U.S. patent number 3,728,706 [Application Number 05/076,149] was granted by the patent office on 1973-04-17 for system for indicating aerosols in the atmosphere.
This patent grant is currently assigned to General Signal Corporation. Invention is credited to Michael Suchomel, William C. Tipton.
United States Patent |
3,728,706 |
Tipton , et al. |
April 17, 1973 |
SYSTEM FOR INDICATING AEROSOLS IN THE ATMOSPHERE
Abstract
A fire detector having an ionization sensing means which changes
the level of an output signal in accordance with sensing the
presence of smoke in the atmosphere has been provided. A detecting
means selectively responsive to the output signal generates an
alarm control signal and the improvement includes a filter means
for rendering a detecting means operable if uninhibited to generate
the alarm signal irrespective of an output signal from the sensing
means. Inhibiting means normally operable to inhibit the filter is
rendered ineffective in response to the output signal of the
sensing means and the inhibiting means therefore checks the
operability of the detecting means.
Inventors: |
Tipton; William C. (Newark,
NJ), Suchomel; Michael (Mountainside, NJ) |
Assignee: |
General Signal Corporation
(Rochester, NY)
|
Family
ID: |
22130220 |
Appl.
No.: |
05/076,149 |
Filed: |
September 28, 1970 |
Current U.S.
Class: |
340/517; 250/381;
340/629; 250/356.1; 340/533 |
Current CPC
Class: |
G08B
17/113 (20130101); G08B 17/11 (20130101) |
Current International
Class: |
G08B
17/11 (20060101); G08B 17/113 (20060101); G08B
17/10 (20060101); G08b 017/10 (); H01j
039/28 () |
Field of
Search: |
;340/237S
;250/43.5D,44,83.6FT |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Myer; Daniel
Claims
What is claimed is:
1. A fire detector including; ionization sensing means for
increasing the level of an output signal relative to a normal
threshold value in accordance with sensing the presence of smoke in
the atmosphere, and detecting means selectively responsive to the
output signal for generating an alarm control signal wherein the
improvement comprises:
filter means effective if uninhibited for generating the alarm
signal; and
inhibiting means including a voltage level detector governed by the
output signal, operable to inhibit the filter means only if the
output signal is within a normal voltage range,
whereby an alarm signal is generated when the output signal is
above the voltage range as when smoke is present in the chamber or
when the output signal is below the voltage range as when there is
a malfunction in the ionization sensing means.
2. The fire detector of claim 1 wherein said detecting means
comprises: a field effect transistor amplifier having its input
governed by the output signal and switching means including a
plurality of electronic gates energized in accordance with an
output of the amplifier for generating said alarm output
signal.
3. The fire detector of claim 2 wherein said inhibiting means
includes: charging means coupled to a first of said gates, said
first gate being responsive to the level of the output of the
amplifier and said charging means to periodically trigger said gate
upon each accumulation of energy beyond the level of said output of
the amplifier.
4. The fire detector of claim 3 wherein said charging means
includes a resistor and capacitor combination coupled to said first
gate.
5. The fire detector of claim 4 wherein said filter means
comprises: timing means responsive to the periodic triggering of
said first gate producing a pulse for each occurrence.
6. The fire detector of claim 5 wherein said timing means includes
a charging circuit coupled between a second and third of said
gates, said charging circuit being discharged through said second
gate when said second gate is in a conductance state, said second
gate conductance state being governed by said triggering of said
first gate; and the third gate being turned on to produce a signal
when said charging rate is less than said triggering rate of said
first gate.
7. The fire detector of claim 6 wherein said detecting means
includes: a fourth of said gates, operative to produce said output
alarm signal in response to the conductance state of said third
gate.
8. A system as defined in claim 1 further comprising: centrally
located control means including line wires coupled to each of a
plurality of said fire detectors for indicating said alarm
condition.
9. A system as defined in claim 8 wherein said indicating means
comprises: current sensitive means responsive to a change in
current occasioned by the occurrence of said alarm signal for
providing said indication including a relay energized in accordance
with said current increase.
10. A system as defined in claim 8 further comprising: an end of
line code sender connected across the line wires for terminating a
line circuit connecting the detectors to the centrally located
control means including:
an oscillator powered by energy applied to the line wires for
generating pulses for transmission over the line wires to the
centrally located control means, and
centrally located receiver means responsive to said pulses for
indicating a trouble condition upon the cessation of pulses.
11. A system as defined in claim 10 wherein said receiver means
comprises:
switching means having a conductance state in accordance with said
pulsed energy signals,
timing means including a resistor and capacitor charging circuit
discharged each time said switching means is in its conductance
state, said charging circuit having a discharge time greater than
the normal frequency of said pulsed energy; and
means responsive to a charged condition of said charging means for
indicating a trouble condition upon the cessation of pulses.
12. The fire detector of claim 1 wherein said ionization sensing
means comprises: two ionization chambers coupled serially across a
source of low voltage including a reference chamber and a measuring
chamber, said reference chamber being substantially isolated from
smoke particles establishing a reference potential for said
detector and said measuring chamber serially coupled adjacent to
said reference chamber for producing a change in the level of said
reference potential thereby providing said output signal.
13. The fire detector of claim 1 comprising:
a base member;
a cylindrical tube of insulative material mounted thereto;
a common electrode plate secured coaxially within said tube, said
electrode coupled to said detector means;
a reference electrode substantially parallel with said common
electrode movably mounted to said base for establishing the
reference potential in accordance with the spacing of said common
electrode and said reference electrode;
a measuring electrode mounted to said base at the opposite end of
said tube in spaced relation with the opposite side of said common
electrode, said measuring electrode having openings therein for
admitting smoke particles.
14. The fire detector of claim 13 wherein said ionization chamber
includes a source of alpha particle emitting material disposed on
each side of said common electrode for ionizing the atmosphere with
said chambers and producing an ionization current in accordance
with a potential imposed across the reference and measuring
electrodes.
15. The fire detector of claim 14 wherein said measuring electrode
and the associated side of said common electrode are in a spaced
relation such that the walls of the tube tend to columnate the
alpha particles towards said measuring electrode for uniform
ionization of the atmosphere.
Description
BACKGROUND OF INVENTION
This invention relates to a fire detector system and in particular
to the system for detecting the presence of combustion products,
aerosols in the atmosphere.
Modern fire detection equipment generally utilizes an ionization
chamber which uses a radio-active source to ionize the atmosphere
within the chamber and a measurement of the variation in voltage
across the chamber is indicative of the condition of the
atmosphere. If the voltage across the chamber increases, it
indicates that the atmosphere for a fixed temperature and pressure
has mixed therein atmospheric aerosols or ionized particles which
tend to combine with the ionized particles produced by the radio
active source and reduce the current through the chamber. Other
factors, however, may provide a variation in voltage which would
give false indication of a fire condition. A draft, for example,
caused by a slamming door might disturb the atmosphere within the
chamber such that an alarm is sounded. Variations in atmospheric
pressure also effect the characteristic of the chamber. As is well
known in the art, two chambers can be used; one open to the
atmosphere for receiving or detecting the presence of aerosols and
another substantially closed to atmospheric aerosols. The closed
chamber is a reference for compensating the system under changes in
the atmospheric pressure. The system still may, however, be
susceptible to false triggering and noise transients which may be
received by the equipment.
Generally it is desirable to connect a plurality of fire detectors
in a specific area and link them together to a receiver which would
activate the alarm signal. However, noise, loading and supervisory
problems have made installing such fire detectors on a system-wide
basis with a central indication very difficult and costly.
In order to provide for a truly safety oriented system, supervision
of the circuits joining multiple sensing units and individual
trouble detection in each of the units is necessary. Trouble may be
corrected as soon as it occurs. Discovery should not be delayed by
the frequency of periodic checking, but rather continuous
monitoring of the fire detection apparatus should be conducted.
It is therefore an object of the present invention to provide an
arrangement which substantially obviates one or more of the
difficulties of the described prior arrangements.
It is another object of the present invention to provide a
simplified fire detection system.
It is yet another object of the present invention to provide a safe
fire detection system.
It is still another object of the present invention to provide an
economically manufactured installed and maintained fire detection
system.
SUMMARY OF INVENTION
There has been provided a fire detection system including
ionization sensing means for changing the level of an output signal
in accordance with sensing the presence of smoke in the atmosphere.
Detecting means selectively responsive to the output signal
generates an alarm control signal and the improvement includes
filter means for rendering the detecting means operable if
uninhibited to generate the alarm signal irrespective of an output
signal for the sensing means. Inhibiting means including a voltage
level detector is also provided which is normally operable to
inhibit the filtering means and it is rendered ineffective when a
voltage level governed by the output signal is above or below a
predetermined normal voltage range.
For a better understanding of the present invention, together with
other and further objects thereof, reference is had to the
following description taken in connection with the accompanying
drawings, while its scope will be pointed out in the appended
claims.
FIG. 1 is a schematic diagram of the fire detector including the
sensing means, detector circuitry and the improved apparatus of the
present invention.
FIG. 2 is a diagram of the control unit which may be remotely
located for central monitoring of a number of detectors coupled in
parallel.
FIG. 3 is a diagram of the supervisory circuit coupled in parallel
to the last unit in the multiple detector system.
FIG. 4 is a drawing showing the structure of the sensing unit.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The combustion products smoke detection system consists essentially
of three units; a detector 11, control unit 15, and an end-of-line
signal sender 16.
A detector unit 11 produces an output when combustion products are
sensed by an ionization chamber 10.
A control unit 15 supplies regulated, short circuit protected, 16V
D.C., to operate from one to 10 detectors connected in parallel. In
addition, it indicates a trouble condition if the line continuity
is broken, or if the 16V D.C. is lost for any reason. "FIRE"
indication condition is provided when a detector 11 output, signals
the presence of combustion products.
An end-of-line sender unit 16 furnishes current pulses to a line L
which are received by a trouble detection portion of the control
unit 15 thus, monitoring continuity.
DETECTOR
The detector 11 circuit operates on a relatively low input voltage
in the order of 16V D.C., applied to terminals 17 and 18. Numerals
20 and 21 indicate ionization chambers of sensing means 10
activated by a radio-active material 19 which emits alpha
particles.
Chamber 21 is open to the atmosphere and exposed to combustion
particles. Chamber 20 is the reference chamber closed to aerosols
which is used to nullify the effects of atmospheric changes. The
voltage across the ionization chambers 20 and 21 connected in
series is filtered by capacitor 22.
Particulate products of combustion entering the open sensing
chamber 21 hinder the normal ion current flow through the chamber
caused by alpha particles from the radio-active material 19,
resulting in a rise of voltage across the chamber 21. The input
stage of the detector 11 circuitry is a dual-gate MOSFET (Metal
Oxide Semiconductor Field Effect Transistor) 23, connected as a
source follower. The drain D electrode of FET 23 is connected to
the positive potential, the gate electrode G1, is connected to the
junction of chambers 20 and 21, the source electrode S is connected
to resistor 24, and the remaining electrode G2, is connected to the
source. In this connection the source S voltage follows the voltage
at gate G1, which is the chamber 21 voltage. It should also be
noted that a resistor 24 is selected to provide stable temperature
characteristic to the response of FET 23 and is connected between
electrode S and negative terminal 17. Capacitor 25 prevents damage
to FET 23 should gate G1 suddenly go positive.
The next stage of circuitry functions as a voltage level detector.
Transistor 26 is a Programmable Unijunction Transistor (PUT) and
with resistor 27 and capacitor 28 forms an oscillator or inhibit
means. Resistor 29, rectifier 30, and resistor 24 set a reference
voltage level for PUT 26. Capacitor 28 charges through resistor 27
until its voltage exceeds the reference level which is the voltage
at the gate electrode G3 of PUT 26 connected to resistors 29, 31
and rectifier 30 by about 0.3V. At this point, conduction begins
between the anode A1 and the cathode C1. Capacitor 28 then
discharges through the base emitter junction of transistor 32 until
its voltage drops to a cut off level at which conduction cannot be
maintained. Cut off then restores the anode A1 to cathode C1
blocking of PUT 26 and capacitor 28 may recharge through resistor
27. This charging and discharging of capacitor 28 continues,
forming a free running oscillator. A voltage divider formed by
resistors 37, 38 and potentiometer 39 is a sensitivity adjustment
for the detector. The voltage at the slider of potentiometer 39
sets the maximum voltage that capacitor 28 can charge to. Rectifier
33 permits discharge of capacitor 28 if it exceeds the voltage set
at potentiometer 39. If the voltage that capacitor 28 charges to is
not greater than the gate voltage by 0.3V, conduction of PUT 26
does not take place, and oscillations cease. This can occur if the
gate G3 voltage of PUT 26 is raised high enough as is the case when
the sensing chamber 21 voltage rises from the entrance of
particulate matter created by combustion. This, therefore, inhibits
the filter means 12 described further in the discussion. A fire
condition, therefore, causes the oscillations of PUT 26, resistor
27 and capacitor 28 to cease.
Each time capacitor 28 discharges through PUT 26, it biases
transistor 32 into conduction. Filter 12 comprises capacitor 40,
resistor 41 and transistor 32. When transistor 32 conducts,
capacitor 40 which is charged through resistor 41 discharges
through rectifier 42. Since transistor 32 conducts periodically,
capacitor 40 is discharged periodically. During the non-conduction
periods, capacitor 40 recharges through resistor 41. The discharge
of capacitor 40 depends on the repetition rate of the pulses gating
transistor 32 from capacitor 28, and the charge of capacitor 40
depends on the time constant of resistor-capacitor 41-40
combination. It should be noted that resistor 35 also provides bias
voltage for transistor 32. The voltage that appears on capacitor 40
depends on the relation of this repetition rate to the
resistor-capacitor 41-40 time constant. The average value of
voltage on transistor 32 is in the order of 2V. The
resistor-capacitor 41-40 combination in cooperation with transistor
32 therefore filters out signals of less than a predetermined
rate.
Transistor 43 is a Programmable Unijunction Transistor (PUT) with
resistors 44 and 45 setting the voltage reference level on gate G4.
If the pulses which make transistor 32 conduct, stop, capacitor 40
will charge to a higher voltage than the average. When this voltage
exceeds the reference set by resistors 44 and 45 by approximately
0.3V, conduction takes place from anode A2 to cathode C2 and
capacitor 40 discharges through PUT 43 and resistor 46. The voltage
across resistor 46 is coupled to the gate G5 of Silicon Controlled
Rectifier SCR 47 through resistor 48 and triggers conduction from
its anode A3 to cathode C3. This then effectively grounds resistor
49 and lights a fire indication lamp 50, and also any external
light (not shown) that may be connected across terminals 51 and 52.
Resistors 49 and 53 are dropping resistors for low voltage lamps
such as lamp 50. Resistor 54 and capacitor 56 form a filter network
to prevent firing of (SCR) 47 from spurious transients. Rectifier
57 allows the fire indication lamp 50 to light immediately if the
line should be connected in reverse polarity. Capacitor 58 is a
filter capacitor to filter R.F. transients from the unit. Diode 62
coupled between capacitor 58 and resistor 44 is provided for
polarity protection for the semi-conductors if the apparatus of
FIG. 1 is inadvertently connected in reverse. Resistors 59 and 60
assure closure of the Fire Sense Relay 61 in the control unit 15
should the fire indication lamp 50 be open circuited.
The control unit 15 contains the voltage regulated supply, fire
sensing circuitry, trouble circuitry, power failure sensing and
emergency power transfer circuitry.
POWER INPUT
Normal power is supplied to the control unit through a suitable
fuse 156 and transformer 67 which is full wave rectified by bridge
68 and filtered by capacitor 69. Emergency battery power is applied
to terminal 70 and externally switched, full-wave rectified D.C.
power may be applied to terminal 71. Resistor 77 limits the surge
current due to the charging of the filter capacitor 69. When AC
power is applied, relay 73 is energized thus opening normally
closed contacts 74 and closing normally open contacts 75. This
opens the battery input and connects the AC power to the control
unit. In addition, if A.C. power is lost for any reason, relay 73
deenergizes and closes contacts 152 for producing an A.C. power
loss signal. Diode 76 blocks battery voltage from re-energizing
relay 76 through resistor 77 if AC power is lost.
VOLTAGE REGULATOR
Zener diode 78 and resistor 79 form a voltage divider. The zener
voltage of 78 is about 10V. Resistors 80 and 81 in parallel with
zener diode 78 furnish a reference voltage for the base of
transistor 82. Since the base voltage is fixed, the drop across
resistor 83 is also fixed producing constant emitter current.
Neglecting base current the emitter and collector currents are
equal and fixed and transistor 82 forms a constant current
generator. The collector current of transistor 82 supplies the base
of transistor 84 and the collector of transistor 85. Transistors 84
and 86 are Darlington connected current amplifiers. Zener diode 87
and resistor 88 are in series across the output. The junction of
zener diode 87 and resistor 88 is at approximately 10V, with
respect to ground, and is a reference for the emitter of transistor
85. Resistors 89, 90 and 91 form a voltage divider across the
output of transistor 86. Resistor 90, which is adjustable, is used
to vary the output. The output is varied in the same manner that
the output is regulated. If the slider on resistor 90 is made more
positive, or the output voltage increases, the base current of
transistor 85 is increased, which causes 85 to conduct more. Since
the collector current of transistor 82 is constant, the additional
current must come from a decrease in base current of transistor 84
and in turn a decrease in the collector current of transistor 86.
This decrease in current is accomplished by an increase in drop
across 86. The output voltage will therefore decrease, since
transistor 86 is in series with the supply and load. For decrease
in the slider voltage of resistor 90 or a decrease in output
voltage, the opposite sequence occurs with a resulting increase in
the output voltage. Capacitor 92 is used as a filter capacitor.
OVER-CURRENT PROTECTION
Over-Current Protection is provided to prevent damage due to an
accidental short circuit of the output terminals. Output current
passes through resistor 93 which is connected to the emitter of
transistor 94 and base of transistor 94 through current limit
resistor 95. An increase in output current of transistor 86
increases the drop across resistor 93 and increases the current
through transistor 94 until saturation occurs. The increased
current draws base current from transistor 84 thus decreasing the
drive of transistor 86 and limits its output current.
FIRE SENSING
A parallel combination of relay 61, resistors 96 and 97 is placed
in series with the detectors 11. When a detector 11 is activated
due to the presence of smoke, there is an increase in current due
to the added load of the fire indicating lights 50 and the
resistors 59 and 60 in the detector 11. This increase of current
actuates the sensing relay 61. The current drawn per detector
ranges from a quiescent current of about 2.5 ma to a fire condition
current of 30 ma minimum. The relay 61 is calibrated to close with
an increase of 20 ma by closing switch 100 (which connects the
resistor 98 and 99 across the supply) and increasing the resistance
with resistor 96 until relay 61 pulls in. This step is performed
with all the detectors 11 connected. When switch 101 controlled by
relay 61 closes, voltage is impressed across resistors 102 and 103
applying voltage to the gate G6 of silicon control rectifier (SCR)
104 causing anode-cathode conduction. This applies voltage across
relay 105 energizing it. One set of contacts 109 of relay 105 is
available for fire indication, and the other set 110 is used to
short out the fire sensing relay, 61 to protect it from over
current. Diode 106 prevents inductive voltage spikes when relay 105
deenergizes. Resistor 107 and capacitor 108 limit the rate of rise
of voltage applied to SCR 104 to prevent accidental triggering.
Diode 111 is a "Stabistor" diode used to bypass the surge currents
of the detectors from relay 61. For a 0.5V pickup voltage of relay
61, diode 111 conducts less than 1 ma.
TROUBLE CIRCUIT
The trouble circuit detects the pulses placed on the line L
connecting all detectors 11 by the "End of Line Sender " 16 unit
located at the last detector. The primary of transformer 112 is
placed in series with the control unit 15 output terminal 113. The
pulses which appear across the secondary of transformer 112 are
rectified by diode 115, filtered by capacitor 116, and impressed
across resistors 117 and 118. A portion of this voltage is applied
to the base of transistor 119. Transistor 119 which is
non-conducting is turned on by the incoming pulses. When 119 is on,
capacitor 120 charges through resistor 121 and when transistor 119
is off, capacitor 120 discharges through resistor 122. The
discharge time constant is chosen to be much longer than the
repetition rate of the input pulses from the sender 16. The average
value of the voltage at the junction of resistors 121, 122 and
capacitor 120 which is tied to the gate G7 of field effect
transistor FET 124 is about 8-9V. This reduces the gate voltage
from the no signal condition. Since the source terminal S2 follows
the gate G7 except for a small voltage difference, the source
voltage is reduced also. Connected to the source S2 is a series
combination of resistors 125, 126 and a 10V zener diode 127. The
source voltage under these conditions is not sufficient to overcome
the zener voltage and cause conduction through this path. The
junction between diode 127 and resistor 126 is at a low voltage, so
that transistor 128 is cut off with a resulting high voltage on the
collector. Transistors 129 and 130 are in series with the "Trouble
Relay " 131. In a normal ready condition, both transistors 129 and
130 will be conducting and 131 will be energized. Rectifier 139
coupled across the coil of relay 131 acts as an electrical surge
suppressing device. Transistor 129 will be conducting when
transistor 128 is cut off since resistor 132 is supplying base
current. Should the pulses fail to be received for any reason,
transistor 129 would cut off since transistor 128 would divert its
base current. This would cause the "Trouble Relay " 131 to
deenergize, indicating trouble. Transistor 130 is maintained
conducting by the current through the fire lamp 133, wire 133a and
resistors 134, 135 and 136. A portion of this current supplies the
base of transistor 130 to maintain it conducting. This supervisory
current through the lamp 133 is not sufficient to light it. If the
lamp 133 is removed or opens, transistor 130 will cut off and
trouble relay 131 will become deenergized. In summary, "Trouble"
will be indicated when relay 131 deenergizes opening normally
closed contacts 151 for providing a trouble signal, this occurring
from loss of pulses received from the line L, the loss of the lamp
133, or the loss of the regulated positive voltage for the
system.
The diodes 137 and 138 are connected to the anode of SCR 104 which
is caused to conduct when a fire signal occurs. This places the
diodes 137 and 138 at near ground level as the anode of SCR 104
would then be at a low voltage. The lamp 133 with its protective
fuse circuit 140 will then light as almost full supply voltage is
across it and resistor 134, its dropping resistor. Also, "Trouble
Relay " 131 will be locked out giving priority to the fire
signal.
END-OF-LINE SENDER
The End-of-Line Sender 16 generates pulses and places them on the
lines L connecting the detector 11 to the control unit 15. The unit
is connected to the line at the most remote detector. The operation
is as follows: diode 141 provides protection against connecting the
leads in reverse polarity. Transistor 142 is a "Unijunction
Transistor" which, with resistor 143 and capacitor 144 forms an
oscillator. The pulses are developed across resistor 145 and the
divider formed by resistors 146 and 147. The pulse voltage across
resistor 147 is applied to the base of transistor 148. The emitter
current of 148 supplies enough drive to the base of transistor 149
to saturate it. Thus, almost the full 16V supply is placed across
resistor 150. The supply will be loaded by about 100 ma for the
duration of the pulse. The pulse duration is set for about 500
microseconds and it occurs periodically every 500 milliseconds.
FIG. 4 shows a preferred structure of a typical detector 11 having
sensing means 10 which is used to sense the presence of aerosols in
the atmosphere and produce an output signal indicative of a fire
condition. The sensing means includes a base number 160 which has
an adjustable reference electrode 161 appropriately coupled or
mounted thereto. Cylindrical housing 162 is fitted over the
reference electrode 161. A common electrode 163, which is a bobbin
shaped body of conductor having upper and lower surfaces A and B
respectively, is fit into the housing 162. On each of the surfaces
of the cylindrical electrode 163 is deposited a small amount of
radio-active material, which in the preferred embodiment, is
Americium 241 and is designated by the reference 164.
The reference chamber 20 is that volume disposed between the
reference electrode 161 and the lower surface B of the common
electrode 163. The adjustment of the distance between reference
electrode 161 and surface B sets up a reference potential. This is
substantially isolated from the atmosphere such that no aerosols
enter the chamber 20 and interfere with the current produced by the
radio-active source 19.
The sensing chamber 21 is formed between the upper surface A of the
common electrode 163 and sensing electrode 168 mounted above the
common electrode 163. This chamber is substantially opened to the
atmosphere and may receive aerosol particles present in the
atmospheric medium.
The upper surface A of the common electrode 163 is recessed in the
cylindrical housing 162 in order to deflect radiation emitted by
the source 19 towards the sensing electrode 168. The cylindrical
housing 162 is composed of teflon material and serves as an
absorber of alpha particles which may be emitted at angles away
from the sensing electrode 168. This cylindrical housing 162,
therefore, serves to columnate or direct the radio-active emissions
toward the sensing electrode 168. Another reason for the use of
teflon in the construction of the cylindrical 162 is that the
teflon has a very high insulative quality and facilitates the use
of FET 23 to detect extremely low level changes in ionization
potential due to presence of smoke aerosols.
A printed circuit board 165 is mounted about the housing 162 to the
base 160 and includes the components of the electronic circuitry
shown in FIG. 1. The field effect transistor 23 is mounted to the
board 165 and coupled to the common electrode 163 through a hole in
the cylinder 162.
To the base 160 is mounted a cover 166 for protecting the circuitry
and internal components from dust and the like. Warning lamp 50 is
coupled to the output of the circuitry on the printed circuit board
165. The warning lamp 50 is mounted in the cover 166 and gives
visual indication of a fire condition. This is convenient for
checking where a fire or trouble condition exists in a specific
chain of detectors. Over the cover 166 is mounted the sensing
electrode 168 and between the electrode 168 and wind shield 170 is
a screen 169 used to protect the sensing electrode and to permit
the free flow of atmosphere into the sensing chamber 21.
It can be seen from the drawing that the construction of the
sensing unit 10 is simplified and that the reference electrode 161,
sensing electrode 168 and common electrode 163 may be coupled to
the input of the detector circuit mounted on the printed circuit
board 165 very readily. The chambers 21 and 20 are also designed
for ease of manufacture because they are one next to the other
rather than one inside of the other as in other detectors in the
art. It is apparent from the construction of the chambers in the
present invention that assembly time of the unit is greatly reduced
and the efficiency of the device is unaffected.
The system shown in the present disclosure therefore provides for
accurate sensing of a fire condition as well as warning of a
malfunction in any of the units or a break in the transmission line
between the central indication unit and any one of the sensing
units remotely located. The system is provided with a safety
feature for eliminating the condition whereby a false alarm is
produced, and the construction of the sensing means has been
greatly simplified in order to reduce the cost of manufacture and
reduce maintenance cost by making the unit reasonably inexpensive
and readily replaceable.
While there has been shown what is considered to be the preferred
embodiment of the present invention, it will be obvious to those
skilled in the art that certain modifications and changes may be
made without departing from the true spirit and scope of the
invention, and it is intended in the appended claims to cover all
such modifications and changes within the scope of the appended
claims.
* * * * *