Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Smoke detectors of various types such as ionization
and photo-electric are finding increasing use in both residen-
tial and industrial structures. These detectors are responsive
to a predetermined concentration of smoke to provide an output
signal to sound an alarm. For example, one type of detector,
utilizing light reflected from smoke particles, onto a photo-
responsive device, has circuit parameters such that an alarm is
sounded when the smoke concentration reaches a density that
will obscure between 1% and 2% of the light in a column of
smoke 1 foot long.
Smoldering fires often take considerable time to
produce a smoke concentration of 1%, and it would be desirable
to have the alarm actuated at a lesser concentration, for ex-
ample .2% to .4%. However, a smoke detector with such a low
sensitivity is subject to false alarms from various causes,
such as tobacco smoke, cooking smoke, dust, and industrial fumes.
SUMMARY OF THE INVENTION
A smoke detector is provided which is responsive,
during conditions of high ambient light such as daylight or
normal artificial light, to a predetermined smoke concentra-
tion, such as 1%, to provide an output signal to sound an
alarm and which is provided with means for increasing the
sensitivity of th~ device during periods of low ambient light,
such as at nighttime or in un-illuminated industrial spaces,
so that it produces an output signal at a substantially lesser
concentration, for example .2%.
In one embodiment of the invention, the increase in
sensitivity is accomplished photo-electrically; that is, a
photo-responsive device viewing the ambient light is so con-
nected into the detector circuit as to increase and decrease
the sensitivity of the detector with decreases and increases,
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respectively, of ambient light. The photo-responsive device
may be, for example, a photo-resistive cell, a photo-genera-
tive cell, a photo-transistor, etc.
Since the causes of most false alarms are generated
by the activity of people in the space being monitored, and
since in the normal installation when there is no illumination,
either natural or artificial, present, it is unlikely that
there will be any activities being carried on to generate
smoke or dust. ~ence during periods of darkness the sensiti-
vity of the detector can be increased without danger of afalse alarm occurring from such causes. Such increase in
sensitivity at night is particularly desirable since statis-
tics show that 75% of deaths due to smoke occur between 11 P.M.
and 6 A.M.
In an~ther embodiment of the invention, the change in
sensitivity may be accomplished by a switch operated either
manually or automatically.
According to a broad aspect of the present invention,
there is provided a particle detector, in which means is pro-
vided responsive to the presence of particles to produce anoutput signal. The detector has at least two modes of opera-
tion and is responsive in a first mode to a predetermined con-
centration of particles to produce an alarm signal at a parti-
cle concentration substantially less than said predetermined
concentration.
According to a further broad principle of the
invention, a particle detector is responsive to the presence
of particles to provide an output signal, in which means is
provided responsive to increases and decreases in ambient
light level to decrease and increase, respectively, the sen-
sitivity of the detector.
In accordance with a still further broad principle
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of the invention, a smoke detector has means for actuating
signal means at a smoke concentration of a predetermined level
wherl the ambient light is at one intensity, and for actuating
the signal m~ans at a substantially lesser smoke concentration
when the ambient light is at a lower intensity.
In accordance with a particular embodiment of the
invention there is provided, a particle detector, in which
means is provided responsive to the presence of particles to
produce an output signal, said detector having two modes of
operation and being responsive in a first mode to a predeter-
mined concentration of particles to produce an alarm signal,
and being responsive in a second mode to produce an alarm
signal at a particle concentration substantially less than said
predetermined concentration, in which the detector shifts
automatically from one mode to another with a substantial
change in the ambient light level.
From a different aspect and in accordance with the
invention there is provided, a smoke detector, comprising a
light source illuminating a space, a first photo-responsive
device and associated electrical circuitry producing an output
signal in response to the receipt by said photo-responsive
device of light reflected from said smoke particles, means
responsive to an output signal above a predetermined value to
energize alarm actuating means, and a second photo-responsive
cell positioned to view ambient light, said second photo-
responsive device being so connected into said circuitry as
to cause a given smoke concentration to produce a greater output
signal when the ambient light level decreases below a pre-
determined intensity compared to the output signal produced
by said given smoke concentration when the ambient light level
has a substantially higher intensity.
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In accordance with a further embodiment of the
invention there is provided an ionization smoke detector,
comprising a circuit including an ionization chamber having a
pair of electrodes and a radioactive source and connected across
power leads in series through a junction with a loading
element, a gating device having a gate electrode connected to
said junction, said gating device being responsive to an input
signal from said junction of a predetermined value to cause
conduction through said gating device, a pboto-responsive device
viewing ambient light, said photo-responsive device being so
connected into said circuit that when the ambient light level is
low, the signal at said gating device for a given amount of
smoke is substantially higher than when the ambient light level
is high.
In accordance with a still further embodiment of the
invention, a smoke detector comprising detector means responsive
to the presence of smoke to provide an output signal which is a
function of smoke concentration, amplifier means receiving said
output signal and producing an amplified output signal which is
a function of the value of the output signal from the detector
means, level detector means responsive to the amplified output
of a predetermined level to actuate alarm energizing means,
and photo-responsive means so associated with the detector
that when the ambient light falling on said cell decreases to
a predetermined value, the alarm is energized at a lower
smoke concentration than the concentration at which the alarm
is energized under conditions of substantial ambient light.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of the optical compo-
nents of a photo-electric detector of the type with which the
present invention can be utilized.
Fig. 2 is a schematic diagram of an electrical circuit
of a photo-electric detector embodying the features of the
~r invention.
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Fig. 3 is a schema~ic diagram of an electric circuit
of another form of photo-electric detector embodying the
features of the invention.
Fig. 4 is a schematic diagram of an electric circuit
of an ionization smoke detector embodying the features of the
invention.
Fig. 5 is a graph used by Underwriters~ Laboratories
illustrating the maximum and minimum permitted build-up rates ~-
of smoke that can be used in a test of detector response to a
smoldering fire, with percent obscuration per foot plotted
against time.
Fig. 6 is a schematic diagram of a modified form of
the electrical circuit of Fig. 2 in which the photo-resistive
cell Cv has been replaced by a switch Sl, which may be con-
trolled by a timer T.
DESCRIPTION OF THE ILLUS~RATED EMBODIMENTS
Fig. 1 of the drawing is a schematic diagram of the
arrangement of the physical components of a photo-electric
smoke detector with which the present invention can be utilized,
comprising a support block 10, carrying a light source L posi-
tioned to illuminate smoke particles S appearing in the space
in front of the block, and a photo-responsive device C viewing
the volume illuminated by the light. The light source L may
be of incandescent, glow tube, or electronic (light emitting
diode) origin, and the photo-responsive device may be photo-
resistive or photo generative, as desired for a particular cir-
cuit. The light may be pulsing or continuous.
The block 10 and the associated components may be
enclosed in a suitable housing to allow the entrance of ambient
atmosphere but to exclude ambient light. Housings for this
purpose are well known in the art.
Referring to Fig. 2, there is illustrated a schematic
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diagram of an electrical circuit of a smoke detector embodying
the lnvention, comprising a light source Ll, a photo-resistive
cell Cl connected in series across a power source with a re-
sistor Rl through a junction Jl, which junction provides the
input to an amplifier Al, the output of which is connected to
the gate of a controller switch SCR, the anode and cathode of
which are connected in series with an alarm device Al across
the power source. The presence of smoke in the viewing area
of the cell Cl illuminated by the light Ll cause Cl to drop in
resistance raising the voltage at Jl. When the Jl voltage
reaches a predetermined value the output of the amplifier be-
comes high enough to trigger SCR into conduction energizing
the alarm Al.
The above-described portion of the circuit is in
common U90 in several types of smoke detectors.
To render said circuit automatically responsive to
a decrease in ambient light to increase its sensitivity, a
photo-resistor cell Cv is connected in parallel with resistor
Rl and in series with limiting resistor R2, said cell being
physically positioned to view ambient light in the vicinity of
the detector.
During daylight hours, or when the space being moni-
tored by the detector is artificially illuminated, the resis-
tance of the cell Cv is low. The value of R] and R2 and the
circuit parameters of the amplifier Al are such that a pre-
determined smoke concentration (such as 2%) is required to
lower the resistance of Cl to a value such that the voltage at
Jl rises to a value that will actuate the alarm.
However, as the intensity of the ambient light de-
creases, the resistance of cell Cv increases, thereby increas-
ing the resistance between ~1 and ground. Hence the resistance
drop of cell Cl required to produce the required alarm voltage
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at Jl is less, and said resistance drop is produced by a lesser
smoke concentration. In total darkness, the resistance of cell
Cv is substantially infinite, so that the voltage at J2 is
determined only by the relative resistance of Cl and Rl.
Referring to Fig. 3 there is illustrated another form
of electronic smoke detector circuit embodying the features of
the invention, which comprises a light emitting diode light
source L2, a photo-voltaic cell C2 capacitor coupled to an am-
plifier A2. The amplifier output is connected to the input
of a level detector LD, the output of which is connected to
the set terminal of a bi-stable switching device such as a
flip-flop FF. The flip-flop output is fed through an integra-
tor T to an alarm actuating device K.
A pulse generator P supplies pulses to energize the
light emitting diode L2, to turn on the level detector LD, to
re-set the flip flop FF and to the normally closed electronic
switch Sl, connected from the signal lead to ground, to open
said switch during the time that L2 is emitting light and the
level detector is energized.
Light reflected from smoke particles illuminated by
L2 pulse falls on the photo-voltaic cell C2 causing a voltage
~ulse which is amplified by the amplifier A2, the output of
which, if of sufficient magnitude, passes through the level
detector to the set terminal of the flip-flop to provide an
output signal to the integrator, which signal continues until
the beginning of the next pulse. The discriminator D con-
verts the pulse from the pulse generator into a spike pulse
to the flip-flop re-set terminal at the beginning of each pulse
cycle, so that at the start of each pulse, the flip-flop out-
put is turned off. Two or more consecutive output signals into
the integrator may be required to actuate the alarm.
The above-described smoke detector circuit is des-
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cribed and claimed in my U.S. Patent 3,946,241 issued May 23,
1976, and as shown therein, provides means for making minor
adju~;tments in the sensitivity of the device, to compensate
for variations in the electrical characteristics of the various
components, said means comprising voltage divider resistors
Rl and R2 connected across the power source, with their junc-
tion connected to one of the inputs to the level detector, and
a variable resistor R3 connected across the power source, with
the tap thereof connected to the other input to the level
detector. The sensitivity adjustment provided by resistors
R3 is accomplished during the manufacturing process and is not
thereafter adjusted, except possibly during repair of the unit.
To enable the sensitivity of the circuit to increase
automatically when the ambient light decreases, a photo-resi~~
tive cell Cv, which is positioned to observe ambient light, is
connected in series with a resistor R4 between one of the am-
plifier output terminals and ground.
During periods of high ambient light, such as during
daylight or when artificial illumination is being used, the
cell Cv is at a low resistance, so that the resistance from
the signal lead to ground is substantially that of the resis-
tor R4.
Therefore during periods of high ambient light, a
portion of the amplifier A2 output flows to ground, and the
circuit parameters are such that when the concentration of
smoke (in high ambient light conditions) reaches the level at
which the alarm is to be sounded, the amplifier output is high
enough to cause the level detector to provide an output signal
to the flip-flop.
However, under conditions of darkness, the resistance
of cell Cv increases so that the resistance of the path to
ground through R4 and Cv is substantially infinite. A lesser
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output from amplifier A2 is therefore required to provide the
signal level required to produce an output signal from the
level detector, and since this smaller amplifier output can be
produced by a lesser concentration of smoke viewed by the
cell C2, the detector is more sensitive under conditions of
low ambient light.
Referring to Fig. 4 of the drawing, there is illus-
trated a smoke detector of the ionization type embodying the
features of the invention, comprising an open ionization
chamber Zl and a closed ionization chamber z2 connected in
series through a junction J2 across a power source, said junc-
tion being connected to the gate of a field effect transistor
FET, the source-drain path of which is connected across the
power source. The output signal from the field effect trans-
istor is taken across the resistance between the source elec-
trode S and the negative supply terminal, which signal is
applied to the gate of a silicon-controlled rectifier SCR, the
anode-cathode path of which is connected in series with an
alarm K across the power source.
The power source provides a constant ionization cur-
rent through both ionization chambers. When there is no smoke
present in the ambient air, the-voltage at junction J2 has a
predetermined value.
Under these conditions the current through the
source-drain path of the field effect transistor is either
zero or has a value such that the voltage drop across the load
resistance (between S and the negative supply terminal) does
not exceed the breakdown voltage of the zener diode ZD. Hence
no signal is applied to the gate of SCR.
When smoke enters the open ionization chamber Zl,
its impedence is increased and ionization current will de-
crease, which causes the voltage at junction J2 to increase,
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which increases the potential at gate electrode G of the FET.
The current in the source-drain path of the FET therefore in~
creases, which increases the voltage across load resistors
RLl and RL2. If the amount of smoke is sufficient to produce
the required voltage between the source electrode S and the
negative supply, the SCR will be triggered into conduction,
actuating the alarm K.
The above-described portion of the circuit is a
standard circuit used in commercial ionization detectors.
To render said circuit automatically responsive to a
decrease in ambient light to make the detector more sensitive,
a photo-resistive cell Cv is connected in parallel with load
resistor RL2.
The cell Cv is positioned to be exposed to ambient
light, so that during daylight hours, or when the space being
monitored i9 artificially illuminated, its resistance is low,
and during the period in which the space being monitored is
dark, its resistance is high.
Therefore, during periods of high ambient light more
current is required through the FET source-drain path to pro-
- vide the necessary SCR trigger voltage, than is required dur-
ing periods of darkness, since the resistance from J2 to the
negative side of the power line is greater during low ambient
light than during high ambient light.
The amount of smoke at ionization chamber Zl neces-
sary to provide the SCR trigger voltage is therefore less when
the ambient light is low than when the ambient light is high.
In each of the above-described modifications of the
invention, the range of sensitivity of the detector is main-
tained between predetermined upper and lower limits by theassociated circuitry, so that even if the resistance of the
cell varies from zero to infinity, the sensitivity of the de-
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vice will not go above or below said predetermined limits.
For example in Fig. 2, the maximum sensitivity, occurring
when the photo-cell Cv is dark, cannot exceed a value deter-
mined by the resistor Rl, and the minimum sensitivity, occurr-
ing when the cell Cv is exposed to ambient light and therefore
at a low resistance, cannot go below a value determined by
resistor R2.
Referring to Fig. 5, there is illustrated a graph
illustrating maximum and minimum permitted rates of smoke
build-up permitted in an Underwriters' Laboratory standard
smoldering fire test. With the illustrated rates of build-
up, a detector set to alarm at a smoke density of 2.0% obscura-
tion per foot will alarm, at the maximum rate build-up in
about 6 minutes, and at the minimum build-up rate in about 20
minutes.
Thus the increase in sensitivity from a 2% alarm
point to a ,2% alarm point gives ah alarm 27 minutes earlier
at the maximum permitted test rate and 34 minutes earlier at
the minimum permitted test rate.
As previously stated, 75% of deaths from residence
fires occur at night, and it has also been determined that 75%
of residence fires are of the smoldering type. Hence the
shortened alarm time is obviously very desirable.
Many modifications of the herein-described embodi-
ments of the invention may be made, since it will be apparent
to one skilled in the art that the photo-resistive device Cv
can be incorporated into the illustrated circuits in various
ways, with the same result.
Although the above-described embodiments of the in-
vention utilize a photo-resistive cell as the sensitivity con-
trol responding to ambient light, other photo-responsive de-
vices may, with suitable changes in circuitry, be used, such
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as photo-voltaic devices, photo-diodes, photo-transistors, etc.
Referring to Fig. 6, there is illustrated another
embodiment of the invention, which is a modified form of the
embodiment illustrated in Fig. 2, in which a switch Sl is
included in the circuit in series with resistor R2, replacing
the photo-resistive device Cv. The other components of the
circuit are identical to those in the embodiment of Fig. 2.
The switch Sl may be mechanical or electronic, and may be con-
trolled by a timer T.
When it is desired that the detector have a low sen-
sitivity such as when the space being monitored by the detec-
tor is occupied, the switch Sl is closed. However, w~en the
space is unoccupied the switch Sl is closed (manually or by
the time T), increasing the sensitivity of the detector in the
manner described in connection with the embodiment of Fig. 2.
In like manner, the photo-resistive device Cv of
the embodiments of Figs. 3 and 4 may be replaced by mechanical
or electronic switches, operable in any desired manner.
Although in the above-described embodiments of the
invention, the switch is so connected into the circuit that
opening the switch increases the sensitivity of the detector,
it will be understood that if desired the switch could be
connected so as to operate in the reverse manner, in which
the detector would be in the more sensitive condition when
the switch is closed.
Since other modifications in the above-illustrated
~and described embodiments of the invention may be made by one
skilled in the art without departing from the scope of the
invention, it is intended that all matter disclosed herein be
interpreted in an illustrative and not a limiting sence.
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