Note: Descriptions are shown in the official language in which they were submitted.
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Smoke detectors that utilize various electronic means
' to provide an alarm signal when ambient smoke concentration reaches
a predetermined level are widely used.
One disadvantage of such detectors is the fact that
although it' might be desirable for them to provide an alarm at
a smoke concentration as low as .2%, as a practical matter in the
manufacture of commercial detectors it is necessary that they be
- calibrated so that a smoke concentration of at least 2% is re-
quired before the ala'rm will be actuated, to avoid false alarms
from ambient smoke from non-fire sources such as tobacco smoke,
severe dust conditions, cooking smoke, etc Hence a smoldering
fire in a situation where there is some air circul'ation so that
'; the smoke concentration never reaches 2% may never be detected
by the device.
Another disadvantage of such detectors that utilize a
photo-resistive device as the detecting eleme~t is the fact'that
' the response time of the photo-resistive device, when suddenly
exposed to light reflected from smoke particles, is a function
of the intensity of the light falling thereon, since the response
' 20 of such devices is analogous to the charging time of a capacitor,
' which is a function of the applied voltage.
At a low smoke Concentration, which provides a low re~
flected light intensity, an un~uly long time will be required for
i the cell to reach a predetermined response level required to act-
,~ uate the alarm. Also, a high smoke concentration that appears
`~ suddenly, such as may result when smoldering combustible material
.J' bursts into flame, may pass the detector so suddenly that the
smoke concentration drops so rapidly that the reflected light
1 level drops below the intensity required for alarm before the photo-
`~ 30 responsive device has reached the alarm condition.
~ This situation is particularly troublesome in detectors
`! in which light emitting diodes are used as the lig ~ ource, since ~ ~
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the light output of such devices, when operated at a current levellow enough to ensure a long service life, is much lower, perhaps
by a factor of 20, than the light output of an incandescent bulb.
In a detector with an LED light source and a photo-resistive `
cell calibrated to provide an alarm at a continuous smoke level
of 2% (defined as the amount of~smoke necessary to reduce the
light intensity 2% in a column 1 foot long), a smoke concentra-
tion of just over 2% may require 10 minutes or more for the resist-
ance of the photo-resistive cell t,o drop to the alarm point.
A smoke detector in which means is provided for detect-
ing the rate of change of smoke concentration, and for Froducing
a signal when the rate of change exceeds a predetermined value.
In one embodiment of the invention the signal may act-
uate an alarm device.
In other embodiments of the invention, the rate detect-
ing means may be incorporated into a smoke detector of the type
which provides an alarm when the smoke concentratlon reaches a
predetermined level, and the signal from the rate detecting means
is utilized to increase the sensitivity of the smoke detector,
so that it is capable of providing an alarm at a smoke concentra-
tion substantially less than the predetermined level. In one
modification of this embodiment of the invention, in which a
photo-responsive device views smoke particles illuminated by a
light source, the signal from the rate detecting means is utilized
to cause an increase in the intensity of the light source.
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~he invention will now be described with reference to
the accompanying drawings which show a preferred form thereof and
wherein:
Figure 1 is a view in side elevation, partly in sec-
tion of a portion of a smoke detector
assembly of a type for which the invention
is designed for use.
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Figure 2 is a schematic representation of an
electronic circuit embodying the feat-
ures of the invention, in which the rate
circuit is connected so as to actuate
the alarm directly when the rate of change
of reslstance of the photo-cell exceeds
a predetermined value.
Figure 3 is a view in side elevation, partly in
section of a portion of another type of
smoke detector for which the invention is
designed for use.
Figure 4 is a schematic representation of an elec-
tronic circuit embodying the features of
the invention, in which the rate circuit
is connected so as to increase the out-
put of the light source when the rate of
; change of resistance of the photo-cell
exceeds a predetermined value.
Figure S is a modified form of the circuit of
Figure 4, in which timing means is actu-
ated by the rate circuit to maintain thè
increased light output for a predeter-
mined period of time.
Figure 6 is a schematic representation of a modi-
, fied form of smoke detector circuit, in
which the rate circuit is so connected as
to change the smoke cell calibrating resist-
or when the rate of change of resistance of
the cell exceeds a predeterrnined value.
Figure 7 is a schematic representation of an elec-
tronic circuit in which the rate circuit
is used in a smoke detector of the type
having a photo-voltaic cell as the detect-
or element.
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Figure 8 is a schematic representation of an elec-
tronic circuit in which the rate circuit
is used ln a smoke detector of the type
utilizing an ionization chamber as the
detector element.
Figure 9 is a schematic representation of an elec-
tronic circuit similar to Figure 8, in
which the rate circuit is utilized to
increase the sensitivity of the ionization
chamber.
Referring to the drawing, there is illustrated in Fig-
ure l, a schematic representation of a smoke detector of the type
utilizing a photo-resistive device to respond -to light reflected
from smoke particles in a light beam.
The detector includes a support block 10 which may be
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- provided with a pair of apertures 12 and 14 which extend from
the ends of the block and open to the forward face 16 thereof in
spaced relation.
A light source 18 is disposed rearwardly to aperture 12
to cooperate with lens 20 in the aperture to project a beam of
light from the forward end of the aperture. A photo-resistive
cell Cl and suitable focusing lens 22 is disposed in aperture 14.
The apertures 12 and 14 may be disposed at an angle of about 135
to take advantage of the "forward scatter" effect.
, Referring now to Figure 2, there is illustrated an
electronic circuit for use with the smoke detector components of
Figure 1.
I The smoke detector cell Cl is connected across a suit-
-~ able power source P, which may be regulated by suitable means Rv, ~ -
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said cell Cl being in series with a resistor Rl through a junc-
tion Jl. The junction Jl is connected to the input of an ampli-
fier Al, the output of which is connected to the control elec- ~ -
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trode of an SCR, the anode-cathode circuit of which is in series
with an alarm device A. The light source 18 is also connected
across the power source.
The above-described portion of the circuit of Figure 2
is a typical smoke detector circuit well known in the art and
operates in the following manner. When smoke appears in the light
beam, light from the light source 18 reflects therefrom onto the
cell Cl causes the cell, which is normally dark, to drop in re-
sistance, raising the voltage at the junction Jl, which voltage
is amplified by Al. When the resistance of cell Cl drops to a
predetermined value, the output of amplifier A1 is sufficient to
fire SCR and energize the alarm.
However, the above-described portion of the circuit
will actuate the alarm only if the smoke level reaches the pre-
determined level, for example, 2%.
To enable the detector to respond to lower smoke con-
centrations without increasing the possibility of false alarms,
and to increase the speed of response of the detector, a rate
~ detecting circuit is connected in parallel with the amplifier Al
: 20 between the junction Jl and the SCR trigger.
' The rate circuit is designed to provide an output sig-
nal to the trigger of SCR when the rate of increase of voltage
at junction Jl e~ceeds a predetermined ~alue. Circuits that de-
tect and respond to the rate of change of a voltage are well
known in the art. In the illustrated em~odiment the circuit com-
prises an electronic differentiator A2 whose output voltage to
, the SCR is proportional to the rate of increase (differential)
of the input voltage from Jl. If the rate of increase of voltage
at junction Jl is below a predetermined value, t~e output of the
electronic differentiator is insufficient to trigger SCR into
' conduction. In a typical embodiment of the invention the elec-
: trical parameters of A2 are such that a smoke concentration in-
creasing at a rate of less than .05% per minute is ignored, but
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smoke concentration increasiny at a faster rate will provide
a voltage ramp at Jl having a slope or differential great enough
that the output of A2, which i9 proportional to the slope or
differential of the input voltage, is high enough to trigger SCR
into conduction and actuate the alarm.
The frequency response of the amplifier is such that
it is capable of amplifying frequencies much higher than would
ever result from sudden puffs of smoke. However, such high fre-
quency response is undesirable in that it would allow 60 cycle
voltage and voltage transients to trigger the alarm. For this
reason the frequency response of the amplifier is intentionally
reduced by the use of a suitable RC network with values so as to
cause the frequency response of the amplifier to roll off above
about 30 cycles/second.
' Referring now to Figure 3, there is illustrated a view
in side elevation, partly in section of a portion of a smoke de-
; tector assembly similar to that shown in Figure 1, with the ad-
dition of means to be described for regulating the intensity of
the light source. In addition to the components illustrated in
Figure 1, the device of Figure 3 includes a photo-resistive cell
C3 mounted in the support block 10, and a light pipe, such as an
- acrylic rod 24, extends from the cell upwardly into the light
beam to conduct light therefrom onto ~he surface of the photo-cell.
~n adjusting screw 26 is provided to vary the amount of light re-
`1 ceived by the cell C3.
,l Referring now to Figure 4, there is illustrated anotherform of smoke detector utilizing a rate detecting circuit.
, The circuit of Figure 4 and the assembly of Figure
3 are similar to that disclosed and claimed in United States
patent 4,011,458, issued March 8, 1977, William J. Milnowski,
one of the co-inventors of this application, and comprises a
smoke detector cell Cl connected in series with a resistor Rl
across a suitable power source P. The junction
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Jl between the cell Cl and the resistor Rl develops a voltage
which is a functlon of the resistance of the cell Cl, which vol-
tage is applied to the input of a differential amplifier Al. The
output of Al is applied to the trigger electrode of an SCR.
The light intensity regulating cell C3 i9 also connect-
ed across the power source in series with resistor R2 through
a junction J2, which is connected to the input of a differential
amplifier A2 q'he output of A2 is connected to the base of
transistor Tl, the emitter-collector path of which is in series
with the light source 18 across the power source P.
The operation of the circuit of Figure ~ is fully des-
cribed in the above identified U.S. patent and may be
summarized as follows. An increase in light output of 1~ (such as
might be caused by line voltage variations), will reduce the re-
sistance of C3, increasing the voltage at Jl and at the amplifier -~
input, (which is an inverting amplifier) which reduces the bias on
the transistor Tl to reduce the current through the light 18 and
thereby reduce in light output.
As illustrated in Figure 4, a rate detecting electronic
differentiator A3 is provided, with its input connected to Jl and
its output connected to J2 through a diode Dl.
The rate circuit operation is similar to that of Fi-
gure 2 in that it~ ouput i~ a function of the rate of change of
the voltage at junction Jl. However, in the modification of Fi-
gure 4, the electronic differentiator A3 include~ an inverter, so
that the output thereof decreases with increasing rate of change
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of input voltage. As de~cribed in the above-identified U.S.
patent, amplifier A2 maintains the voltage at J2 at 1/2 of the sup-
~3 ply voltage. In a typical ca~e the supply voltage i5 5 volts, 90
that the voltage at J2 is 2.5 volt~. Amplifier A3 iY also de-
signed to have an output of 1/2 of the supply voltage when the
input is negligible. When the voltage at Jl increases as a result
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1~67597
of light reflected from smoke particles onto cell Cl, the output
voltage of A3 drops in proportion to the rate of increase of the
Jl voltage If the rate of voltage increase at Jl is great enough,
the output voltage of A3 drops to a value such that current flows
from J2 through the diode Dl and the amplifier A3 to ground. This
additional circuit-path drops the voltage at J2 which increases
the bias on transistor Tl, increasing the output of light 18. The
increased light output reflected from the smoke onto cell Cl caus-
es a further drop in voltage at Jl, so that with sufficient ini-
tial rate of increase of voltage at Jl, the output voltage of A3drops substantially to ground.
If the detector is calibrated to provide an alarm
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through amplifier Al when the voltage at junction Jl corresponds
to a smoke concentration of 2% (which, in the illustrated embodi-
ment i9 one half of supply voltage), the increase in output of the
light source to the maximum as a result of the response of the
` rate circuit as above described, converts the detector from a 2%
detector to a detector of much greater sensitivity. In a parti-
cular embodiment of the invention the light output may be in-
creased by the rate circuit so that the detector will become, at
maximum light output, a,2% detector.
The rate circuit operates so rapidly in response to a
~` voltage increase at ~1 due to smoke entering the detector that
seldom, if ever, does the detector contain sufficient smoke to go
into alarm when the light output is increased, however, assuming
that the amount of smoke in the housing continues to increase at
a rate fast enough to maintain the rate circuit in operation, the
alarm will be actuated by amplifier Al when the voltage at Jl
, reaches 1/2 of the supply voltage which voltage is provided by
.2% smoke.
However, in the event that rate of increase of smoke
concentration, after the initial plume, is not sufficient to
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~67597
maintain the rate circuit in operation although smoke remains in
the ambient atmosphere, but at a level of lees than 2%, the de-
tector may not go into alarm since thelight output reduces to its
original intensity as the rate of increase of voltage at Jl de-
creases.
To insure that the alarm will be actuated in such cir-
cumstances, there is illustrated in Figure 5 a schematic diagram
of a modified form of the circuit of Figure 4, in which a timing
device T is inserted between the output of amplifier A3 and the
diode Dl.
The device T, which may be a one shot mono-stable multi-
vibrator is converted to a conductive state by an output of pre- ~
determined level from the amplifier A3 and remains in the conduct-
ing state for a predetermined period of time after the output from
the amplifier A3 has dropped, so that the light will remain at the
higher output for a short perio~ of time, after the rate of in-
crease of smoke concentration has dropped, so that the detector
will remain at the higher sensitivity long enough for the smoke
concentration to reach 2% and for the cell to respond thereto.
Referring to Figure 6 of the drawing, there is shown a
portion of the circuit of Figure 4 illustrating another method of
utilizing a rate circuit of the type described, wherein the rate
circuit A3 actuates a switch Sl which is connected in series with
a resistor Rs between the junction Jl and ground. The resistor
Rs and resistor Rl have values such that their combined resistance
in parallel is equal to the resistance of cell Cl when 2% smoke
is in the detector.
When the rate of voltage increase at Jl exceeds a pre-
determined value, the rate amplifier A3 output opens switch Sl,
thereby raising the voltage at Jl and in effect increasing the
s~ensitivity of the detector.
As in the embodiment of Figure 5, atimer T may be pro-
vided to hold the switch Sl open for a predetermined time, so the
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detector will remain in the sensitive condition for a time suffi-
cient to allow the smoke to build up in the housing, and to give
the photo-responsive device time to react to the light reflected
- from the smoke.
Although the invention has been described as it is
applied to smoke detection utilizing photo-resistive cells, it
may also be used with detectors utilizing photo-generative cells,
- photo-transistors or photo-diodes. The response of such devices
is much faster than that of photo-resistive cells, and hence the
rate detecting circult is ordinarily not necessary to compensate
for slow response time, however, the use of a rate circuit with
such devices allows the creation of a very sensitive detector, ;~
responsive, for example, to ,2% smoke without substantial false
alarm problems, with a back-up response at 2% smoke which operates
regardless of the rate of rise of smoke concentration.
~he circùit for such a device utilizing a photo-tran-
sistor or a photo-diode would be similar to that of Figure 1, with
the photo-transistor or photo-diode substituted for the cell Cl,
with the values of other components of the circuit being adjusted
to suit the electric characteristics of the photo-responsive de-
vice being used.
The principle of the invention may be utilized with a
detector utilizing a photo-generative device in the manner shown
in Figure 7, wherein the voltage generated by the photo-generating
cell Cg in response to Iight ~rom source L reflected therein
from smo~e particles is applied to a rate detecting circuit A2 of
the type previously described, which actuates alarm I~ when the
rate of change at junction Jl exceeds a predetermined value. A
back-up amplifier Al may also be provided to actuate the alarm
, 30 when the voltage at junction Jl reaches a value corresponding to
2% smoke. Although in the embodiment of Figure 7, the rate cir-
cuit actuates the alarm directly, (as in the embodiment of Figure
' 2) it will be apparent that if desired, the rate circuit could be
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utilized to increase the output of the light source, as in the
embodiment of Figure 4, and a timing device such as is illustrated
in Figure 5 could be included in the output of amplifier A2
The principle of the invention may also be utilized in
detectors operating on the ionization principle as illustrated
in Figure 8. Such detectors utilize the principle that the cur-
rent flow through the ionization chamber decreases when smoke or
combustion aerosols enter the chamber.
In Figure 8, the detector comprises an ionization cham-
ber lC containing two spaced electrodes El and E2 and a radio-
active substance R. The electrodes El and E2 are connected in
series with a resistor Rl across a power source P through a junc-
tion Jl, which is connected to the input of an electronic dif-
ferentiator Al, the output of which is connected to to the gate
of an SCR. The junction Jl is also connected to the gate of a
field effect transistor, the source-drain path of which is in
series with a resistor R2 through junction J2. Junction J2 is
also connected to the gate of the SCR
As smoke enters chamber IC, the current between the
electrodes decreases so the voltage at junction Jl rises. If
the rate of rise of the voltage at junction Jl exceeds a pre-
determined value, the output of electronic differentiator ~1
rises to a value sufficient to trigger SCR into conduction, ac-
tuating alarm K. If the rate of increase of smoke is not fast
enough to provide sufficient output from Al to actuate the alarm,
then the alarm will be actuated when the smoke corlcentration
reaches a predetermined level, by conduction ~hrough the source-
drain path of the FET raising the voltage at J2 to a value suf-
ficient to trigger the SCR into conduction.
Referring now to Figure 9 of the drawing, there is
illustrated a modified form of the circuit of Figure 8 in which
the source-drain path of the FET is connected in serles with a
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pair of resistors R2 and R3. Junction J2 between the resistors
is connected to the gate of the SCR.
An electronic switch Sl, which is normally open, is
connected in parallel with resistor R3 and the switch Sl is op-
erated by the output of the electronic differentiator Al,
The value of resistors R2 and R3 may be selected so that
if the voltage at junction Jl increases showly due to slowly in-
creasing smoke concentration in the chamber IC, the current
through thé source-drain path of the FET increases slowly, in-
creasing the voltage at junction J2 which eventually reaches a
value that will cause conduction in the SCR to actuate the alarm.
The combined resistance value of R2 and R3 in series
may be such that the SCR conducts when 2% smoke is in chamber IC.
However, when the rate of increase of smoke concentra-
tion in the chamber is great enough, the output of electronic
differentiator Al closes switch Sl, cutting R3 out of the circuit,
'~ and thereby increasing the voltage at J2, The value of resistor
R2 may be selected so that with resistor R3 shorted out of the
circuit the voltage at J2 will be sufficient to cause the SCR to
conduct when only .2% smoke is in the housing. As in previous
embodiments of the invention, a timer T may be incorporated into
,
the circuit to maintain the switch in the open conditi.on for a
predetermined time after it has been opened by th~ electronic
differentiator Al.
The above-described embodiments of the invention have
` the advantage of providing a smoke detector which normally op-
', erates at a standard sensitivity of 2% smoke, yet converts to a
,i high sensitivity condition when a predetermined rate of increase
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;~ of smoke concentration is detected. The detector is thereby
substantially immune to false alarms from relatively high levels
of ambient smoke from non-fire sources yet is almost instantly
converted to the sensitive mode of operation by a sudden
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increase in smoke concentration at a rate whlch is unlikely to
result from any source other than an incipient fire,
Although the above embodiments of the invention all
utilize the back-up means for actuating the alarm at the pre-
determined smoke concentrations, it will be understood that in
some situations only the rate-responsive means for actuating
the alarm may be provided
Since certain changes obvious to one skilled in the art
may be made in the herein described embodiments of the invention
without departing from the scope thereof, it is intended that all
matter contained herein be interpreted in an illustrative and not
a limiting sense.
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