition, each reference voltage generating means can having a
plurality of adjustable potentiometers which are connected into the
comparing circuit by a switch so that the level of reference
voltage can be quickly changed by merely throwing a switch for
monitoring different levels of current.
With the system according to the invention, a plurality of current
levels can be easily monitored by comparing the output voltage to a
plurality of reference voltages and by generating an alarm signal
responsive to the output voltage rising above or falling below the
reference voltages. Discriminator means are employed to control the
alarm signals generated when the output voltage has risen above or
fallen below two or more reference voltages to produce a single
alarm representative of the particular level of current in the
subscriber line.
Still further, with the electronic monitoring system according to
the invention, an AC signal can also be monitored simultaneously
with the DC current. For this purpose, the current monitoring
apparatus has a means for detecting the AC signal in the subscriber
line, means for comparing the frequency of the AC signal with
predetermined minimum and maximum frequency values and generating
an alarm signal which activates an alarm when the frequency thus
detected falls below a minimum frequency value or rises above a
maximum frequency value. Desirably, the frequency is converted to
an output voltage representative of the frequency of the signal in
the subscriber line, and this output voltage is then compared to
reference voltages which represent minimum and maximum frequency
values for a signal on the subscriber line. By this system, the
frequency tolerances can be adjusted, and the frequency levels can
be easily changed for monitoring different frequencies in the
subscriber line.
In the monitoring system according to the invention, many different
alarms can be used. Preferably, however, an audible alarm signal is
sounded when any of the alarm signals are generated, and a separate
visual alarm, such as a light bulb, is activated for each of the
alarm current values in the subscriber line. Thus, a separate light
bulb will be lighted when the current in the subscriber line
reaches each of the separate predetermined current values for the
subscriber line. Additional visual alarms are employed for the AC
monitoring system.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described with reference to the
accompanying drawings in which:
FIG. 1 is a schematic representation of one embodiment of the
invention; and
FIG. 2 is a schematic representation of a second, more
sophisticated, embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, and to FIG. 1 in particular, a
subscriber station 12 is electrically connected to a monitoring
station at a distant point through means such as a telephone line
16. An input DC voltage is applied at terminal 18 for the
subscriber 12. For purposes of illustration, the input voltage will
be described as a negative input DC voltage which requires only a
single line to the subscriber. A positive voltage input can also be
used but this voltage may require a return line. A voltage sensor
14 is provided at the monitoring station to detect the current flow
through the telephone line 16. The voltage sensed by the voltage
sensor 14 is applied to an attenuating network 20 to reduce the
voltage to an acceptable level. The output from the attenuating
network 20 is applied to a subtractor circuit 26 which includes
filtering networks 22 and 24 which filter noise from the voltage.
The output from the subtractor circuit 26 is applied to the
negative input to a high gain comparator 28 through line 68. An
open line reference voltage means 30 produces a reference voltage
which is applied to the positive input to the comparator 28. As
used herein with respect to comparators and amplifiers, the terms
"positive" and "negative" are intended to designate non-inverting
and inverting inputs respectively to such devices.
The output from the subtractor circuit 26 is also applied to the
positive input to a second comparator 32 which compares the output
voltage with a reference voltage produced by a short reference
voltage means 34. The reference voltage from voltage means 34 is
applied to the negative input to the comparator 32.
The comparator 28 generates an alarm signal when the voltage in
line 68 drops below the reference voltage from voltage means 30.
Similarly, the comparator 32 generates an alarm signal when the
output voltage rises above the reference voltage from voltage means
34.
The output from the comparator 28 is connected to an inverter 36
for inverting the output voltage from the comparator 28. The output
from the inverter 36 is connected to a latching circuit 40 which in
turn is connected to an audible alarm circuit 44. Similarly, the
output from the comparator 32 is connected to an inverter 38. A
latching circuit 42 is connected to the output of the inverter 38
and has its output connected to the audible alarm circuit 44.
More specifically, at the subscriber's station, the telephone line
18 is connected to an adjustable resistance 46 such as a
potentiometer. The resistance at the subscriber station can thus be
adjusted by the resistance 46. In addition, the subscriber's
station can comprise a pair of such potentiometers alternately
connected into the line by a switch (not shown) to alternately set
different levels of current in the line.
The voltage sensor 14 comprises a resistance 48 and leads 50 and 52
to detect the voltage drop across the resistance 48. Lead 50 is
connected to resistance R.sub.1 which is connected to ground
through resistance R.sub.2. Lead 52 is connected to resistance
R.sub.3 which is also connected to ground through a resistance
R.sub.4. Resistances R.sub.1, R.sub.2, R.sub.3 and R.sub.4 comprise
the attenuating network 20. The attenuated voltage is taken from
the attenuating network 20 by leads 54 and 56.
Lead 54 is connected to input line 62 through filter 24. Lead 56 is
connected to input lead 64 through filter 22. The filters 22 and 24
remove noise from the attenuated voltage.
The subtractor circuit 26 includes the filters 22 and 24, a linear
operational amplifier 58 and a pair of RC filters 60 and 66. RC
filter 60 is connected to input line 62. RC filter 66 is connected
between input line 64 and the output line 68 for the amplifier 58.
The combination of the amplifier 58 and the filter 66 makes an
active filter network.
The comparator 28 includes a linear operational amplifier 86 having
negative and positive input leads and a feedback resistor 88
between the amplifier output and the positive input. The positive
feedback through resistor 88 makes the amplifier 86 stable and less
sensitive to noise. The result is that the comparator is stable and
relatively immune to noise.
The open line reference voltage means 30 comprises an input
terminal 70 at which a negative DC voltage is applied. A pair of
parallel adjustable potentiometers 72 and 74 are connected to the
input terminal 70 and alternately determine the open line reference
voltage through switch 76. For purposes of illustration, the
potentiometer 72 having resistance R.sub.6 is designated as the
"day" potentiometer and the potentiometer 74 having resistance
R.sub.8 is designated as the "night" potentiometer. Resistances
R.sub.5 and R.sub.7 limit the current through potentiometers 72 and
74, respectively. The voltage means 30 produces an output voltage
which is representative of a low tolerance level of current flowing
through the telephone line 16. This reference voltage can be
quickly changed by throwing switch 76. The voltage from voltage
means 30 is applied to the positive input to the amplifier 86.
The negative input terminal of the amplifier 86 is connected to the
output voltage line 68 of the amplifier 58. The function of the
comparator 28 is to compare the reference voltage from voltage
means 30 with the output voltage from the amplifier 58. So long as
the output voltage from amplifier 58 is greater (more negative when
negative input voltages are used) than the voltage from the voltage
means 30, the output from the amplifier 86 will be positive.
However, when the output voltage from the amplifier 58 is less
(less negative) than the voltage from the voltage means 30, then
the output from the amplifier 86 will be negative.
The short reference voltage means 34 is substantially the same as
the open line reference voltage means 30 except that the output is
at a higher voltage level. Voltage means 34 comprises a terminal 78
at which is applied a negative DC voltage. A switch 84 alternately
connects the reference voltage lines to the voltage terminal 78
through potentiometers 80 and 82. Potentiometer 80 is comprised of
resistance R.sub.10 and, for purposes of illustration, is called
the "day" potentiometer. Potentiometer 82 is comprised of
resistance R.sub.12 and, for purposes of illustration, is called
the "night" potentiometer. Resistances R.sub.9 and R.sub.11 limit
the current in potentiometers 80 and 82, respectively. Different
values are set on potentiometer 80 and 82 so that a different
output voltage results when the switch 84 changes from
potentiometer 80 to potentiometer 82.
The comparator 32 is comprised of a linear operational amplifier 90
having negative and positive input terminals and a single output
terminal. A feedback resistor 92 is connected across the positive
input terminal and the output terminal of the amplifier 90.
The output voltage from the voltage means 34 is applied to the
negative input terminal of the amplifier 90. The output voltage in
line 68 from the amplifier 58 is applied to the negative input
terminal of the amplifier 90.
Comparator 32 functions in a manner similar to comparator 28. When
the output voltage from the voltage 32 is greater (more negative)
than the output voltage from amplifier 58, then a positive output
voltage results from the comparator 32. On the other hand, when the
output voltage from amplifier 58 is greater (more negative) than
the voltage from voltage means 30, then a negative output voltage
results from the comparator 32.
The inverters 36 and 38 are identical in structure and function.
For this reason, identical numerals have been used to designate
like parts. For purposes of this description, inverter 36 will be
described, it being understood that inverter 38 functions in the
same manner. The inverter comprises a PNP transistor 94 whose base
96 is connected to the output of the comparator 23 and to ground
through resistance R.sub.13. The emitter 98 of the transistor 94 is
connected to ground. The collector 100 of the transistor 94 is
connected through resistance R.sub.14 to a terminal 102 to which is
applied a negative voltage. When the output from the comparator 28,
for example, is positive, the transistor is off and no current can
flow from the emitter 98 to the collector 100. On the other hand,
when the output voltage from the comparator switches to negative,
the transistor is turned on and the ground is transferred from the
emitter 98 to the collector 100 and current flows from the emitter
98 of the transistor into the latching circuit 40.
Similarly, when the output from the comparator 32 is positive, the
transistor of inverter 38 is off and there is no current flow to
the latching circuit 42. When the output from the comparator 32 is
negative, the transistor of the inverter 38 is switched on and
current then flows to the latching circuit 42.
Latching circuits 40 and 42 are identical in construction and
function. Hence, identical numerals have been used to describe like
parts of each of the circuits. For purposes of brevity, only one
such latching circuit will be described in detail. The latching
circuit 40 comprises SCR 106 whose gate 108 is connected to the
output from the inverter 36 through a diode 104. A voltage terminal
110 has a negative voltage applied thereto and is connected to the
cathode of the SCR 106 through a light 112 which acts as a visual
alarm. In addition, the voltage at terminal 110 is applied to the
gate 108 through a capacitor 114. The anode of the SCR is connected
to ground through a reset switch 116 which is normally closed. The
voltage terminal 110 is also connected to the output from the
inverter 36 through a resistor R.sub.16. If desirable, a resistance
and capacitor in series (not shown) can be connected across the SCR
to provide suppression of the rate effect for the SCR, preventing a
false firing of the SCR due to a fast rate of voltage change. Still
further, a resistor R.sub.18 can be shunted across the light 112 to
permit latching in event the light 112 burns out. A diode 118 is
connected to the output from the latching circuit 40.
In operation of the latching circuits 40 and 42, when there is a
negative voltage at the gate 108 of the SCR 106, the SCR will be
off, no current will flow through the light 112 and no current will
flow from the output from the latching circuit 40. However, when a
positive voltage is applied to the latching circuit from the
inverter, the SCR 106 will be switched on. Current will then flow
through light 112 and through diode 118 to the audible alarm
circuit 44. Once the SCR is turned on, it will stay on until reset
by switch 116.
The audible alarm circuit 44 has a voltage terminal 120 to which a
negative DC voltage is applied. A resistor R.sub.15 is connected
between the output from the latching circuits 40 and 42 and a
buzzer 126 or other similar audible alarm. A diode 124 is connected
across the buzzer 126. The diode 124 and the buzzer 126 are
connected to the voltage terminal 120 through a variable resistor
128. A capacitor 122 is connected to resistance R.sub.15 and to the
voltage terminal 120 across the buzzer and variable resistor
circuit.
The buzzer operates whenever there is current flowing through
resistor R.sub.15 to the voltage terminal 120 from a latching
circuit.
In operation of the system illustrated in FIG. 1, a predetermined
normal current level is set at the subscriber station 12 with the
adjustable resistance 46. This can be one value for daytime and
another value for night time. For example, a current value of 20
milliamps can be used for daytime monitoring and a current of 15
milliamps can be used for night time monitoring. The value of the
current flowing through the line 16 is sensed by the voltage sensor
14. The voltage between leads 50 and 52 is proportional to the
value of current flowing through the line 16. This voltage is then
applied to the subtractor circuit 26 through attenuator 20 and
filters 22 and 24. The output from the subtractor circuit 26 is a
voltage value which is proportional to the flow of current through
line 16. The open line reference voltage means 30 is set at a value
which is proportional to a minimum value of current flow through
line 16. For example, if a current flow of 20 milliamps through
line 16 is represented as a voltage of 5 volts at line 68, the
output voltage from the voltage means 30 can be set at four volts
which would represent a current flow through line 16 of about 16
milliamps. The output voltage from voltage means 30 is controlled
by adjusting the day potentiometer 72. So long as the value of the
voltage in line 68 is greater than the value of the output voltage
from voltage means 30, the output from the comparator 28 will be
positive. Thus, assuming that a negative voltage is applied at
voltage terminals 18 and 70, the output from the comparator 28 will
be positive so long as the absolute value of the voltage in line 68
is greater than the absolute value of the voltage from voltage
means 30. In this condition, the transistor 94 will be off, a
negative voltage will be applied to the latching circuit 40 and the
audible alarm circuit 44 will be in the inactive state.
In the event that someone tampers with the monitoring line, or the
line 16 is cut, the current flow through line 16 will drop and the
voltage across resistor 48 at voltage sensing means 14 will drop.
Therefore the output voltage from the subtractor circuit 26 will
drop, making the voltage from the voltage means 30 more negative
than the voltage in line 68. As soon as the voltage in line 68
drops below (less negative) the voltage from the voltage output
means 30, the comparator 28 will switch from a positive output to a
negative output, thereby activating the transistor 94, causing a
positive voltage to be applied to gate 108 of the SCR 106. The SCR
will then be latched on, causing current to flow through the light
112, giving a visual alarm, and into the audible alarm circuit 44,
thereby activating the buzzer 126. The light 112 will indicate the
presence of low current, or no current, in line 16 at the
monitoring station. A check can then be made of the subscriber to
see whether anyone is burglarizing the subscriber or otherwise
tampering with the alarm system.
The over current control works in a similar manner. So long as the
value of the voltage in line 68 is less (less negative) than the
value of the voltage from the voltage means 34, the output from the
comparator 32 will be positive and the transistor 94 will be off.
The latching circuit 42 will be inactive as a negative voltage is
applied to the gate 108. The light 112 of the latching circuit 42
will therefore be off and the audible alarm circuit 44 will also be
inactive.
In the event that a short occurs in the line 16, or in the event
that tampering with the subscriber alarm system causes the current
to increase, the voltage drop across resistor 48 of the voltage
sensor 14 will be increased, thereby reflecting a higher voltage at
the output voltage line 68 of the subtractor circuit 26. As soon as
the voltage in line 68 increases (becomes more negative) to a value
greater than the voltage from the voltage means 34, the output from
the comparator 32 will switch negative, thereby causing the
transistor 94 of the inverter 38 to turn on causing a positive
voltage to be applied to gate 108 of SCR 106 in the latching
circuit 42. This positive voltage causes the SCR to switch on,
causing current to flow through the light 112 of latching circuit
42 and into the alarm circuit 44, thereby activating the buzzer
126. The light 112 of the latching circuit 42 will be lit, giving a
visual alarm, whereas the corresponding light of the latching
circuit 40 will not be lit. This indicates to the monitor that a
short or high current condition is present at the subscriber.
With the system illustrated in FIG. 1, the tolerance levels, i.e.,
that deviation from the normal current which will produce an alarm,
can be adjusted. For example, the lower tolerance level can be
adjusted by simply adjusting the potentiometer 72 of the voltage
means 30. The upper tolerance level can be adjusted by simply
adjusting the potentiometer 80 of the voltage means 34.
In monitoring systems, it is desirable to switch the current levels
from time to time to minimize successful tampering with the alarm
system. Frequently, the current levels are different for days and
nights. In the system described above, the current monitoring
levels can be easily changed by readjusting the adjustable
resistance 46 at the subscriber station 12, thereby altering the
level of current flow through the line 16. The new normal current
level will thereby be established at a predetermined value. The
change at the monitoring station takes place very easily by simply
throwing switch 76 of voltage means 30 and switch 84 of voltage
means 34. When the switch is thrown for the "night" potentiometer
74 of voltage means 30, a new low level monitoring voltage will be
established. Similarly, when switch 84 is thrown to establish
contact with the "night" potentiometer 82, a new high tolerance
voltage will be established. For example, if the normal daytime
current level is at about 20 milliamps, the night time level can be
switched to 16 milliamps. The low tolerance level can be switched
from a voltage representing 16 milliamps to a voltage representing
12 milliamps. Similarly, the upper tolerance level of current can
be adjusted by the voltage means 34 to represent a current of 20
milliamps for night time monitoring and 24 milliamps for daytime
monitoring. Thus, the in example given, for daytime operation the
normal current level will be 20 milliamps and an alarm will be
sounded and displayed when the current level drops below 16
milliamps or rises above 24 milliamps. For night time monitoring
with the current level at 16 milliamps, the alarm will be sounded
and displayed when the current drops below 12 milliamps or rises
above 20 milliamps.
Reference is now made to FIG. 2 which illustrates a more
sophisticated alarm system according to the invention. In FIG. 2,
like numerals have been used to designate like parts. In this more
sophisticated system, the subscriber station 12 has a plurality of
potentiometers 46a, 46b, 46c, and 46d all connected to the
subscriber line 16. Each of the potentiometers represents a
different sensing device at the subscriber station. Potentiometers
46a, 46b and 46c are connected respectively to switches S.sub.1,
S.sub.2 and S.sub.3 which are normally open. The switches S.sub.1,
S.sub.2, S.sub.3, and S.sub.4 represent sensing devices such as
photo cells, heat detectors, transistors, smoke detectors and the
like and for purposes of illustration are shown in the open
position. Potentiometer 46d has current normally flowing
therethrough and establishes the current monitoring level for the
system. Each of the potentiometers 46a, 46b, 46c and 46d are
adjusted so that given current values will result in line 16 when
any one of potentiometers 46a, 46b, or 46c are joined in parallel
with potentiometer 46d by closing switches S.sub.1, S.sub.2, or
S.sub.3. In the event that switch S.sub.4 is closed, a short
results.
An input DC voltage, preferably negative, is applied to terminal 18
and the current in the subscriber line 16 is sensed by the voltage
sensor 14. The voltage is attenuated in attenuating network 20 and
the output is passed through voltage followers 130 and 132 and then
applied to a subtractor network 26. Voltage followers 130 and 132
act as buffers between the attenuating network 20 and the
subtractor network 126. The output voltage from the subtractor
network 26 appearing at line 68 is directly porportional to the
current flowing through the subscriber line 16. This output voltage
is applied to comparators 28, 28a, 32a and 32. Voltage output means
30 and 30a provide a reference voltage for comparators 28 and 28a
respectively and reference voltage means 34a and 34 provide
reference voltages for comparators 32a and 32 respectively. The
output from comparator 28 is applied to inverter 36 whose output is
applied to latching circuit 40 which includes a visual alarm (not
shown). The output from latching circuit 40 is connected with the
audible alarm circuit 44 which contains an audible alarm activated
by a signal from the latching circuit 40.
The output from comparator 28a is applied to inverter 36a. The
output from inverter 36a is applied to AND gate 140 through a delay
circuit 134. The delay circuit 134 can be any suitable well known
delay circuit such as an RC network. The output from the inverter
138 is also applied to AND gate 140 which applies its output signal
to latching circuit 40a which in turn is connected to the audible
alarm circuit 44. Latching circuit 40a is of similar construction
to the latching circuit 40, described more fully in FIG. 1, and
contains a visual alarm such as a light.
Comparators 28 and 28a function in the same manner as the
comparator 28 of the embodiment illustrated in FIG. 1. Comparator
28 compares the voltage appearing in line 68 and the voltage from
output voltage means 30. Comparator 28a compares the voltage
appearing in line 68 with the voltage from voltage means 30a.
Voltage means 30 and 30a are set at different levels for monitoring
different current levels. For example, voltage means 30 would be
set with a less negative voltage than voltage means 30a. The output
from comparator 28a would switch negative when the current in the
subscriber line dropped to a first predetermined value, and
comparator 28 would be switched negative when the current in the
subscriber line 16 dropped to a second predetermined value. By way
of illustration, if the normal current monitoring level is at 20
milliamps, the voltage from voltage means 30a can be set so that
comparator 28a switches to a negative output when the current drops
below 16 milliamps. The voltage from voltage means 30 can be set so
that the comparator 28 switches to a negative output when the
current in line 16 drops below 12 milliamps.
When the voltage at line 68 drops to the first predetermined level,
the comparator 28a switches negative thereby sending a negative
signal to inverter 36a. The output from 36a is delayed in delay
means 134 and then passed to AND gate 140. In the event that both
AND gate inputs, i.e., from delay 134 and from inverter 138 are
positive, the AND gate 140 will be open, permitting the signal to
be transmitted to latching circuit 40a and causing a voltage to be
sent to the audible alarm circuit 44a, thereby producing an audible
alarm. At the same time, the light in the latching circuit 40a will
go on, thereby giving a visual alarm which is indicative of a
particular current in the subscriber line 16.
The delay means 134, the inverter 138, and the AND gate 140
function as a discriminator to distinguish between a current at the
first low level and the second low level. For example, if the
current has dropped to the first predetermined level but not to the
second predetermined level, the output from the comparator 28a will
be negative, thereby producing a positive voltage from the inverter
36a. A positive voltage will thereby be transmitted to AND gate
140. At the same time, the output from the comparator 28 will be
positive, thereby producing a negative output from the inverter 36
which maintains the latching circuit 40 in the off condition.
Therefore the light in latching circuit 40 will be off. The
negative voltage from inverter 36 is inverted in inverter 138
thereby producing a positive voltage which is applied to AND gate
140. The AND gate thus sees two positive voltages and opens to
permit the voltage to be applied to latching circuit 40a, thereby
activating the latching circuit, lighting the light of latching
circuit 40a, and producing an audible alarm from the alarm circuit
44.
When the current drops from the normal level below the second
predetermined level, the output from the comparator 28 will switch
negative, thereby producing a positive output voltage from inverter
36, which voltage, when applied to latching circuit 40, causes
latching circuit 40 to switch on, thereby lighting the light in
latching circuit 40 and activating the audible alarm circuit 44. At
the same time, the positive voltage from the inverter 36 is again
inverted in 138 and applied as a negative voltage to AND gate 140,
preventing the signal from passing therethrough to latching circuit
40a. When the current drops from the normal monitoring current to
the second predetermined level, the delay means 134 will delay the
positive signal to AND gate 140 and will thus prevent application
of the positive signal to AND gate from the output of inverter 138.
Thus, the voltage is prevented from being transmitted through the
AND gate 140 to the latching circuit 40a and latching circuit 40a
will not be switched on. When the current drops to the second
predetermined level, an audible alarm sounds, the light of latching
circuit 40 will be lit and the light of latching circuit 40a will
not be lit. As explained above, the drop in current level results
from closing of one of the switches S.sub.1, S.sub.2, or S.sub.3,
or from tampering with the alarm system.
The over current alarm system works in a similar manner. The output
from the voltage means 34a represents a first current level in
subscriber line 16 above the normal current, and the output voltage
from voltage means 34 represents a second current value higher than
the first predetermined current level in the subscriber line 16.
The output from comparator 32a is applied to inverter 38a. The
inverted voltage passes through a delay 136 and is applied to AND
gate 144. Similarly, the output from comparator 32 is applied to
inverter 38 and the inverted signal from inverter 38 is again
inverted in inverter 142 and applied to AND gate 144. When the
voltage polarity from the inverter 142 and from delay 136 are
positive, the positive voltage is passed through AND gate 144 and
to latching circuit 42a. When the voltage applied to latching
circuit 42a is positive, the latching circuit 42a is turned on,
thereby lighting a light in the latching circuit and applying a
voltage to the alarm circuit 44 for activating an audible alarm.
The latching circuit 42a is of a construction identical with
latching circuit 42 which is illustrated in detail in FIG. 1.
The output voltage from inverter 38 is also applied to the latching
circuit 42 which, when turned on by a positive voltage, will apply
a voltage to the alarm circuit 44.
The over current side of the circuit operates in a manner similar
to the under current side described above. At the normal level, the
output from comparator 32a will be positive, thereby producing a
negative signal from the delay means at AND gate 144. The voltage
from comparator 32 will also be positive, thereby producing a
positive voltage at AND gate 144. Since the polarities of the
voltages are opposite at AND gate 144, no voltage will be passed
therethrough to latching circuit 42a which will therefore be off.
In addition, the negative voltage from inverter 38, appearing at
the input to latching circuit 42, will cause the latching circuit
42 to remain off. Similarly, latching circuits 40 and 40a will also
be off.
When the current in the subscriber line 16 rises to a first
predetermined level, the comparator 32a will switch negative and
apply a negative voltage to inverter 38a. A positive voltage is
delayed in delay means 136 and then applied to AND gate 144. At the
first predetermined current level above the normal current, the
comparator 32 will have a positive output voltage, which when
inverted first by inverter 38 and second by inverter 142 will
appear as a positive voltage at AND gate 144. The AND gate will
then pass the positive voltage to latching circuit 42a which will
be thereby activated to light the light therein and to apply a
voltage to the audible alarm circuit 44. At this time, the negative
voltage from inverter 38 will be applied to latching circuit 42
which will consequently be off.
When the current in line 16 rises from normal current to a current
above the second predetermined level, such as when a short occurs,
the output voltage at line 68 will appear greater (more negative)
than the output voltage from the voltage means 34. The comparator
32 will then switch to a negative output, causing a positive
voltage to appear at the input to the latching circuit 42, thereby
switching on the latching circuit 42 and activating the buzzer 44.
At the same time, the output from the comparator 32a will switch
negative, thereby producing a positive signal at the input to AND
gate 144. However, the positive output voltage from inverter 38
will be again inverted to a negative voltage in inverter 142 and
the negative voltage is applied to AND gate 144. This opposite
polarity of the voltages at AND gate 144 prevents the voltage from
being applied to latching circuit 42a and prevents the latching
circuit 42a from being activated. Thus when the current in the
subscriber lines reaches the second predetermined level above the
normal current, the audible alarm will be on and the light of the
latching circuit 42 will be on. At the same time, the lights in the
latching circuits 42a, 40a, and 40 will be off.
As set forth above, the various current levels can be set by
potentiometers 46a, 46b, 46c, and 46d. The normal current through
the subscriber line 16 is set by the potentiometer 46d. At this
time, switches S.sub.1, S.sub.2, S.sub.3, and S.sub.4 will be
opened. In the event of one condition, such as the presence of
smoke, one of the signals, for example S.sub.1 will close, thereby
giving a deviation from the normal current in line 16 to a
predetermined value. For example, the potentiometer 46a can be set
so that the current flowing through line 16 will drop below the
second predetermined current value when switch S.sub.1 is closed,
thereby activating the latching circuit and turning on the light in
that latching circuit. Similarly, potentiometer 46b can be set so
that when switch S.sub.2 is closed, the current drops below the
first predetermined current level, thereby activating latching
circuit 40a. Similarly, potentiometer 46c can be set so that when
switch S.sub.3 is closed, the current in line 16 rises above the
first predetermined current level and activates the latching
circuit 42a, thereby turning on the light in the latching circuit.
Obviously, when switch S.sub.4 is activated, the subscriber line 16
will be shorted, thereby raising the current in the subscriber line
above the second predetermined level, and thereby activating the
latching circuit 42a to turn on the audible alarm and to light the
light of 42a. In order to protect the monitoring equipment, a
current limiting device (not shown) can be installed in line 16 to
limit the current when the current in line 16 rises above a certain
value. Such current limiting devices are well known and will not be
further described herein.
The lights in the latching circuits 40, 40a, 42a and 42 can be all
of different colors so that each color represents a different
condition at the subscriber station.
As a further means to detect tampering with the subscriber line and
the monitoring system, an AC signal can be impressed upon the DC
current and detected at the monitoring station. For this purpose,
an AC signal generator 150 can be installed at the subscriber
station to impart an alternating current signal to the DC current.
The signal generator is preferably variable so that the AC signal
can be varied for different alternating currents at different
times. For example, one frequency can be used for daytime
monitoring and another frequency can be used for night time
monitoring. The signal generator can be of conventional design and
impart a sinusoidal signal on top of the DC current or can be a
conventional chopper network which alternates the DC current.
At the monitoring station, a lead 152 picks off the signal from the
output from the attenuator 20. The DC signal is blocked by
capacitor 154 and the AC signal is amplified in an AC amplifier
156. The signal is then converted to a DC voltage which is
representative of the frequency of the signal in line 152 by a
frequency to voltage converter 158. This converter comprises a one
shot multivibrator 160, a filter network 162 and a DC amplifier 164
having an RC filter 166 applied between the output and the negative
input of the amplifier 164. The output from the converter 158 thus
is a direct current voltage which is directly proportional to the
frequency of the signal detected in line 152. The output voltage
from the converter 158 is applied to a negative input to a
comparator 170 and to a positive input to comparator 180. A
reference voltage means 168 of similar construction to the
reference voltage means 30 applies a reference voltage to the
positive input to the comparator 170, which voltage is
representative of the low tolerance level of the frequency of the
signal in subscriber line 16. The output voltage from the
comparator 170 is inverted by inverter 172 and the inverted voltage
is applied to a latching circuit 174 having a light 176. The
inverter 172 can be of the same construction as the inverter 36 and
the latching circuit 174 can be of the same construction as the
latching circuit 40.
Similarly, a reference voltage means 178 applies an upper limit
reference voltage to the negative input to comparator 180. The
output voltage from comparator 180 is inverted in inverter 182 and
the inverted voltage is applied to a latching circuit 184 having a
light 186. The construction and operation of the inverter 182 is
the same as that of the inverter 38 illustrated in detail in FIG.
1. The outputs from the latching circuits 174 and 184 are applied
to the audible alarm circuit to produce an audible alarm when
either of the latching circuits are activated.
The latching circuit 174 is activated to produce an audible alarm
and to illuminate the light 176 for a visual alarm when the voltage
applied to the input thereof is positive. Under normal conditions,
the output from comparator 170 will be positive, and the voltage,
when inverted, will apply a negative voltage to latching circuit
174. When the frequency drops below a predetermined value, such as
when the line 16 is open, the comparator 170 will switch to a
negative output, thereby causing a positive voltage to be applied
to the latching circuit 174. The light 176 will be illuminated, and
the audible alarm in circuit 44 will be activated.
Similarly, when the frequency of the signal in the subscriber line
16 rises above a predetermined limit, the output from the
comparator 180 will switch negative, thereby applying a positive
voltage (by virtue of inverter 182) to the latching circuit 184.
This positive voltage will switch the latching circuit on, thereby
illuminating the light 186 and thereby activating the audible alarm
in the alarm circuit 44.
All of the visual alarm lights desirably are of a different color
so that the person who is monitoring the subscriber can very easily
detect what condition is causing the audible alarm and can initiate
an appropriate remedy. For example, if smoke is causing the alarm,
the monitor can easily see this condition and can dispatch fire
engines to the subscriber. On the other hand, if switch S.sub.2 for
example, was set for a hold up at the subscriber 12, then an
appropriate light would indicate that a hold up was taking place.
The monitor could then quickly dispatch police officers to the
subscriber 12.
The frequency monitoring system provides increased sophistication
for the current monitoring device. The frequency monitoring system
is used primarily to detect tampering with the system when the
current monitoring system fails. For example, in the event that
someone can by electrical equipment determine what the normal level
of current is in line 16, it is possible for that person to cut off
the alarm system while stil maintaining the normal monitoring
current. They may not, however, detect the exact frequency of the
AC voltage and thus be unable to duplicate that frequency. In
addition, they may not be able to duplicate the monitoring
frequency. In any case, when a small predetermined variation occurs
in the monitoring frequency, an alarm is sounded at the monitoring
station. In addition, the monitoring frequency may change between
the time the frequency is detected and the time the false frequency
is impressed on the monitoring current, resulting in the sounding
of an alarm.
With the use of the inventive system, many modifications to the
monitoring system are possible. The tolerances of the monitoring
current can be adjusted easily. Multiple levels of current can be
detected for multiple conditions at the subscriber station with a
single subscriber line. The sophisticated system makes it very
difficult to tamper with the alarm system. Further, the electronic
system, eliminating normal relays and the like, can be constructed
of more reliable electronic components which are also smaller in
physical volume and weight, thus, a compact, more reliable,
monitoring station results from the electronic monitoring
system.
The system has been described with reference to the use of negative
voltages at the voltage terminals. When negative voltages are
applied to the voltage terminals, a single subscriber line can be
employed. Obviously, positive voltages can also be employed in lieu
of negative voltages but positive voltages may require a return
line from the subscriber.
A particular latching circuit has been described. Other latching
circuits such as a flip-flop circuit can be employed in lieu of the
described latching circuit without departing from the scope of the
invention.
In the above described system, it is desirable to limit the current
in the line in the event a short occurs, for example, to avoid
damaging of the electrical components in the subscriber line. For
this purpose, a current limiting device, such as a transistor, can
be placed in the subscriber line and coupled directly to the output
of the over current comparator 32 so that the transistor switches
to a less conducting or non-conducting state when the comparator 32
switches negative.
Reasonable variation and modification are possible within the scope
of the foregoing disclosure, the drawings, and appended claims
without departing from the spirit of the invention.
* * * * *