U.S. patent number 3,699,569 [Application Number 05/221,721] was granted by the patent office on 1972-10-17 for security system for indicating fire, intrusion or the like.
This patent grant is currently assigned to Minnesota Mining & Manufacturing Company. Invention is credited to John H. Lee.
United States Patent |
3,699,569 |
Lee |
October 17, 1972 |
SECURITY SYSTEM FOR INDICATING FIRE, INTRUSION OR THE LIKE
Abstract
A fire-intrusion system of the type including at least one
electrically continuous closed-loop for detecting the presence of
an intruder and at least one electrically continuous closed-loop
for detecting a fire wherein breaches in security indicative of
fire and intrusion are sensed by a change in potential at a point
in their respective loops. Potential sensing circuits are provided
which are responsive to a changed or abnormal potential to indicate
a fire, intrusion or inoperative circuit condition.
Inventors: |
Lee; John H. (Woodbury
Township, Washington County, MN) |
Assignee: |
Minnesota Mining &
Manufacturing Company (St. Paul, MN)
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Family
ID: |
22829064 |
Appl.
No.: |
05/221,721 |
Filed: |
January 28, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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839007 |
Jul 3, 1969 |
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Current U.S.
Class: |
340/508; 340/521;
340/512; 340/651 |
Current CPC
Class: |
G08B
19/00 (20130101); G08B 29/02 (20130101) |
Current International
Class: |
G08B
19/00 (20060101); G08B 29/00 (20060101); G08B
29/02 (20060101); G08b 019/00 (); G08b
029/00 () |
Field of
Search: |
;340/409,420,226,227.1,221,273,274,253,256,213.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Myer; Daniel
Parent Case Text
CROSS REFERENCES
This application is a continuation-in-part of our pending
application, Security System For Indicating Fire, Intrusion Or The
Like, U.S. Ser. No. 839,007 Filed July 3, 1969, now abandoned.
Claims
What is claimed is:
1. An electronic security system for detecting a breach in security
within a closed-loop circuit having a plurality of sensors which
may short or open said closed-loop in response to said breach
wherein the improvement comprises an electronic device responsive
to said sensors for producing an alarm signal when one of said
sensors is opened or shorted, said electronic device
comprising:
a. a bistable multivibrator means connected in said closed-loop
having a first normal state representing that said closed-loop
circuit is neither open nor short circuited and a second state
representing that said circuit is either short or open circuited,
said multivibrator means adapted to provide as said alarm signal an
output signal representative of said second state and including
first and second cross-coupled transistors, said first transistor
being in a conducting state during said multivibrator first normal
state;
b. a source of potential;
c. a unidirectional conducting device connected between a point on
said closed-loop circuit and to the base of said first transistor,
said unidirectional conducting device being in a non-conducting
state when the potential with respect to ground of said point on
said circuit is at a normal level;
d. potential dividing means connected between said source of
potential and said point; and
e. a switching transistor having a base coupled to said potential
dividing means and having collector and emitter leads coupled in
series between said source of potential and the base of said first
transistor, the normal potential at said point causing said
switching transistor to conduct to provide current to the base of
said first transistor;
said unidirectional conducting device switching to a conducting
state to divert current from the base of said first transistor in
response to said closed-loop becoming short circuited and said
switching transistor switching to a nonconducting state to block
current flow to the base of said first transistor in response to
said closed-loop becoming open circuited, said multivibrator means
switching to said second state to produce said alarm signal
whenever the base current flow of said first transistor is diverted
or blocked.
2. An electronic security system for detecting breach in security
within a closed-loop circuit having at least one sensor which may
short said closed-loop in response to a said breach comprising:
a bistable multivibrator means coupled to said closed-loop circuit
and having a first normal state representing that said circuit is
not short circuited and a second state representing that said
circuit is short circuited, said multivibrator means adapted to
provide an output alarm signal representative of said second
state;
a first bistable switching means coupled to said closed-loop
circuit having a first normal level in which said first bistable
switching means remains so long as said closed-loop circuit is
closed, and switching to a second level in response to sensing an
open-loop, and producing a system malfunction signal when switched
to said second level; and
a second bistable switching means coupled to said closed-loop
circuit having a first normal mode in which said second bistable
switching means remains so long as said closed-loop circuit remains
closed, and said second bistable means switching to a second mode
in response to said closed-loop circuit shorting to ground and
producing said system malfunction signal when switched to said
second mode; and wherein said first and second bistable switching
means are coupled to said bistable multivibrator means for
preventing production of said output alarm signal when either a
second level or second mode is produced.
3. An electronic security system according to claim 2 further
comprising:
supervision means coupled to said first and second switching means
and responsive to the presence of said system malfunction signal
therefrom to produce sensible indications for supervisory and
maintenance personnel that said system is malfunctioning.
4. An electronic device for detecting abnormal conditions in a
closed-loop circuit, which circuit comprises a first portion
normally at a substantially uniform first potential and a second
portion normally at a substantially uniform second potential
different from said first potential and other than zero,
comprising:
a. first means for sensing a change in potential at a point on said
first portion of the loop produced when said first and second
portions become substantially short circuited, for switching from a
normal state to an alarm state in response to said change and for
providing an alarm signal when switched to said alarm state;
b. second means for sensing a change in potential at a point on
said first portion of the loop produced when the potential at said
point rises above that of said substantially uniform first
potential, and for providing an output signal in response to said
sensed change in potential; and
c. third means for sensing a change in potential at a point on said
second portion of the loop portion produced when said potential
becomes approximately zero, and for providing an output signal in
response to said sensed change in potential.
5. An electronic device according to claim 4, wherein said third
means is further coupled to said first means to prevent said first
means from switching to said alarm state when said third means
switches to said second state and produces said output signal.
6. An electronic device according to claim 4, further comprising an
inhibiting means coupled to said first means for holding said first
means in said alarm state until a reset signal causes said first
means to switch back to said normal state.
7. An electronic device according to claim 4, wherein said first
means comprises
a bistable multivibrator having a pair of cross-coupled
transistors, including a first transistor normally residing in a
conducting state and having a base lead coupled to a point on said
first closed-loop portion, the normal potential at said point
providing current flow through said base lead to maintain said
first transistor in a conducting state but the potential at said
point, when at the same potential as the potential of said second
portion, switching said first transistor to the nonconducting
state.
8. An electronic device according to claim 7, wherein said third
means comprises a switching transistor having its collector and
emitter leads connected in series with a current source, having its
collector lead also connected to said first multivibrator
transistor base lead, and having its base lead connected to a point
on said second portion of said closed loop, the normal potential of
said second portion maintaining said switching transistor in a
conducting state and an abnormally low potential at said second
portion causing said switching transistor to switch to a
nonconducting state, to divert current normally flowing through
said switching transistor to said base lead of said first
multivibrator transistor to prevent said multivibrator transistor
from switching to a nonconducting state.
9. An electrical device according to claim 4, wherein said second
means comprises a switching transistor having its collector and
emitter leads connected in series with a current source and having
its base lead coupled to said point in said first portion of said
closed-loop circuit so as to be in a normally conducting state but
turning off in response to an increase in potential at said point
to provide an output signal.
10. A limited current drain device coupled to a closed-loop circuit
for providing a momentary indication indicative of a closed circuit
condition existing in said closed-loop circuit comprising:
a. energy source means;
b. switch means having first and second normally closed switches
mechanically coupled to operate together as a unit, said first
switch connected in series to said closed-loop circuit;
c. a series circuit connected to said energy source including a
current limiting resistor having one end connected to said energy
source, a normally open switch means, a light indicating means and
said second normally closed switch, the current flow from said
energy source means through said series circuit when completed by
closure of said normally opened switch means causing a limited
current drain on said energy source;
d. a capacitor;
e. a switching circuit means connected across said current limiting
resistor and to said capacitor, said switching circuit means
connecting said capacitor to said energy source when said normally
open switching means is open to charge said capacitor, said
switching circuit means connecting said capacitor to said other end
of said current limiting resistor when said normally open switch
means is operated to the closed position and said second normally
closed switch is closed, to provide a discharge path for said
capacitor, said light indicating means providing a momentary
indication in response to current flow produced by said discharge
of said capacitor, said momentary indication being indicative of
the position of said switch means and therefore indicative of a
closed circuit condition existing in said closed-loop circuit.
11. A limited current drain device according to claim 10 wherein
said switching circuit means comprises a first transistor having
emitter and collector leads connected across said current limiting
resistor, having a base lead connected to said capacitor and having
a conduction state controlled by said capacitor, and a second
transistor having its emitter and collector leads connected in
series between said energy source and said capacitor and having its
base coupled to the emitter of said first transistor; said second
transistor conducting to rapidly charge said capacitor when said
normally open switch means is in its open state and said capacitor
when charged causing said first transistor to momentarily conduct
to pass current from said capacitor to said lamp when said normally
open switch means is closed and said second normally closed switch
is closed.
12. An electronic security system for indicating breaches in
security within at least one closed-loop circuit comprising:
a. sensing means for sensing breaches in security in said at least
one closed-loop circuit;
b. at least one alarm signal producing means coupled to said
sensing means for producing an alarm signal and response to said
sensing; and
c. indicating means coupled to said at least one alarm signal
producing means for producing a characteristic security breach
indication in response to a said alarm signal, wherein said
indicating means comprises:
1. source switch means responsive to a said alarm signal;
2. characteristic security breach signal producing means connected
to said source switch means for activating said signal producing
means when said alarm signal is present said signal producing means
comprising:
saw tooth generator means for generating a saw tooth voltage
signal;
tone generating means coupled to said saw tooth generator for
producing an output, the frequency of which varies in response to
the amplitude of said saw tooth voltage signal; and
sound generating means coupled to said tone generating means for
producing as a said characteristic security breach indication a
warbling tone in response to said varying frequency output.
13. An electronic security system according to claim 12 wherein
each of said at least one closed-loop circuit has associated
therewith at least one sensing means and wherein the sensing means
associated with each closed-loop circuit is coupled to a separate
alarm signal producing means, wherein a first of said alarm signal
producing means is capable of producing a first alarm signal, and a
second of said alarm signal producing means is capable of producing
a second alarm signal; and wherein said source switch means further
comprises means responsive to said second alarm signal to
deactivate said tone generator, and wherein said characteristic
signal producing means further comprises a free running
multivibrator means for producing a series of pulses, which
multivibrator is capable of being activated by said switching means
in response to said second alarm signal, and wherein said
multivibrator is coupled to said tone generator such that said
series of pulses is transformed into a series of intermittent burst
of signals having a characteristic frequency within each burst to
produce an intermittent tone in said sound generating means as said
characteristic security breach indication when said second alarm
signal is present.
Description
FIELD OF THE INVENTION
Generally, the invention relates to electronic security systems
and, more specifically, it relates to electronic devices responsive
to detecting an abnormal potential in a normally closed security
loop to indicate the nature of the condition causing the abnormal
potential.
BACKGROUND OF THE INVENTION
A great need exists for protecting residences and businesses from
unauthorized entry and from fires. Many devices for use in such
security or fire-intrusion systems are commercially available. For
example, electrically conductive tape to be applied to windows, and
various tamper-proof locks and other means for indicating opening
of a door are available. Also, fire detection devices such as smoke
sensors and temperature sensors such as bi-metallic strips are well
known. Such devices are commonly connected to form an electrically
continuous circuit, commonly referred to as a normally closed or
closed-loop circuit. Such systems also include means for sensing an
abnormal potential in the closed-loop (most commonly a potential
corresponding to either a short or open circuit condition) and for
providing an indication corresponding to the nature of the abnormal
potential. Serious deficiencies of prior art sensing means include
excessive power consumption, unreliability, and cost. The sensing
means of many prior art systems employed relays. They draw
relatively high operating currents, are considerably more expensive
than solid state switches and generally are less reliable than
solid state devices. Other systems which employed solid state
circuitry proved unreliable because they were overly sensitive; for
one thing, they generated an alarm, a false alarm, in response to
spurious or noise signals.
Further, a problem of some prior systems, partly because of their
large power consumption, is that they were connected to receive
their operating energy from the ordinary house wiring and could
thus be rendered inoperative by cutting this connection.
BRIEF SUMMARY OF THE INVENTION
In one embodiment, the invention comprises an electronic security
system for sensing breaches in security within a closed-loop
circuit, such breaches being recognized as an open or short circuit
condition between two portions of the closed-loop circuit. The
system includes a bistable multivibrator means having a first
normal state representing that said closed-loop circuit is neither
open or short circuited, and a second alarm state representing that
said circuit is either open or short circuited. The multivibrator
comprises first and second cross-coupled transistors wherein the
first transistor is in the conducting state when the multivibrator
is in the first normal state. The system also includes a
unidirectional conducting device and a switching transistor both of
which are connected between the closed-loop circuit and the
multivibrator to sense said open or short circuit conditions and to
cause the multivibrator to change states in response thereto. The
unidirectional conducting device is connected between a point on
the closed-loop circuit and the base of the first transistor such
that the unidirectional conducting device is in a nonconducting
state when the potential with respect to ground at that point on
the circuit is at a normal level. The switching transistor has a
base coupled to a potential divider in series with the closed-loop
circuit, and collector and emitter leads coupled in series between
a source of potential and the base of the first transistor such
that the normal potential at the point on the closed-loop causes
the switching transistor to conduct to provide current to the base
of the first transistor. In operation the unidirectional conducting
device switches to a conducting state to divert current from the
base of the first transistor in response to the closed-loop
becoming short circuited and the switching transistor switches to a
nonconducting state to block the current flow to the base of the
first transistor in response to the closed-loop becoming open
circuited. The multivibrator means is thus caused to switch to the
second state to produce an alarm signal whenever the base current
flow of the first transistor is diverted or blocked.
In another embodiment of the invention the device detects breaches
in security in the closed-loop circuit corresponding to open and
short circuit conditions and also other abnormally high and low
potentials. This embodiment is particularly suited for use with the
closed-loop of a fire alarm system. Two portions of the closed-loop
are normally maintained at two different potentials with respect to
ground. A condition indicative of a fire is signaled by shorting
the two closed-loop portions together. It is usually required in
such fire alarm systems, for all malfunctions causing other
abnormal potentials within the closed-loop, that a positive
indication different than that provided by the fire alarm signal be
provided whenever such malfunctions occur within the system. The
device includes a bistable multivibrator means similar to the
bistable means of the afore-mentioned embodiment. The bistable
multivibrator means in this embodiment is coupled directly to one
portion of the closed-loop and is used to directly detect a short
condition of one portion of the closed-loop to another portion,
which short circuit condition is indicative of a fire condition.
The multivibrator means are adapted to provide an output alarm
signal representative of the second or alarm state.
This embodiment further provides first and second bistable
switching means coupled to the closed-loop circuit for sensing
malfunctions. The first bistable switching means has a first normal
level in which the first bistable means remains so long as the
closed-loop circuit is closed, and switches to a second level in
response to sensing an open-loop condition to produce a system
malfunction signal. The second bistable switching means has a first
normal mode in which it remains so long as the closed-loop circuit
is closed and switches to a second mode in response to the
closed-loop circuit becoming shorted to ground to produce the
system malfunction signal. The first and second bistable switching
means are coupled to the bistable multivibrator means to prevent
production of the output alarm signal when either the second level
or second mode are present.
In another embodiment of the invention, a limited current drain
device coupled to the closed-loop circuit provides a momentary
indication of a closed circuit condition existing in the
closed-loop circuit. This device includes switch means having first
and second normally closed switches mechanically coupled together,
wherein the first switch is connected in series in the closed-loop
circuit while the second switch is connected in series with an
energy source, a current limiting resistor, a normally open switch
means and a light indicating means. The device further includes a
capacitor and a switching circuit. The switching circuit is
connected across the current limiting resistor and to the
capacitor, connects the capacitor to the energy source when the
normally open switching means is open, to charge the capacitor, and
connects the capacitor to the current limiting resistor when the
normally open switch means is closed, thereby allowing a discharge
path for the capacitor through the light indicating means. This
provides a momentary indication of the position of the switch means
and therefore an indication of a closed circuit condition existing
in the closed-loop circuit.
A still further embodiment of the invention provides an electronic
security system for indicating breaches in security within at least
one closed-loop circuit wherein alarm signals produced by different
alarm signal producing means are coupled to indicating means. The
indicating means include source switch means responsive to a given
alarm signal and characteristic security breach signal producing
means, in turn coupled to the source switch means, for producing a
particular characteristic breach indication depending upon the
source of the alarm signal present.
DESCRIPTION OF PREFERRED EMBODIMENTS
Having thus generally described exemplary embodiments of the
abnormal potential sensors of the present invention, a preferred
embodiment of the present invention will be described in greater
detail with reference to the accompanying drawing.
The FIGURE is a combination block diagram and schematic wiring
diagram illustrating a security system including three exemplary
embodiments of abnormal potential sensing circuits and associated
indicating means.
The particular circuits shown are specifically intended for use
with a security system for a residence. The system is shown to
include a perimeter alarm circuit 10, a perimeter protection loop
12, a perimeter check loop 13, and a perimeter check circuit 14.
The perimeter protection loop 12 may include motion detector
devices (such as those described in U.S. Pat. No. RE.27,067; and
co-pending U.S. application, Ser. No. 191,917, both of which are
assigned to the same assignee as the present invention), contact
switches for use on doors and windows, door locks, pressure mats,
invisible ray devices and window locks. When all such devices in a
loop such as perimeter protection loop 12 and perimeter alarm
circuit 10 are in their normal condition, that is, with the doors
and windows locked, and with the motion detectors operating but not
detecting a moving object, they form a continuous closed-loop
circuit comprising first and second portions providing first and
second potentials with respect to ground at terminals 16 and 18
respectively. Terminal 16 it should be noted is at ground
potential. A resistor 19 is connected between terminals 16 and 18
to provide the desired potential difference between terminals 16
and 18. Perimeter check circuit 14 provides means for momentarily
checking for an open circuit in the perimeter protection loop 12 so
that one may be sure that all of the system sensors are in their
proper non-alarm condition prior to activating the system. For
purposes of illustration only one sensor 15 which includes the
normally closed switch 17A is shown used in the perimeter
protection loop 12. Because security systems often operate from a
self-contained energy source, such as a wet or dry cell battery, it
is desirable that such a checking circuit, when activated, provide
only an insignificant current drain from the energy source.
Otherwise, one intending to compromise the system need only hold
the switch of such a circuit in an activated position until the
energy source is depleted to a point insufficient to operate the
remainder of the system.
Also included in the system are a fire alarm circuit 20 and its
associated fire protection loop 22 and a supervision means 24. Fire
protection loop 22, like the perimeter protection loop 12, is a
normally electrically continuous circuit having at least two
portions at different potentials with respect to ground, one
potential provided at terminal 26 and another potential provided at
terminal 28. Resistor 19B is connected between terminals 26 and 28
to provide the potential difference between terminals 26 and 28.
The sensors employed within the fire protection loop 22 are well
known in the art. For purposes of illustration only one sensor
having a normally open switch 2 which is connected in parallel with
resistor 19B is shown. Switch 2 closes in response to a fire
condition. They may be devices such as smoke sensors, heat
detecting devices, or devices which sense for both heat and smoke.
One example of such a smoke and heat detecting device is a
Pyrotector D-C Smoke and Heat Detector, Pyrotector Model No.
30-286. Fire alarm circuit 20, in addition to providing an
indication that a sensor of loop 22 has detected a condition
indicative of a fire, must also provide a positive indication of
any other abnormal potential in the closed-loop. Supervision means
24 provides such an indication.
The system also includes a siren generator shown generally as 30, a
speaker 32 and a horn 34, both coupled to the siren generator 30.
The siren generator 30 responds to a signal from the perimeter
alarm circuit 10 to produce a warbling tone from the speaker 32 and
horn 34 and responds to a signal from the fire alarm circuit 20 to
produce an intermittent tone of a single frequency. A reset timer
36 is provided for resetting the perimeter alarm circuit 10 and
fire alarm circuit 20 to their respective normal state a
predetermined time after switching into their alarm state, provided
an alarm condition no longer exists.
The operation of the system will now be described beginning with
the perimeter check circuit 14. The perimeter check circuit 14
includes a resistor 52, a lamp 38, a normally open manually
operated switch 40, and the perimeter check loop 13, all of which
are connected in series between the system's energy source 42,
which for the illustrated embodiment may be a 12-volt battery, and
ground. The sensor 15 for the perimeter protection loop 12,
includes the normally closed switch 17A, connected in series in the
perimeter protection loop 12, and another normally closed switch
17B connected in series in the perimeter check loop 13. Switch 17B
is shown linked to switch 17A and can therefore be used in check
loop 13 to check the position of sensing switch 17A. Resistor 52
limits the current drawn from said energy source upon closure of
switch 40 to an insignificant level. With switch 40 open, and with
no charge stored in capacitor 48, the voltage at node 50 will cause
transistor 46 to conduct, thereby passing current to charge
capacitor 48. Capacitor 48 charges rapidly until transistor 46 is
turned off, while transistor 44 remains off due to the lack of a
potential difference across the transistor. Transistor 44 has its
base connected to the emitter of transistor 46, its collector
connected to the collector of transistor 46, and its emitter
connected to node 50 and the base of transistor 46. Upon closure of
switch 40, and assuming that switch 17B is closed, the emitter of
transistor 44 is grounded while the base is forward biased by the
charge on capacitor 48, the effect of which is to cause transistor
44 to conduct, resulting in a relatively high current flow through
transistor 44, closed switch 40, thence through the lamp 38 and
switch 17B to ground. This energizes the lamp 38, provided the
sensor 15 of perimeter protection loop 12 is in its normally closed
condition so that switch 17B of sensor 15 is closed, and provides
an indication that the perimeter protection loop is in a normal
operating condition. The relatively large current is applied only
so long as the charge on capacitor 48 is sufficient to forward bias
transistor 44. As the charge on capacitor 48 is dissipated,
transistor 44 ceases to conduct. The current through lamp 38 is
then limited by resistor 52 to a very low value, on the order of 60
microamps for so long as switch 40 is closed. When switch 40 is
released, capacitor 48 rapidly recharges so that another check may
be immediately made, if desired. If during such a check the sensor
15 of perimeter check loop 13 is not in its normal operating
condition so that switch 17 is open, lamp 38 will not be energized
and the nonoperating sensor thus detected.
Operation of the perimeter alarm circuit 10 begins upon closure of
switch 54. The perimeter alarm circuit 10 is shown to comprise a
bistable multivibrator shown generally as 56 and a voltage
potential sensing means shown generally as 58. Upon closure of
switch 54, multivibrator 56 is set to its normal state, i.e., with
cross-coupled transistors 60 and 62 conducting and nonconducting
respectively.
A damping capacitor 72 is provided between the collector and base
of normally conducting transistor 60 to prevent multivibrator 56
from changing from a normal state to an alarm state in response to
noise or otherwise spurious signals.
Voltage potential sensing means 58 is coupled by lead 64 to
terminal 18 of the perimeter protection loop 12, which terminal is
at about 6 volts. The potential of the other portion of the loop
12, which may be sensed at terminal 16, is normally at about ground
potential or zero volts. Under normal conditions, a diode 66 and
transistor 68 of the voltage potential sensing means 58 are
non-conducting and conducting respectively. Diode 66 is connected
between lead 64 and the multivibrator 56. Should the voltage at
terminal 18 rise to about the voltage of energy source 42, as would
happen should the perimeter protection loop 12 become open
circuited, the voltage at the base of transistor 68 will then be
the same as the voltage at its emitter, and the transistor 68 will
be turned off. With transistor 68 turned off, the current to the
base of transistor 60 is cut off and multivibrator 56 is switched
from its normal state to an alarm state, i.e., with transistor 62
conducting and transistor 60 nonconducting. When multivibrator 56
switches to its alarm state, a signal is provided on lead 70 to
siren generator 30. The perimeter protection loop 12 may also
utilize a number of sensors each of which have normally open
switches, connected in parallel with resistor 19A. Thus, a sensor
which includes a normally open switch is shown connected in
parallel with resistor 19A. When such a sensor responds to an alarm
condition the switch 1 closes to connect terminal 16 directly to
terminal 18. Assuming switch 17A is closed, as would occur when the
security in the perimeter protection loop is breached, the current
normally flowing to the base of the transistor 60 is diverted
through diode 66 to ground. With the base current thus diverted,
transistor 60 turns off to switch the multivibrator 56 into its
alarm state, thereby also providing a signal on lead 30.
The fire alarm circuit 20 includes a bistable multivibrator circuit
80 much like multivibrator 56 of the perimeter alarm circuit 10,
and control switch 5 similar to switch 54 of perimeter alarm
circuit 10 which must be closed to have circuit 20 operate.
Bistable multivibrator 80 has a pair of cross-coupled transistors
82 and 84 which normally are in a conducting and nonconducting
state, respectively. The base of the normally conducting transistor
82 is coupled through resistors 150 and 152 to a point on the fire
protection loop 22, specifically to terminal 28. With the fire
protection loop 22 in its normal state, current flows through the
loop 22 to ground and also to the base of transistor 82 to maintain
it in a conducting state. When switch 2 closes in response to a
fire condition the current in loop 22 increases substantially to
reduce the potential at terminal 28. This reduces the base current
to transistor 82 to a level sufficient to turn the transistor 82
off, thus switching multivibrator 80 to a second or alarm state,
wherein transistor 84 conducts, due to the increase in potential at
the collector of transistor 82. When transistor 84 conducts, a
diode 85 which connects with the collector of transistor 84 and to
terminal 28 via resistor 150 conducts to inhibit the current from
flowing to the base of transistor 82 thereby holding multivibrator
80 in its alarm state. In its second or alarm state, fire alarm
circuit 20 provides an output signal on lead 86 to the siren
generator 30.
It is a requirement of fire alarm circuits in general that,
whenever the voltage potential in a fire protection loop changes to
some abnormal level other than that indicative of a fire condition,
that an indication be provided to indicate the the circuit is not
operating properly. Moreover, when such an abnormal condition
exists, the circuit is not to provide its normal alarm condition.
Fire protection loop 22 therefore includes means for sensing three
types of conditions. These three conditions are fire, open
circuits, and short circuits to ground which could exist anywhere
in the loop 22. Accordingly, multivibrator 80 is provided with an
additional transistor 88, the base of which is coupled through
resistor 154 to the low potential side of the fire protection loop
22, while the collector of transistor 88 is coupled through
resistors 98 and 152 to the multivibrator transistor 82 in a manner
to provide current to the base of transistor 82 whenever an open
circuit or like condition in the fire protection loop 22 interrupts
the normal supply of current to the base of transistor 88.
The fire alarm circuit 20 is also provided with a high potential
sensor designated generally as 90 and a low potential sensor
designated generally as 92. As shown, the high potential sensor 90
includes a PNP transistor 94 having its base coupled through a
resistor 153 to terminal 28. Accordingly, should terminal 28
suddenly rise to an abnormally high potential with respect to
ground, e.g., the voltage of the energy source 42, the
corresponding voltage increase at the base of transistor 94 will
cause the transistor to lose its bias current thereby turning the
transistor 94 off. Such a rise to an abnormally high potential
could, for example, be caused by a short circuit from terminal 28
to the energy source 42 or by an open circuit in the loop 22. When
transistor 94 turns off, an output signal is provided on lead 96 to
the supervision means 24 in the form of cessation of the current
normally flowing through transistor 94 to the means 24.
The low potential sensor 92 includes NPN transistor 88 and NPN
transistor 99 together with associated coupling resistors 151, 157
and 159. Transistor 88 has its base coupled through resistors 154
and 155 to terminal 26, to normally provide a bias current
maintaining transistor 88 in a conducting state. Should the fire
loop 22 potential drop to an abnormally low potential, such as if
terminal 26 was shorted to ground, the base current to transistor
88 is removed, causing the transistor 88 to turn off. When
transistor 88 turns off, the potential at the collector of
transistor 88 rises to cause current to flow through resistor 98 to
multivibrator 80 to maintain it in its normal state, as previously
described. The increased potential at the collector of transistor
88 is also coupled to the base of NPN transistor 99 through
resistor 157 to cause the transistor 99 to conduct. This produces a
signal on lead 96 to the supervision means 24 in the form of
cessation of the current normally flowing to the supervision means
24 from the transistor 94, such current being diverted to ground
through transistor 99 rather than to the supervision means 24. The
supervision means 24 provides only an indication of whether or not
there is an open circuit, a short circuit to ground, or a short
circuit to the energy supply source 42 in the fire protection loop
22. The supervision means 24 may be monitored by maintenance
supervisors, a change in current supplied to the means 24 being
interpreted as an indication of a malfunction in the loop. On the
other hand, a short circuit across the line, i.e., between
terminals 26 and 28, as is provided by closure of sensor switch 2,
produces an alarm signal at the collector of transistor 82 which is
indicative of an actual fire condition.
The siren generator 30 is shown to comprise a tone generator 100,
saw tooth generator 102, a source switch 104, a free running
multivibrator 106, a series of input gates 108 and circuits 110 for
driving the horn 34. The bases of transistors 112 and 114 of the
input gates 108 are coupled by resistors 111 and 113 to lead 70
respectively. Breach of security in the perimeter protection loop
12 causes an output signal to be present on lead 70 which switches
transistors 112 and 114 to a conducting state. Transistor 114, when
conducting, continuously provides biasing current to the base of
PNP transistor 115 of source switch 104 via resistors 116, 119 and
121. Consequently, transistor 115 turns on, which in turn causes
NPN transistor 117 to conduct, thereby providing a source of
current on lead 120 for enabling saw tooth generator 102, tone
generator 100, and horn driving circuits 110. When the saw tooth
generator 102 is thus enabled, a ramp signal is generated on lead
123, which in turn is coupled to the tone generator 100. Tone
generator 100 is shown to comprise a free running multivibrator
125, the frequency of which varies according to the amplitude of
the ramp signal on lead 123. The effect of the output of saw tooth
generator 102 thus varying the frequency of tone generator 100, is
to produce a warbling tone from speaker 32 and horn 34.
In the fire alarm mode of operation, a signal from fire alarm
circuit 20 on lead 86 is coupled to the bases of transistors 112,
116 and 118 via resistors 105, 107 and 109. This causes the
transistors 112, 116 and 118 to conduct. Transistor 114 remains cut
off. With transistor 114 cut off and transistor 112 conducting, the
free running multivibrator 106 is enabled, thereby intermittently
supplying bias current to transistor 115 only when transistor 120
of the free running multivibrator is conducting. The effect of this
intermittent bias current is to cause the source switch 104 to turn
on and off, thus providing an intermittent current on lead 120,
which in turn only allows intermittent operation of tone generator
100 and horn driving circuits 110. Saw tooth generator 102 is
continuously disabled by the conduction of transistor 118 to stop
the warbling effect of tone generator 100. With the source switch
104 turning on and off, and the tone generator 100 oscillating at a
fixed frequency during the on periods of switch 104, siren
generator 30 provides an on-off tone to the speaker 32 and horn
34.
Reset timer 36 is also enabled in response to a signal on either
lead 70 or 86. As shown, the collector of transistor 112 is coupled
via lead 118 to the base of transistor 122 in the reset timer. When
transistor 112 turns on, transistor 122 does likewise, thus
permitting current flow through resistors 124 and 126 to store a
charge in capacitor 128. When the charge on capacitor 128 exceeds
the voltage at the base of transistor 130, (determined by a voltage
divider comprising resistors 132, 134 and 136), the transistor 130
conducts. This in turn forward biases emitter follower transistor
138. Current then flows through transistor 138 and its emitter
resistor 140 to produce a positive pulse on lead 142, which lead is
shown to be coupled to the bases of the normally conducting
transistors of multivibrators 80 and 56. If the condition which
caused one of the multivibrators to change states, i.e., the
conditions which turned on transistor 110, no longer exists, the
multivibrator will be switched back to its normal state.
In the embodiment depicted in the drawing, the following component
values are preferred.
RESISTORS
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19A 33K 153 10K 244 33K 19B 33K 154 10K 246 33K 52 220K 155 1K 250
6.8K 98 100K 157 100K 252 6.8K 105 220K 159 33K 254 100K 107 220K
200 33K 256 100K 109 220K 202 220K 262 56.OMEGA. 111 220K 206 10K
266 6.8K 113 220K 208 33K 268 10K 116 2.2K 209 33K 274 100.OMEGA.
119 6.8K 210 100K 278 1.2K 121 6.8K 212 47K 280 2.2K 124 220K 214
33K 282 1K 126 470K Pot. 216 33K 288 6.8K 127 470.OMEGA. 218
22.OMEGA. 290 15K 132 15K 224 10K 294 1K 134 15K 226 33K 300 10K
136 33K 228 33K 304 15K 140 100.OMEGA. 232 33K 308 2.2K 150 100K
237 33K 310 3.9K 151 33K 238 100K 314 2.2K 152 33K 242 33K 316 1K
324 33K 326 220K
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transistors
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44, 46, 60, 62, 82, 84, 88, 99, 112, 114, 116, 120, 138, 251, 270,
277, 284, 306 2n3394 68, 94, 115, 122, 130, 272 2n5354 117, 298,
302, 318 2n3414 320 2n4921
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capacitors
__________________________________________________________________________
48 125.mu.f 264 320.mu.f 72 .22.mu.f 276 125.mu.f 128 1000.mu.f 286
0.05.mu.f 240 .22.mu.f 292 0.05.mu.f 256 10.mu.f 328 10.mu.f 260
10.mu.f
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DIODES
__________________________________________________________________________
66, 85, 220, 222, 296 1N914 322 1N4001 204, 330 Fuse 1/2 amp
__________________________________________________________________________
* * * * *