U.S. patent number 3,587,082 [Application Number 04/719,145] was granted by the patent office on 1971-06-22 for holdup alarm system.
This patent grant is currently assigned to American District Telegraph Company. Invention is credited to Frederick G. Hill, Anthony C. LaMartina, Jr., Manfred W. Muehter.
United States Patent |
3,587,082 |
Muehter , et al. |
June 22, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
HOLDUP ALARM SYSTEM
Abstract
A holdup alarm system in which the premises to be protected are
coupled to a central station via a transmission line, the central
station having a source of direct current potential, a break
indicator and a ground indicator connected in series with the line.
The protected premises is provided with a normally closed loop
circuit connected to the transmission line. The loop circuit
includes a manually operable holdup switch having break contacts
normally included in the loop circuit and connected to the high
side of the transmission line. The holdup switch also has make
contacts arranged, upon operation of the holdup switch, to be
connected to the low side of the transmission line. An electronic
switch is coupled between the high and low sides of the
transmission line when the holdup switch is operated. An astable
multivibrator is energized when the holdup switch is operated and
operates the electronic switch to successively open and ground the
transmission line at a repetition rate controlled by the
multivibrator. Means also are provided to prevent a holdup alarm
from being given upon an accidental break or ground in the wiring.
Means also are provided to supervise the line and associated wiring
against open and ground faults.
Inventors: |
Muehter; Manfred W.
(Livingston, NJ), Hill; Frederick G. (Yonkers, NY),
LaMartina, Jr.; Anthony C. (Ridgewood, NY) |
Assignee: |
American District Telegraph
Company (Jersey City, NJ)
|
Family
ID: |
24888910 |
Appl.
No.: |
04/719,145 |
Filed: |
April 5, 1968 |
Current U.S.
Class: |
340/286.04;
340/532; 340/533; 340/650; 340/693.1 |
Current CPC
Class: |
G08B
21/0297 (20130101) |
Current International
Class: |
G08B
21/00 (20060101); G08B 21/02 (20060101); G08b
029/00 (); G08b 019/00 () |
Field of
Search: |
;340/409,276 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Habecker; Thomas B.
Claims
What we claim is:
1. Holdup alarm apparatus for use in an electrical protection
system at a protected premises, said system having a local
protection circuit and being coupled to a central station over a
transmission line having a high side and a low side, the central
station having a current sensitive line break indicator and a
source of direct current potential coupled in series between the
high and low sides of the transmission line, the holdup alarm
apparatus comprising:
a. a manually operable switch having break contacts interconnecting
the high side of said transmission line and one end of said local
protection circuit and having make contacts;
b. means interconnecting the other side of said local protection
circuit and the low side of said transmission line;
c. an electronic switching device;
d. means including said make contacts coupling said electronic
switching device between the high and low sides of said
transmission line when said manually operable switch is operated;
and
e. an astable multivibrator operatively coupled to said electronic
switch alternately to open and close said electronic switch thereby
alternately to intercouple and uncouple the high and low sides of
said transmission line at a rate determined by the repetition rate
of said multivibrator alternately to cause current to flow in said
transmission line and to break said transmission line thereby to
subject said break indicator at said central station alternately to
periods of current flow and periods of reduced current flow as a
signal indication of operation of said manually operable
switch.
2. Holdup alarm apparatus for use in an electrical protection
system at a protected premises, said system having a local
protection circuit and being coupled to a central station over a
transmission line having a high side and a low side, the central
station having a current sensitive break indicator and a source of
direct current potential coupled in series between the high and low
sides of the transmission line, the holdup alarm apparatus
comprising:
a. a manually operable switch having break contacts interconnecting
the high side of said transmission line and one end of said local
protection circuit and having make contacts;
b. means interconnecting the other side of said local protection
circuit and the low side of said transmission line;
c. an electronic switching device;
d. means including said make contacts coupling said electronic
switching device between the high and low sides of said
transmission line when said manually operable switch is
operated;
e. an astable multivibrator operatively coupled to said electronic
switch alternately to open and close said electronic switch thereby
alternately to intercouple and uncouple the high and low sides of
said transmission line at a rate determined by the repetition rate
of said multivibrator;
f. means including said break contacts, when closed, to render said
multivibrator unoperated; and
g. means including said make contacts, when closed, to render said
multivibrator operative whereby said electronic switch is operated,
alternately to cause current to flow in said transmission line and
to break said transmission line thereby to subject said break
indicator at said central station alternately to periods of current
flow and periods of reduced current flow as a signal indication of
operation of said manually operable switch.
3. Holdup alarm apparatus as set forth in claim 2 in which said
electronic switch comprises a transistor having a first electrode
coupled to the high side of said transmission line, a second
electrode coupled through said make contacts to the low side of
said transmission line and a third electrode coupled to said
multivibrator whereby a change in condition of said multivibrator
changes the conductive condition of the circuit including said
first and second electrodes.
4. Holdup alarm apparatus as set forth in claim 3 in which a Zener
diode is intercoupled between said second electrode and said make
contacts, said Zener diode being poled to break down when said
transistor is conductive between said first and second
electrodes.
5. Holdup alarm apparatus as set forth in claim 4 in which a
capacitor is coupled in parallel with said Zener diode and to said
multivibrator to provide operating potential for the latter when
said transistor is not conductive.
6. Holdup alarm apparatus as set forth in claim 5 in which said
multivibrator comprises second and third transistors operatively
coupled between said second electrode of said first transistor and
said make contacts, so as to become alternately conductive and
nonconductive in sequence, one of said second and third transistors
being arranged, when conductive, to bias said first transistor to
conduction and, when not conductive, to bias said first transistor
to nonconduction.
7. Holdup alarm apparatus as set forth in claim 2 in which said
electronic switch comprises a first transistor and said
multivibrator comprises second and third transistors, said
transistors being operatively coupled to said transmission line to
receive operating potential therefrom, said break contacts being
arranged, when closed, to shunt said operating potential.
8. Holdup alarm apparatus as set forth in claim 7 comprising
biasing means arranged to prevent operation of said multivibrator
upon removal of said shunt, said biasing means being arranged to be
shunted by said make contacts when the latter are closed.
9. Holdup alarm apparatus for use in an electrical protection
system at a protected premises, said system having a local
protection circuit and being coupled to a central station over a
transmission line having a high side and a low side, the central
station having a current sensitive break indicator, a tone
responsive indicator and a source of direct current potential
coupled in series between the high and low sides of the
transmission line, the holdup alarm apparatus comprising:
a. a manually operable switch having break contacts interconnecting
the high side of said transmission line and one end of said local
protection circuit and having make contacts;
b. means interconnecting the other side of said local protection
circuit and the low side of said transmission line;
c. an electronic switching device;
d. means including said make contacts coupling said electronic
switching device between the high and low sides of said
transmission line when said manually operable switch is
operated;
e. an astable multivibrator operatively coupled to said electronic
switch alternately to open and close said electronic switch thereby
alternately to intercouple and uncouple the high and low sides of
said transmission line at a rate determined by the repetition rate
of said multivibrator;
f. means including said break contacts, when closed, to render said
multivibrator unoperated;
g. means including said make contacts, when closed, to render said
multivibrator operative whereby said electronic switch is operated
alternately to cause current to flow in said transmission line and
to break said transmission line thereby to subject said break
indicator at said central station alternately to periods of current
flow and periods of reduced current flow as a signal indication of
operation of said manually operable switch; and
h. a tone generator at said protected premises and coupled in
series with said transmission line to superimpose an alternating
tone signal on the direct current flowing in said line, absence of
said tone being detected by said tone responsive indicator as an
indication of a fault in said transmission line.
Description
BACKGROUND
The present invention relates to electrical protection systems and
more particularly to such systems in which manually operable means
are provided at a protected premises to signal to a central station
the occurrence of a holdup.
As used in the electrical protection field, the term "central
station" usually refers to a particular type of central signal
receiving station. As used herein, however, the term is intended to
have a somewhat broader meaning so as to include also other distant
places where signals can be received and acted upon, e.g., guard
stations and police stations. The term "holdup" usually refers to
an armed robbery and presents a protection situation which must be
responded to with extreme speed, as distinguished, for example,
from the usual burglary which normally will require more time to
consummate and which, while requiring prompt action, is not usually
as urgent as a holdup. However, the term "holdup" is intended to
include generally those emergency situations requiring extremely
rapid response.
It is common for mercantile and industrial establishments to be
provided with protection devices which will be actuated upon entry
of an intruder automatically to signal the fact of such entry to a
central station where an operator will take the necessary action,
e.g., dispatching armed guards or police to the premises so
attacked. When the premises are open for business, some or all of
the protection devices usually will be disabled since it is not
desired that innocent entry or exit be signalled. However, the
protected premises are nevertheless connected to the central
station over the transmission line and, even if no entry protection
devices are operative, supervisory current will flow over the line
to protect against opens on the line.
Where there is a danger of armed robber, i.e., holdup, of a
protected premises, it is customary to have manually operable
switches, usually concealed, which can be operated to signal the
occurrence of a holdup so that prompt corrective action can be
taken by police or other appropriate agencies. Because of the
extreme emergency represented by a holdup situation, it has been
customary to make the holdup alarm signal which is transmitted to
the central station distinctive, i.e., readily distinguishable from
the usual break and ground signals.
Many thousands of holdup alarm systems have been installed as part
of electrical protection systems. A particularly useful example
used for many years has involved the use of a manually operable
switch with both break and make contacts and a relay with a
weighted armature and break contacts connected in series with the
relay coil and the switch make contacts. The supervisory current
flows through the switch break contacts until interrupted by
operation of the switch. The consequent cutting off of supervisory
current actuates the central station break drop. Closing of the
switch make contacts energizes the relay and also restores the flow
of supervisory current, but through the relay coil rather than the
switch break contacts. Energization of the relay opens its break
contacts, again deenergizing the relay and cutting off supervisory
current. The relay will then continue to be successively energized
and deenergized at a rate dependent on the armature mechanical
constants, which typically might involve 3 operating cycles per
second. A special relay or special contacts on the break drop at
the central station respond to the interrupted current flow to
actuate a holdup signalling device at the central station.
While holdup alarm systems of the character described have afforded
highly satisfactory service, they lack the extreme reliability and
trouble free operation which can be obtained with properly designed
solid state circuitry.
The principal object of the present invention has been the
provision of a novel and improved holdup alarm system.
More particularly, it has been an object of the invention to
provide such a system which is extremely reliable and trouble free
and which requires a minimum of maintenance.
Still another object of the invention has been the provision of
such a system which will not transmit a spurious holdup alarm
signal in the event of an accidental break or ground.
Another object of the invention has been the provision of such a
system which affords positive supervision against accidental opens
or grounds on the transmission line.
Other and further objects, features and advantages of the invention
will appear more fully from the following description of the
invention.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a holdup alarm
apparatus for use in an electrical protection system at a protected
premises, the system having a local protection circuit and being
coupled to a central station over a transmission line, the central
station having a current sensitive break indicator, a current
sensitive ground indicator and a source of direct current potential
coupled in series between the high and low sides of the
transmission line. The holdup alarm apparatus at the protected
premises comprises a manually operable switch having break contacts
interconnecting the high side of the transmission line and one end
of the local protection circuit, the manually operable switch also
having make contacts. The other side of the local protection
circuit is connected to the low side of the transmission line. An
electronic switching device is arranged to be coupled between the
high and low sides of the transmission lines through the make
contacts when the latter are closed. An astable multivibrator is
operatively coupled to the electronic switch alternately to open
and close the electronic switch thereby alternately to intercouple
and uncouple the high and low sides of the transmission line at a
rate determined by the repetition rate of the multivibrator. The
break contacts of the manually operable switch are arranged
normally to render the multivibrator unoperated. Means including
the make contacts of the manually operable switch, when closed,
render the multivibrator operative whereby the electronic switch is
operated alternately to cause current to flow in the transmission
line and to break the transmission line thereby to subject the
break indicator at the central station alternately to periods of
current flow and periods of no current flow as a signal indication
of operation of the manually operable switch.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described in connection with the appended
drawing which illustrates an electrical protection system embodying
the invention.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Referring now to the drawing, there are shown a central station 10,
a protected premises 11 and a transmission line 12 intercoupling
the central station and protected premises. The transmission line
is shown as a two conductor line which might typically be a leased
telephone line. However, a single conductor line with ground return
can be used in place of a two conductor line, as is well known in
the art.
The central station 10 is provided with a break drop 13, a ground
drop 14 and a central station battery 15, all connected in series
between the high and low sides of the transmission line. The term
high side is intended to refer to the high potential side while the
term low side is intended to refer to the low potential or ground
side of the line. The system shown is intended to operate with a
positive potential on the high side and a grounded negative, but as
will be explained below, a positive grounded source can be used
with appropriate changes.
The break drop 13 is a current sensitive device which responds to a
drop in line current below a predetermined value to give a break
signal. The ground drop 14 is a current sensitive device which
responds to an increase in line current above another predetermined
value to give a ground signal. Such devices are well known in the
art in the form of relays and in electronic form. In addition to
responding to a decrease in line current, the break drop should
respond also to alternate periods of current flow above the break
level and below the break level, commonly called a "police call"
signal, to actuate an alarm device at the central station.
Typically the repetition rate or frequency used for a "police call"
signal is of the order of 3 cycles per second. If galvanometer-type
drops are used at the central station, as is common, it will be
desirable to use a separate break drop relay for the "police call"
signal.
In accordance with a further aspect of the invention, a tone
responsive device 16 may be included in series with the central
station drop for a purpose to be described below.
At the protected premises the high side of the transmission line 12
is connected through a current limiting resistor 17 and a path
designated A-B-C-D-E to a conventional burglar alarm circuit 18,
the other end of the burglar alarm circuit 18 being connected by a
conductor 19 to the low side of the transmission line. A tone
generator 20 is shown connected in series between point E and
circuit 18. The purpose of tone generator 20 will be described
below. The tone generator may be omitted and point E connected
directly to circuit 18.
The burglar alarm circuit 18 may be of any type adapted to be
connected in a series circuit between the high side and the low
side of a transmission line. Typically the circuit 18 might include
a control set, door and window contacts, foil, photoelectric beams,
sound responsive systems, and capacitance responsive systems. The
control set will usually provide for a "day" protection and a
"night" protection condition. In the "day" protection condition
some or all of the protective devices will be shunted and a normal
day supervisory current will flow through line 12. In the "night"
protection condition all of the protective devices will be
connected in the circuit and a normal "night" current will flow
through line 12. Adjustable resistors are provided in the control
set to permit adjustment of the "day" and "night" values of
current.
As is well known in the art, an alarm may be transmitted by opening
the current path, thereby decreasing the current flow through line
12, by grounding the current path, i.e., shunting all following
elements and thereby increasing the current flow through line 12,
or by opening and then grounding the current path. Distributed
constants of the line may provide a current flow even with an open
circuit at the protected premises.
For the purpose of understanding the operation of the invention,
the burglar alarm circuit 18 could be considered as a resistor
selected to provide a normal value of current flow in line 12.
The path A-B-C-D-E comprises a conductor 21, break contacts H1-B
and H2-B of manually operable holdup switches H1 and H2,
respectively, a conductor 22 and diodes D6, D7 and D8. The return
path includes tone generator 20 (if provided), burglar alarm
circuit 18 and conductor 19 to the low side of the central station
line. The conductor 19 is connected to the fixed terminals of make
contacts H1-M and H2-M of holdup switches H1 and H2,
respectively.
The holdup alarm switches H1 and H2 are of a type commonly used in
holdup alarm systems and comprise an armature normally made with a
fixed contact and together forming the "break" contacts, the
armature being arranged when the switch is manually operated to
leave the fixed contact of the "break" contacts and make with
another fixed contact which, together with the armature, forms the
"make" contacts. The holdup alarm switches usually will be hidden
and will be placed where they can be operated surreptitiously by a
hand, arm, foot or other part of a body. After operation the
switches preferably lock into their make positions. Any number of
holdup alarm switches may be provided, e.g., at each teller's
position in a bank. The break contacts of the switches are all
connected in series. The fixed terminal of each of the make
contacts is connected to the conductor 19.
When one of the holdup alarm switches is operated, it is desired
alternately to increase and decrease the transmission line current
at a fixed repetition rate, e.g., 3 cycles per second, which will
actuate the break drop at the central station to produce a holdup
alarm at the central station. For this purpose there are provided
an electronic switching device, comprising a transistor Q1, and an
astable multivibrator 23, comprising transistors Q2 and Q3.
The transistor Q1, which might be of the 2N4036 type, has its
emitter connected to point A and its collector connected through a
conductor 24 to one terminal of a Zener diode D9, which might be of
the 1N4736 (6.8 v.) type and to the positive side of a capacitor
C5. The other terminal of diode D9 and the other side of capacitor
C5 are connected to conductor 22. A resistor R1 intercouples the
base and emitter of transistor Q1. A resistor R2, a capacitor C4
and a resistor R3, in series circuit, intercouple the base and
collector of transistor Q1.
Transistors Q2 and Q3 might be of the 2N2405 and 2N3391 types,
respectively. The emitters of transistors Q2 and Q3 are connected
to conductor 22. The collector of transistor Q2 is connected to the
junction of resistor R2 and capacitor C4 and is coupled to
conductor 24 through diode D1 and resistor R4. Diode D1, and also
diodes D2--D8, might be of the 1N2070 type. The collector of
transistor Q2 is also coupled to the emitter thereof through a
capacitor C3. A capacitor C1, poled as shown, intercouples the
junction of diode D1 and resistor R4 to the junction of resistor R6
and diode D3. Resistor R6, diode D3 and a resistor R9 are coupled
in series between conductors 24 and 22.
A resistor R5, a diode D2 and a resistor R8 are connected in series
between conductors 24 and 22. The base of transistor Q2 is
connected to the junction of diode D2 and resistor R8 and, through
series connected diodes D4 and D5, to point E.
The collector of transistor Q3 is coupled to conductor 24 through a
resistor R7 and, through a capacitor C2, to the junction of
resistor R5 and diode D2. The base of transistor Q3 is connected to
the junction of diode D3 and resistor R9.
A Zener diode D10, which might be of the 1N4762 (82 v.) type is
coupled between points A and E to limit the voltage which may
appear across the electronic components, e.g., to 82 volts, thus
affording protection against surges in line current.
With the circuit in the condition shown, the metallic link A-B-C-D
creates a shunt path between points A and D. Hence there is no
operating potential for the transistors and all will be
nonconductive. Similarly all capacitors will be discharged.
When occurrence of a holdup is to be signalled, the link A-B-C-D
will be broken by operation of switch H1 or switch H2 (or both) and
the consequent opening of break contacts H1-B or H2-B, as the case
may be.
With the shunt link A-B-C-D broken, current will flow from point A
to point E through the parallel combination of the emitter-base
junction of transistor Q1 and resistor R1 and the following three
circuits:
a. resistor R2, capacitor C3, conductor 22 and diodes D6, D7 and
D8;
b. resistor R2, capacitor C4, resistor R3, resistor R5, diode D2,
and diodes D4 and D5;
c. resistor R2, capacitor C4, resistor R3, capacitor C5, and diodes
D6, D7 and D8.
The voltage which appears across diodes D6, D7, D8 reverse biases
the base of transistor Q2 through diodes D4 and D5 and prevents it
from becoming conductive. This feature prevents the multivibrator
Q2, Q3 from starting and producing a false holdup alarm signal in
the event of an accidental break in the line A-B-C-D which connects
the various holdup switches. An accidental ground on line A-B-C-D
cannot cause a false holdup signal because the electronic circuit
would still be shunted by line A-B-C-D. The holdup alarm signal can
be initiated only by the proper sequence of a break followed by a
ground on the line A-B-C-D, as will be described.
The initial current flow through the three paths identified above
is small and will result in only a minor charging of capacitors C3
and C4 and a very minor charging of capacitor C5.
When the armature of the operated holdup switch makes with the
opposite contact, i.e., when contacts H1-M or H2-M are closed,
ground potential is applied to point D. A larger current now flows
through the three paths enumerated above, charging the capacitors
C3, C4 and, by increasing the base current through transistor Q2,
causing the emitter-collector junction of that transistor to become
partially conductive and increasing the base current and
consequently the collector current of Q1 to allow capacitor C5 to
start charging at an appreciable rate. Simultaneously, the
application of ground potential at point D shunts diodes D6, D7, D8
thereby removing the reverse bias from the emitter of transistor
Q2. Since the shunt provided by diodes D4, D5 and the burglar alarm
circuit 18 across the base-emitter circuit of transistor Q2 is not
sufficient to prevent transistor Q2 from becoming conductive, the
multivibrator will be ready to start when capacitor C5 is
sufficiently charged.
The multivibrator, as indicated on the drawing, comprises
transistors Q2, Q3 and their associated capacitors and resistors.
As is the case with any astable multivibrator, either transistor
may become conductive first depending on such indeterminate factors
as transient circuit conditions, component tolerances, etc. Assume
therefore that transistor Q2 becomes conductive first when
capacitor C5 is sufficiently charged to permit base current to flow
to transistor Q2 via the path resistor R5 and diode D2 and via the
path resistor R7, capacitor C2 and diode D2 and capacitors C1 and
C2 are charged in the polarities shown in the drawing.
With transistor Q2 conductive, the collector-emitter circuit
thereof allows sufficient current to flow through the emitter-base
junction of transistor Q1 to cause it to become fully conductive.
The increased current flow through the emitter-collector circuit of
transistor Q1 completes the charging of capacitor C5. Current also
flows through the path consisting of resistor R6, discharging
capacitor C1, diode D1 and the collector-emitter junction of
transistor Q2 to ground at point D. The voltage across capacitor C1
reverse biases the base of transistor Q3 keeping that transistor
nonconductive until capacitor C1 discharges to the point where
transistor Q3 is no longer reverse biased. Transistor Q3 then
begins to conduct, drawing base current via resistor R6 and diode
D3, and current flows through the path resistor R5, discharging
capacitor C2 and the collector-emitter junction of transistor Q3 to
ground at point D. The voltage drop across capacitor C2 reverse
biases transistor Q2 causing it to become nonconductive for the
moment.
When transistor Q2 becomes nonconductive, the base current path for
transistor Q1 is removed (capacitors C3 and C4 are charged by now)
and transistor Q1 becomes nonconductive. With transistor Q1
nonconductive and the line A-B-C-D open at the operated contacts
H1-B or H2-B, the transmission line to the central station is open
and this condition represents the beginning of a break period in
the pulsing holdup alarm signal.
Transistor Q2 remains reverse biased until capacitor C2 becomes
sufficiently discharged to overcome the bias and transistor Q2
conducts again. Because transistor Q1 is now not conducting, base
current for transistor Q2 is supplied from capacitor C5 (whose
function is to provide current for the multivibrator when
transistor Q1 is nonconductive) via the path resistor R7, capacitor
C2 and diode D2. With transistor Q2 conductive, transistor Q1 is
switched on again and current flows from point A to recharge
capacitor C5 and to ground via the breakdown of Zener diode D9. The
Zener diode D9 also performs the functions of establishing a
constant voltage for the multivibrator and limiting the charge on
capacitor C5. Diode D9 might be of the 1N4736 (6.8 v.) type.
The switching on of transistor Q1 marks the end of the break period
and the beginning of the ground period in the signalling sequence.
The transistor Q3 becomes conductive again when the charge on
capacitor C1 overcomes the reverse bias voltage across resistor R6
and the multivibrator continues with transistors Q2 and Q3 turning
each other on and off. Whenever transistor Q2 is conductive,
transistor Q1 is conductive and a ground is applied to the
transmission line 12. When transistor Q2 is nonconductive,
transistor Q1 is nonconductive and a break appears in the
transmission line 12.
Had transistor Q3 become conductive first when the multivibrator
started, the sequence of events would have been the same, but
shifted half a cycle in time. The system will continue to operate
at a nominal repetition rate as determined by the time constant of
the multivibrator until the actuated holdup switch is reset thus
restoring the shunt A-B-C-D. The capacitor charges will then bleed
off through their associated resistors and the system will return
to the normal condition. It is desirable to make the repetition
rate about 3 cycles per second so as to conform to the usual
practice and to permit operation with existing central station
equipment.
The Zener diode D10 provides a bypass from point A to point E to
permit high voltage reverse polarity testing of the regular
protection circuit, as is customary, and to dispose of any high
voltage transients which may appear on the transmission line.
Local annunciation of a holdup alarm may be provided if desired,
e.g., by connecting a diode, a local battery and an alarm bell or
lamp in series between conductors 19 and 22, the diode being poled
to prevent line current from flowing through the alarm device when
none of the holdup switches are operated.
In a typical system the central station battery may supply a
voltage in the range of 52 to 78 volts. Typical values for the
various resistors and capacitors are set forth in the table below.
It should be understood, however, that these values as well as the
various voltages, transistor types and diode types set forth herein
are given only by way of example and should not be taken as
limiting the scope of the invention.
Resistor Ohms
R1 100,000
r2 2,700
r3 1,000,000
r4 18,000
r5 12,000
r6 56,000
r7 3,000
r8 120,000
r8 120,000
capacitor Microfarads
C1 5.6 (35 v.)
C2 22.0 (15 v.)
C3 0.1 (100 v.)
C4 0.5 (200 v.)
C5 200.0 (12 v.)
Should it be desired to operate the system with a positive grounded
central station battery, PNP transistor Q1 would be replaced with
an NPN transistor, e.g., of the 2N2405 type. NPN transistors Q2 and
Q3 would be replaced with PNP transistors, e.g., of the 2N4036 and
2N3906 types, respectively. The polarities of the diodes and
capacitors would be reversed and the values of resistors R2, R4,
R5, R6 and R7 would be changed to 1,800 ohms, 22,000 ohms, 18,000
ohms, 68,000 ohms and 5,600 ohms, respectively.
In order to supervise the system against opens and grounds on the
transmission line and on the shunt link A-B-C-D, a tone generator
20 is shown with its output coil 25 connected in series between
point E and burglar alarm circuit 18. The tone generator
superimposes an alternating signal, e.g., 1,000 cycles per second,
on the normal DC current flow. An open or ground on the
transmission line or on the link A-B-C-D will prevent the tone from
reaching the central station. Presence or absence of the tone is
detected by tone responsive device 16 at the central station, and
absence of the tone is used to signal a fault condition. If it
should be desired to supervise only the transmission line, the tone
generator 20 may be connected between the high side and the
transmission line and resistor 17 or between resistor 17 and point
A.
While the invention has been described in connection with specific
embodiments thereof and in a specific use, various modifications
thereof will occur to those skilled in the art without departing
from the spirit and scope of the invention as set forth in the
appended claims.
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