CA1067175A - Smoke detector - Google Patents
Smoke detectorInfo
- Publication number
- CA1067175A CA1067175A CA295,627A CA295627A CA1067175A CA 1067175 A CA1067175 A CA 1067175A CA 295627 A CA295627 A CA 295627A CA 1067175 A CA1067175 A CA 1067175A
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- Canada
- Prior art keywords
- light
- amplifier
- light source
- voltage
- pulses
- Prior art date
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- Fire-Detection Mechanisms (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
This invention relates to a smoke detector of the type utilizing photo-electric detection of light reflected from smoke particles. The smoke detector includes a light source which is energized intermittently, and a photo-voltaic cell in the path of light of the light source which provides energy pulses in response to the light pulses reflected from smoke particles. A
level detector detects the energy pulses and turns on an alarm actuator which actuates an alarm when smoke is detected. In accordance with the invention, the detector includes a voltage supply for driving the photo-voltaic cell, a capacitor connected across the voltage supply, and an isolating circuit between the capacitor and the voltage supply for isolating voltage pulses occurring at the power supply at least during the time that the light source is emitting light.
This invention relates to a smoke detector of the type utilizing photo-electric detection of light reflected from smoke particles. The smoke detector includes a light source which is energized intermittently, and a photo-voltaic cell in the path of light of the light source which provides energy pulses in response to the light pulses reflected from smoke particles. A
level detector detects the energy pulses and turns on an alarm actuator which actuates an alarm when smoke is detected. In accordance with the invention, the detector includes a voltage supply for driving the photo-voltaic cell, a capacitor connected across the voltage supply, and an isolating circuit between the capacitor and the voltage supply for isolating voltage pulses occurring at the power supply at least during the time that the light source is emitting light.
Description
1067175 `~ `"
This invention relates to a smoke detector.
Many forms of smoke detectors are known that utilize the so-called Tyndall effect, in which light reflected from smoke -particles is detected and the resulting signal amplified to actuate an alarm. Most commercial units utilize a continuously `~
operating incandescent lamp as the light source. Such a detectcr -~
that has achieved great commercial success is disclosed in U, S.
patent 3,382,762 issued January 25, 1966. Smoke detectors based `
on this principle have the disadvantage of high current consump-tion and susceptibility to false alarms due to changing levels of ambient light and changes in line voltage. Hence such devices ^
must be enclosed in a housing that allows diffusion of air into ~
:
the housing without allowing ambient light to enter, the electric-al circuitry must provide means for compensating for changes in line voltage, and the photo-electric detectors must have a high , degree of uniformity and stability. Meeting these requirements adds cGnsiderably to the cost of the device.
- To avoid some of the above disadvantages it has been ; proposed to utilize a flashing light source, such as a gas filled , 20 tube, to reduce the current consumption. It has also been pro-posed to modulate the pulsed light at a predetermined frequency and provide an amplifier that responds only to said frequency.
Such a system is illustrated in U. S. patent 3,316,410 issued April 25, 1967. It has also been proposed that the means amplifying the signal from the light sensitive element should be ;
i~ operative only while the light source is on, so that ambient light changes or electrical disturbances that occur during the period ! the amplifier is off cannot cause a false alarm. However, in such a system, ambient light changes and electrical disturbances that occur while the amplifier is on can nevertheless cause a false alarm. Examples of ambient light changes that can affect ` a detector of this type are flashlights, strong sunlight, turning .''' - ~`' .. -- 1 --' ,:
on of room lights, camera flash bulbs, and lightning. Hence the use of a pulsed light source and a pulsed amplifier as shown in the prior art, although having the advantage of lower power consumption, does little to reduce the possibility of false alarms, and hence to avoid false alarms from such causes the sensitivity of the device must be reduced.
It is often desired for certain installations that such devices be powered from a battery, however one difficulty with a battery power source results from the fact that when the light source is turned on, the internal impedence of the battery causes the voltage at the terminals to drop. If the amplifier lS powered from the same battery, the voltage to the amplifier also drops, ~`
which creates a transient signal in the amplifier which may pro~
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' duce an output signal many times greater than the output signal ; produced by the photo-responsive device when smoke is present, and it is difficult or impossible to separate the two signals.
' The same problem may occur when a plurality of smoke detectors are connected to a loop from a common power source at a central control panel. Since detectors of this type draw very ' 20 little current, one of their great advantages is the fact that `
small wire can be used for connecting them to the central control panel. However since the instantaneous current, on the energiz- 7 -; ation of the light emitting device, may be as high as 7 amperes, . .~
a substantial voltage drop at the terminals of the smoke detector can occur, which would produce a false signal in the amplifier.
Various means have been used to prevent such transient changes in the power supply voltage. For example, it is possible to power the amplifier and the light emitting device from separate batteries. It has also been proposed that the amplifier should be normally off, and turned on only after the light emitting device is turned on. In a loop system powered from a central control panel, it is possible to run separate power supply wires
This invention relates to a smoke detector.
Many forms of smoke detectors are known that utilize the so-called Tyndall effect, in which light reflected from smoke -particles is detected and the resulting signal amplified to actuate an alarm. Most commercial units utilize a continuously `~
operating incandescent lamp as the light source. Such a detectcr -~
that has achieved great commercial success is disclosed in U, S.
patent 3,382,762 issued January 25, 1966. Smoke detectors based `
on this principle have the disadvantage of high current consump-tion and susceptibility to false alarms due to changing levels of ambient light and changes in line voltage. Hence such devices ^
must be enclosed in a housing that allows diffusion of air into ~
:
the housing without allowing ambient light to enter, the electric-al circuitry must provide means for compensating for changes in line voltage, and the photo-electric detectors must have a high , degree of uniformity and stability. Meeting these requirements adds cGnsiderably to the cost of the device.
- To avoid some of the above disadvantages it has been ; proposed to utilize a flashing light source, such as a gas filled , 20 tube, to reduce the current consumption. It has also been pro-posed to modulate the pulsed light at a predetermined frequency and provide an amplifier that responds only to said frequency.
Such a system is illustrated in U. S. patent 3,316,410 issued April 25, 1967. It has also been proposed that the means amplifying the signal from the light sensitive element should be ;
i~ operative only while the light source is on, so that ambient light changes or electrical disturbances that occur during the period ! the amplifier is off cannot cause a false alarm. However, in such a system, ambient light changes and electrical disturbances that occur while the amplifier is on can nevertheless cause a false alarm. Examples of ambient light changes that can affect ` a detector of this type are flashlights, strong sunlight, turning .''' - ~`' .. -- 1 --' ,:
on of room lights, camera flash bulbs, and lightning. Hence the use of a pulsed light source and a pulsed amplifier as shown in the prior art, although having the advantage of lower power consumption, does little to reduce the possibility of false alarms, and hence to avoid false alarms from such causes the sensitivity of the device must be reduced.
It is often desired for certain installations that such devices be powered from a battery, however one difficulty with a battery power source results from the fact that when the light source is turned on, the internal impedence of the battery causes the voltage at the terminals to drop. If the amplifier lS powered from the same battery, the voltage to the amplifier also drops, ~`
which creates a transient signal in the amplifier which may pro~
.~ .
' duce an output signal many times greater than the output signal ; produced by the photo-responsive device when smoke is present, and it is difficult or impossible to separate the two signals.
' The same problem may occur when a plurality of smoke detectors are connected to a loop from a common power source at a central control panel. Since detectors of this type draw very ' 20 little current, one of their great advantages is the fact that `
small wire can be used for connecting them to the central control panel. However since the instantaneous current, on the energiz- 7 -; ation of the light emitting device, may be as high as 7 amperes, . .~
a substantial voltage drop at the terminals of the smoke detector can occur, which would produce a false signal in the amplifier.
Various means have been used to prevent such transient changes in the power supply voltage. For example, it is possible to power the amplifier and the light emitting device from separate batteries. It has also been proposed that the amplifier should be normally off, and turned on only after the light emitting device is turned on. In a loop system powered from a central control panel, it is possible to run separate power supply wires
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for the light emitting devices and the amplifiers. All of these :
expedients involve additional expense that is not acceptable in . .
the majority of smoke detector applications.
The smoke detector disclosed herein comprises a light emitting diode and a photo-generative cell positioned to receive light reflected from smoke in the path of the beam from the light emitting diode. The photo-generative cell is capacitor coupled to an amplifier, the output of which is fed to a level detector such as a differential comparator. The output of the level , 10 detector, which occurs only when the input signal is above a pre~
determined value, is fed to the set terminal of a flip-flop circuit, the output of which is fed to an alarm energizing means.
A pulse generator is provided which pulses, simul-taneously, the light emitting diode on, the level detector on, and provides, through a discriminating circuit, a short pulse to the re-set terminal of the flip-flop. -In a preferred embodiment of the device, the pulse has a duration of about 20 micro-seconds, and a repetition rate of once every two seconds. The amplifier is designed to accept only voltage pulses having a rise time corresponding to a frequency of between 1000 and 100,000 cycles, so that the amplified voltage ; pulse can reach its maximum value and achieve a constant value within the pulse time.
The amplifier is continuously energized, however the level detector is energized only for the duration of the pulse.
During the time between pulses, the level detector is turned off and the signal lead thereof is connected to ground, so that any signal through the amplifier due to random noise during the period that the level detector is off, passes to ground, ; 30 In one embodiment of the invention, to further reduce : the possibility of an alarm from a continuous source of random noise, the output of the flip-flop is fed to an integrating
for the light emitting devices and the amplifiers. All of these :
expedients involve additional expense that is not acceptable in . .
the majority of smoke detector applications.
The smoke detector disclosed herein comprises a light emitting diode and a photo-generative cell positioned to receive light reflected from smoke in the path of the beam from the light emitting diode. The photo-generative cell is capacitor coupled to an amplifier, the output of which is fed to a level detector such as a differential comparator. The output of the level , 10 detector, which occurs only when the input signal is above a pre~
determined value, is fed to the set terminal of a flip-flop circuit, the output of which is fed to an alarm energizing means.
A pulse generator is provided which pulses, simul-taneously, the light emitting diode on, the level detector on, and provides, through a discriminating circuit, a short pulse to the re-set terminal of the flip-flop. -In a preferred embodiment of the device, the pulse has a duration of about 20 micro-seconds, and a repetition rate of once every two seconds. The amplifier is designed to accept only voltage pulses having a rise time corresponding to a frequency of between 1000 and 100,000 cycles, so that the amplified voltage ; pulse can reach its maximum value and achieve a constant value within the pulse time.
The amplifier is continuously energized, however the level detector is energized only for the duration of the pulse.
During the time between pulses, the level detector is turned off and the signal lead thereof is connected to ground, so that any signal through the amplifier due to random noise during the period that the level detector is off, passes to ground, ; 30 In one embodiment of the invention, to further reduce : the possibility of an alarm from a continuous source of random noise, the output of the flip-flop is fed to an integrating
- 3 -.
1 067~75 circuit having a time constant such that at least two consecutive pulses are required to allow a signal to pass from the integrator to the alarm activating device.
Hence during each cycle, the light emitting diode is turned on, the level detector is turned on, and the flip-flop is pulsed to the off condition at the beginning of the pulse. If ~, ' smoke is present, light reflected therefrom onto the photo-generative cell causes a pulse of voltage to appear at the amplifier input. If the amplified pulse at the level detector is of sufficient magnitude to satisfy the requirements of the level detector, a signal passes therefrom to the set terminal of the flip-flop to actuate the alarm.
Ordinary changes in ambient light cannot cause a false alarm, because the amplifier cannot respond to any voltage change ;: . .
at the input with a rate of change corresponding to a frequency ;
of less than about 1000 cyles, and such change would have to occur ;
during the 20 micro-seconds that the pulse is heing applied to ; the level detector. Similarly, random noise that might generate ,: . .:' a signal at the amplifier input sufficient to produce an output hiyh enough to pass through the level detector, would have to occur during the time the level detector is on, and would have to .~ , , . produce a signal at the amplifier input of the proper polarity.
In one embodiment of the invention means is provided ~-for electrically isolating the amplifier from transient voltages ~, from the power supply, by means of suitable filtering means, or by means of an electronic switch which is opened when the light generating device is energized. Capacitor means is provided for powering the amplifier during the period that it is disconnected from the power supply.
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In accordance with a specific embodiment, there is pro-vided, a smoke detector of the type utilizing photo-electric .
detection of light reflected from smoke particles, in which a light source is energized intermittently, first means including a photo-responsive device and an amplifier is provided to pro-duce energy pulses in response to the light pulses reflected from smoke particles, and second means utilizing said energy - pulses to control a signalling device, the improvement comprising a voltage supply for said first means, a capacitor connected across the voltage supply, and third mean~ between the capacitor and the voltage supply for isolating the first means and the `.
capacitor from voltage pulses occuring at the power supply at least during the time that the light source is emitting light.
In accordance with a further specific embodiment, there is provided a smoke detector of the type utilizing photo-~lectric detection of light reflected from smoke particles, in which a pulse generator pulses a light generating device, a photo-voltaic cell is positioned to produce voltage pulses from the pulsed light reflected from smo~e particles, first means for :
amplifying said voltage pulses and for producing an output signal when the pulses are above a predetermined magnitude and means - for utilizing said output signal to control a signalling device, said pulse generator and said first means being supplied by . .
separate leads from a common power source, second means rendering said amplifier operative sub~tantially only during the time the pulse generator is on, a storage capacitor connected across the - lead9 to the first means and an impedence device disposed between the storage capacitor and the power source, whereby when the pulse generator produces a pulse, the first means draws power principally from the storage capacitor, and the impedence means isolates the first means from voltage transients at the power source due to the turning on of the pulse generator and the light generating device.
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In accordance with a further specific embodiment, there is provided a smoke detector having a light source, means ener-gizing the light source intermittently, photo-responsive means positioned to receive light reflected from smoke particles ~:
illuminated by the light source, and amplifier means for amplifying voltage pulses resulting therefrom, a power source, means providing power therefrom to the light source and to the amplifier by first and second power leads, first and second capacitors associated with first and second power leads, respectively, and means between each capacitor and the power .
supply ~or isolating said capacitors from transient voltages : : -occuring at the power supply during the period that the light ~ . -source is emitting light so that during said period the voltages on said first and second capacitors remain substantially equal. ~: .
The invention will now be described with reference to the accompanyiny drawings which show a preferred form thereof and wherein:
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-4b-1()67~75 ,' .:
. Figure l is a schematic diagram of an electrical cir-:: `
cuit for use in a smoke detector embodying the features of the invention, ; Figure 2 is a diagram illustrating the time spacing of the pulses applied to the light emitting diode and the level d~tector. ~-Figure 3 is a diagram illustrating the voltage pulse ~
appearing at the amplifier with 2% smoke in the view of the photo- " -generative cell at various ambient light levels. ~`
Figure 4 is a schematic drawing of a smoke detector , circuit embodying the features of the invention, in which the amplifier is isolated from the power supply, during the time that the light-emitting device is on, by means of a switch which opens -when the light emitting device is turned on so that during the period when the amplifier is likely to receive a signal from the photo-responsive device, the amplifier, and associated equipment, is being powered solely by the storage capacitor.
Figure 5 is a schematic drawing of the detector of . .
, Figure 1 in which choke coils are utilized to isolate the amplifier from voltage transients created when the light emitting device is turned on.
~ Figure 6 is a schematic drawing of the detector of 7`.'' Figure l in which Zener diodes are provided in the separate power leads to the amplifier and the light emitting device.
Referring to Figure 1, there is illustrated an electron-~- ic circuit for use in a smoke detector operating on the reflected light principle. The circuit includes a light emitting diode LED
.
and a photo-voltaic cell C positioned out of the direct line of ; -the beam of light from the light emitting diode. In a preferred embodiment of the invention the cell C is positioned to view a portion of the beam in front of the LED at an angle of about 120 - 135 fro~ the axis of the beam, to take advantage of the well-:~ .
, - 5 -,:
~067~75 known "forward scatter" effect.
The cell C is coupled through capacitor F to an ampli- ~;
fier A, the output of the amplifier being fed to the input of a level detector L, such as a differential comparator. The level :;
detector output is fed to the "set" terminal of a flip-flop circuit FF, the output of which is fed to an alarm actuating ;
device K.
In a preferred embodiment of the device, the different- -' ial comparator is normally off with the signal lead thereof `
clamped to ground by an electronic switch Sl. `
Light emitting diodes presently commercially availableare rated, for example, for a maximum current of 1/2 ampere on a : continuous basis, or for 10 amperes in pulses not to exceed 1 -~
micro-second at 200 pulses per second. However, I have found that such dîodes can be pulsed at 10 amperes for 20 micro-seconds, provided that the pulse repetition rate is much slower, for example 1 pulse every 1 or 2 seconds. As previously mentioned, this pulse duration allows the signal through the amplifier to reach a constant value within the pulse time, so that minor ;`
variations in pulse width will not affect this alarm point.
- For this purpose and for others to appear hereinafter, a pulse generator P is provided, which provides a 20 micro- ~ `
second pulse to the LED every two seconds, and also simultaneously applies a pulse to energize the level detector and to open switch Sl. Hence the differential comparator is energized and its signal lead ungrounded only during the 20 micro-seconds out of each two seconds that the LED is energized.
Simultaneously with the application of the pulse to the LED ;
and the level detector, a pulse is applied to the re-set terminal of the flip-flop circuit through a differentiator D, which converts said pulse to a spike of about 1 micro-second duration, occuring at the beginning of the pulse cycle.
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- - ~06717S
, The operation of the circuit can best be understodd by reference to Figure 2 of the drawing, which is a graph of the response of the various components of the circuit during one pulse. The horizontal scale represents time, and the vertical scale represents response. The vertical scale is arbitrary de-pending on the type of response, and the magnitude of the various curves on the vertical scale have no relation to each other ex-, cept as described hereinafter. -Each cycle begins by the application of a pulse from ; 10 the pulse generator to the LED, the level detector, and the flip- -' flop reset terminal. The pulse to the LED and the level detector ~
i are represented on the diagram by Pl, since they are of the same ^`
duration. They may, of course, be of different magnitudes and different polarities. The pulse appearing at the re-set terminal o~ the flip-flop after passing through the discriminator is re-presented by P2, The application of the pulse to the LED
; produces a light output having a duration and relative intensity represented by curve Ll.
If there is no smoke in the portion of the beam viewed by the cell C, there will be no pulse of voltage generated by the ~`~ cell and hence no output from the amplifier. If the detector is ::
subjected to varying ambient light, the cell will generate a varying D. C. voltage (see Figure 3) which does not cause any amplifier response because of the capacitor coupling between the cell and the amplifier.
If there is smoke pres~nt in the pulsed light beam, a pulsed voltage signal will be produced by the cell, represented by curve Vl of Figure 2, which will be amplified by the amplifier to produce a signal at the input of the differential comparator, which signal will have a magnitude that is a function of the amount of smoke present, To avoid unnecessary alarms from accept-able amounts of smoke and dust in the atmosphere, the differential ". :
. : . .
~ j 1()f~7175 comparator is set to respond only to an amplifier output that ~
corresponds to a predetermined smoke concentration. For example, -in a preferred embodiment of the invention, the differential com~arator is set to respond only if the smoke concentration is ; ;
2%, defined as the amount of smoke that absorbs 2% of a light beam 1 foot long. As illustrated in Figure 2, the amplifier out- -- put level required to permit the output signal to pass through the differential comparator is represented by the horizontal dashed line S.
In a particular embodiment of the invention the differ- -~
ential comparator may have a standby voltage difference between input terminals of abGut 100 millivolts, requiring a signal of over 100 millivolts from the amplifier to produce an output signal.
Means may also be provided at the level detector to adjust the standby voltage difference between terminals, to allow calibration of the system so that the alarm point will be at the desired 2%. In the present embodiment of the invention the calibration is accomplished by providing a voltage divider Rl across the power source, with the junction thereof connected to -one of the inputs of the differential comparator, and providing a variable resistor R2 across the power supply with the center tap thereof connected to the other input.
.. . .
If the amount of smoke in the view of the cell has ~
... .
reached the specified concentration, the amplifier output will be : as shown in curve Al reaching the line S at point Y, thereby producing a differential comparator output represented by line LDl, which applies a signal to the flip-flop set terminal, there-by turning on the flip-flop output (FFl on Figure 2) to energize the alarm.
At the end of the pulse to the LED and the differential comparator, both turn off so that the output from the differential ~ comparator to the flip-flop is turned off. The flip-flop output, : . - ' - ~06~17S ;
.
however, stays on until the beginning of the next pulse, at which --time it is turned off by the pulse through the discriminator in -the manner previously described.
As a greater concentration of smoke appears in the view -~
of the cell, more reflected light is received by the cell, and ~-the output voltage of the pulses applied to the amplifier in-; creases, so that the amplifier output increases and reaches the ; required level S slightly sooner in the pulse cycle, as -illustrated by curve A2, providing differential comparator out-put LD2 and flip-flop output FF2.
Although the amplifier may continue to provide an output for a short time after the end of the pulse to the LED and the differential comparator, no output can exist after the end of the pulse, because the differential comparator is de-energized and the signal lead thereof clamped to ground by switch Sl. - ;
A smoke detector utilizing the above described clrcuit has a number of advantages over detectors of the prior art that have utilized a pulsing light source and a pulsed amplifier. By the use of a pulse of very short duration with a slow repetition rate, an amplifier with a response only to very high rates of change of input voltage, and the use of a pulsed level detector after the amplifier, the occurrence of false alarms due to changing light levels or due to electrical transients is almost completely eliminated.
A change of light level that could actuate the alarm must not only occur at an extremely h gh rate, but its occurrence must coincide with the time in which the level detector is on, - which is only 1/100,000 of the total time.
For example, the turning on of an incandescent light cannot cause a false alarm, since the rate of rise of the light output from an incandescent bulb is much too slow to create a voltage pulse that can pass through the capacitor. Although the .
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resulting increase in ambient light will increase the D.C. voltage at the cell terminals, subsequent pulses of light falling on the cell will cause the cell to generate an output voltage pulse on top of the D. C. voltage (assuming that the ambient light is not so strong as to saturate the cell) which will be detected by the amplifiex.
This effect is illustrated in Figure 3 where curve Va represents the voltage at the cell due to ambient light level and Vp represents the cell voltage during the period that the LED is ... .
illuminated, with 2% smoke present. Since the response of the :-- cell is substantially linear, the sensitivity of the device is not affected by changes in ambient light, since the pulse voltage at 2% smoke remains the same, regardless of the ambient light level, provided that the ambient light level is not so high as to cause saturation of the cell. In the curve of Figure 3, the relative height of the ambient light voltage curve and the height :~ .
of the voltage pulses are necessarily not in proportion, since ~ -the D. C. voltage from ambient light may be on the order of ~1 -volts whereas the additional voltage generated by the pulse of light reflected from smoke particles, at 2% smoke, is only about 600 microvolts.
Although certain light sources, such as lightning, some types of camera flash equipment, and welding apparatus may produce light with a rise time fast enough to be amplified and reach the level detector, such resulting signal not only must be great enough to satisfy the level detector requirements, but also must occur during the 20 micro-seconds that the level detector is on.
The chance of a false alarm from such a source is therefore extremely remote.
; 30 In regard to possible false alarms from random electric-al signals generated in the cell from radio transmitters, tran-sients on the power supply line, and the like, not only must such . .
. ~ - 10 -- 1067~75 :: -~' ;,':
signals occur at the proper instant and generate a signal of adequate magnitude, the signal appearing at the amplifier input must be of the proper polarity.
In the illustrated embodiment of the invention, the signal from the flip-flop is led to an integrator T, comprising a resistor-capacitor network, which integrates pulses received from the flip-flop to provide an output signal to the alarm energizing device R. In one embodiment, the integrator may have a time constant which is at least slightly greater than the total time between pulses, so that two pulses from the flip-flop are required to reach an output level from the integrator to actuate the alarm energizing device.
Although the use of the integrator may not be required in all installations in which the smoke detector is used, it has been found effective in preventing false alarms in locations that are near sources of continuous noige, such as might be produced by arcing electrical apparatus.
Another major advantage of a smoke detector utilizing ,, .
,~ the circuit disclosed herein is its extremely low power consump-tion. Although the pulse to the LED may be of the order of 7 amperes, the short duration of the pulse, and the fact that the level detector is on only during the pulse permits a power consumption of the order of 300 micro-amperes at 6 volts. This power consumption is low enough to allow the device to be operated for over one year on battery power units small enough to be contained within a detector housing, with enough reserve power to ` energize a self-contained alarm.
- Referring to figures 4-6 of the drawing, there is illustrated an electronic circuit for use in a smoke detector of the type operating on the reflected light principle. The circuit includes a light emitting diode LED and a photo-voltaic cell C
positioned out of the direct line of the beam of light from the LED, In a preferred embodiment of the invention the cell C is -positioned to view a portion of the beam in front of the LED at an angle of ab~ut 120 to about 135 to the axis of the beam, to ~ ;
take advantage of the well known "forward scatter" effect.
The cell C is coupled through capacitor F to an amplifier A, the output of which is fed to a level detector L -such as a differential comparator. The level detector output is fed to the set terminal of a flip-flop circuit FF, the output of ;
which is fed to an integrator, which energizes an alarm actuating ; 10 device K.
In a preferred embodiment of the device, the different- -ial comparator is normally off with the signal lead thereof being clamped to ground by an electronic switch Sl.
To energize the LED on an intermittent basis, a pulse generator P is provided which, in addition to providing an energizing pulse to the LED, also simultaneously applies a pulse -to energize the level detector and applies a pulse to the re-set ,' terminal of the flip-flop through differentiator D, which converts the pulse to a spike of voltage applied to the re-set terminal at the beginning of the pulse cycle.
The above described portion of the circuit is similar to that disclosed and described in more detail heretofore in ~ connection with the embodiment of figures 1-3.
- In the modification of the present invention illustrated in Figure 4, one lead Wl is provided from the power source Vl to -,.:; . .
~ the pulse generator, and a second lead W2 is provided from the `
~.... .
~ power source to the amplifier, the level detector, and the flip- -; flop.
The line W2 contains a series connected electronic -switch S2 between the power source and the components energized by said line, and a capacitor Fl is connected between ground and -the line W2 at a point between the switch S2 and the flip-flop.
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In the embodiment of Figure 4, the pulse generator, in addition to the functions previously described, also applies a pulse to the switch S2 to open said switch and thereby disconnect the amplifier and associated equipment on lead W2 from the power source during the time that the LED is energized, so that during -~this period, the amplifier and associated equipment operates solely from the charge stored in the capacitor.
The operation of the device may be summarized as follows:
~ le duration of the pulse which energizes the LED is very short, for example, 20 micro-seconds compared to the repet-ition rate, which is 1 or 2 seconds.
, During the time that the pulse generator is off, capacitors Fl and F2 are charged from the power source V. When the pulse generator applies an energizing pulse to the LED, it simultaneously applies a pulse to the switch S2 to open said switch. The amplifier and other equipment on line W2 are there-fore, at the instant that the LED is turned on, disconnected from the power supply, and the energy necessary to operate the ampli-fier comes only from the capacitor Fl.
As described hereinbefore in connection with the - embodiment of Figures 1-3, pulses of light are reflected there-~ from and fall on the photo-cell C, causing a series of voltage -~ pulses at the input of the amplifier, If the amplified pulses are of sufficient magnitude to satisfy the requirements of the level detector, a series of pulses are applied to the "set"
terminal of the flip-flop, which applies a series of pulses (since the flip-flop is re-set at the beginning of each pulse) to the integrator, to actuate the alarm.
At the end of each pulse to the LED, the switch S2 closes so that the capacitor can recover the small amount of ',: .
, ~ 1 0~;7175 charge used in powering the amplifier and associated equipment.
In the particular illustrated embodiment, the current used to power the LED may be of the order of 7 to 10 amperes.
Although this current is drawn for only 20 micro-seconds, it would nevertheless cause a sudden and substantial drop in the voltage at the amplifier (unless the impedence of the voltage source is so low that it would be impractical for a commercial ;
installation). Without the presence of the switch S2 and capacitor Fl, a substantial drop in supply voltage to the amplifier `
would occur. Such drop in voltage would cause a sudden change in bias voltage in the amplifier, which would be interpreted by the amplifier as a signal, which could either cause or negate an ;' alarm (depending on the phase relationship of the amplifier being used), since the change in supply voltage of such a magnitude , . ...
could cause an output signal from the amplifier which is many times the output signal that would be created by a signal result-ing from light reflected from smoke particles.
If the system is being operated from a loop from a power supply at a central control panel, the isolation of the amplifier from the power supply by the opening of switch S2 during the time that the LED is emitting light also prevents transient voltages -occurring on the loop from generating spurious responses in the -; amplifier.
Referring now to Figure 5, there is illustrated a modified form of smoke detector, with the portion of the circuit shown therein enclosed in the dashed line being substituted for the portion of the circuit enclosed in the dashed line of Figure
1 067~75 circuit having a time constant such that at least two consecutive pulses are required to allow a signal to pass from the integrator to the alarm activating device.
Hence during each cycle, the light emitting diode is turned on, the level detector is turned on, and the flip-flop is pulsed to the off condition at the beginning of the pulse. If ~, ' smoke is present, light reflected therefrom onto the photo-generative cell causes a pulse of voltage to appear at the amplifier input. If the amplified pulse at the level detector is of sufficient magnitude to satisfy the requirements of the level detector, a signal passes therefrom to the set terminal of the flip-flop to actuate the alarm.
Ordinary changes in ambient light cannot cause a false alarm, because the amplifier cannot respond to any voltage change ;: . .
at the input with a rate of change corresponding to a frequency ;
of less than about 1000 cyles, and such change would have to occur ;
during the 20 micro-seconds that the pulse is heing applied to ; the level detector. Similarly, random noise that might generate ,: . .:' a signal at the amplifier input sufficient to produce an output hiyh enough to pass through the level detector, would have to occur during the time the level detector is on, and would have to .~ , , . produce a signal at the amplifier input of the proper polarity.
In one embodiment of the invention means is provided ~-for electrically isolating the amplifier from transient voltages ~, from the power supply, by means of suitable filtering means, or by means of an electronic switch which is opened when the light generating device is energized. Capacitor means is provided for powering the amplifier during the period that it is disconnected from the power supply.
:, '' ;"
: .
~ - 4 -~ 067175 ` ~
In accordance with a specific embodiment, there is pro-vided, a smoke detector of the type utilizing photo-electric .
detection of light reflected from smoke particles, in which a light source is energized intermittently, first means including a photo-responsive device and an amplifier is provided to pro-duce energy pulses in response to the light pulses reflected from smoke particles, and second means utilizing said energy - pulses to control a signalling device, the improvement comprising a voltage supply for said first means, a capacitor connected across the voltage supply, and third mean~ between the capacitor and the voltage supply for isolating the first means and the `.
capacitor from voltage pulses occuring at the power supply at least during the time that the light source is emitting light.
In accordance with a further specific embodiment, there is provided a smoke detector of the type utilizing photo-~lectric detection of light reflected from smoke particles, in which a pulse generator pulses a light generating device, a photo-voltaic cell is positioned to produce voltage pulses from the pulsed light reflected from smo~e particles, first means for :
amplifying said voltage pulses and for producing an output signal when the pulses are above a predetermined magnitude and means - for utilizing said output signal to control a signalling device, said pulse generator and said first means being supplied by . .
separate leads from a common power source, second means rendering said amplifier operative sub~tantially only during the time the pulse generator is on, a storage capacitor connected across the - lead9 to the first means and an impedence device disposed between the storage capacitor and the power source, whereby when the pulse generator produces a pulse, the first means draws power principally from the storage capacitor, and the impedence means isolates the first means from voltage transients at the power source due to the turning on of the pulse generator and the light generating device.
: ~ -4a-~. ~
.... ~ . ... .. ... ~ ....... ... ...
- \ .
In accordance with a further specific embodiment, there is provided a smoke detector having a light source, means ener-gizing the light source intermittently, photo-responsive means positioned to receive light reflected from smoke particles ~:
illuminated by the light source, and amplifier means for amplifying voltage pulses resulting therefrom, a power source, means providing power therefrom to the light source and to the amplifier by first and second power leads, first and second capacitors associated with first and second power leads, respectively, and means between each capacitor and the power .
supply ~or isolating said capacitors from transient voltages : : -occuring at the power supply during the period that the light ~ . -source is emitting light so that during said period the voltages on said first and second capacitors remain substantially equal. ~: .
The invention will now be described with reference to the accompanyiny drawings which show a preferred form thereof and wherein:
''.
'' ' .:
-4b-1()67~75 ,' .:
. Figure l is a schematic diagram of an electrical cir-:: `
cuit for use in a smoke detector embodying the features of the invention, ; Figure 2 is a diagram illustrating the time spacing of the pulses applied to the light emitting diode and the level d~tector. ~-Figure 3 is a diagram illustrating the voltage pulse ~
appearing at the amplifier with 2% smoke in the view of the photo- " -generative cell at various ambient light levels. ~`
Figure 4 is a schematic drawing of a smoke detector , circuit embodying the features of the invention, in which the amplifier is isolated from the power supply, during the time that the light-emitting device is on, by means of a switch which opens -when the light emitting device is turned on so that during the period when the amplifier is likely to receive a signal from the photo-responsive device, the amplifier, and associated equipment, is being powered solely by the storage capacitor.
Figure 5 is a schematic drawing of the detector of . .
, Figure 1 in which choke coils are utilized to isolate the amplifier from voltage transients created when the light emitting device is turned on.
~ Figure 6 is a schematic drawing of the detector of 7`.'' Figure l in which Zener diodes are provided in the separate power leads to the amplifier and the light emitting device.
Referring to Figure 1, there is illustrated an electron-~- ic circuit for use in a smoke detector operating on the reflected light principle. The circuit includes a light emitting diode LED
.
and a photo-voltaic cell C positioned out of the direct line of ; -the beam of light from the light emitting diode. In a preferred embodiment of the invention the cell C is positioned to view a portion of the beam in front of the LED at an angle of about 120 - 135 fro~ the axis of the beam, to take advantage of the well-:~ .
, - 5 -,:
~067~75 known "forward scatter" effect.
The cell C is coupled through capacitor F to an ampli- ~;
fier A, the output of the amplifier being fed to the input of a level detector L, such as a differential comparator. The level :;
detector output is fed to the "set" terminal of a flip-flop circuit FF, the output of which is fed to an alarm actuating ;
device K.
In a preferred embodiment of the device, the different- -' ial comparator is normally off with the signal lead thereof `
clamped to ground by an electronic switch Sl. `
Light emitting diodes presently commercially availableare rated, for example, for a maximum current of 1/2 ampere on a : continuous basis, or for 10 amperes in pulses not to exceed 1 -~
micro-second at 200 pulses per second. However, I have found that such dîodes can be pulsed at 10 amperes for 20 micro-seconds, provided that the pulse repetition rate is much slower, for example 1 pulse every 1 or 2 seconds. As previously mentioned, this pulse duration allows the signal through the amplifier to reach a constant value within the pulse time, so that minor ;`
variations in pulse width will not affect this alarm point.
- For this purpose and for others to appear hereinafter, a pulse generator P is provided, which provides a 20 micro- ~ `
second pulse to the LED every two seconds, and also simultaneously applies a pulse to energize the level detector and to open switch Sl. Hence the differential comparator is energized and its signal lead ungrounded only during the 20 micro-seconds out of each two seconds that the LED is energized.
Simultaneously with the application of the pulse to the LED ;
and the level detector, a pulse is applied to the re-set terminal of the flip-flop circuit through a differentiator D, which converts said pulse to a spike of about 1 micro-second duration, occuring at the beginning of the pulse cycle.
.~ "
~, 6 ";.
- - ~06717S
, The operation of the circuit can best be understodd by reference to Figure 2 of the drawing, which is a graph of the response of the various components of the circuit during one pulse. The horizontal scale represents time, and the vertical scale represents response. The vertical scale is arbitrary de-pending on the type of response, and the magnitude of the various curves on the vertical scale have no relation to each other ex-, cept as described hereinafter. -Each cycle begins by the application of a pulse from ; 10 the pulse generator to the LED, the level detector, and the flip- -' flop reset terminal. The pulse to the LED and the level detector ~
i are represented on the diagram by Pl, since they are of the same ^`
duration. They may, of course, be of different magnitudes and different polarities. The pulse appearing at the re-set terminal o~ the flip-flop after passing through the discriminator is re-presented by P2, The application of the pulse to the LED
; produces a light output having a duration and relative intensity represented by curve Ll.
If there is no smoke in the portion of the beam viewed by the cell C, there will be no pulse of voltage generated by the ~`~ cell and hence no output from the amplifier. If the detector is ::
subjected to varying ambient light, the cell will generate a varying D. C. voltage (see Figure 3) which does not cause any amplifier response because of the capacitor coupling between the cell and the amplifier.
If there is smoke pres~nt in the pulsed light beam, a pulsed voltage signal will be produced by the cell, represented by curve Vl of Figure 2, which will be amplified by the amplifier to produce a signal at the input of the differential comparator, which signal will have a magnitude that is a function of the amount of smoke present, To avoid unnecessary alarms from accept-able amounts of smoke and dust in the atmosphere, the differential ". :
. : . .
~ j 1()f~7175 comparator is set to respond only to an amplifier output that ~
corresponds to a predetermined smoke concentration. For example, -in a preferred embodiment of the invention, the differential com~arator is set to respond only if the smoke concentration is ; ;
2%, defined as the amount of smoke that absorbs 2% of a light beam 1 foot long. As illustrated in Figure 2, the amplifier out- -- put level required to permit the output signal to pass through the differential comparator is represented by the horizontal dashed line S.
In a particular embodiment of the invention the differ- -~
ential comparator may have a standby voltage difference between input terminals of abGut 100 millivolts, requiring a signal of over 100 millivolts from the amplifier to produce an output signal.
Means may also be provided at the level detector to adjust the standby voltage difference between terminals, to allow calibration of the system so that the alarm point will be at the desired 2%. In the present embodiment of the invention the calibration is accomplished by providing a voltage divider Rl across the power source, with the junction thereof connected to -one of the inputs of the differential comparator, and providing a variable resistor R2 across the power supply with the center tap thereof connected to the other input.
.. . .
If the amount of smoke in the view of the cell has ~
... .
reached the specified concentration, the amplifier output will be : as shown in curve Al reaching the line S at point Y, thereby producing a differential comparator output represented by line LDl, which applies a signal to the flip-flop set terminal, there-by turning on the flip-flop output (FFl on Figure 2) to energize the alarm.
At the end of the pulse to the LED and the differential comparator, both turn off so that the output from the differential ~ comparator to the flip-flop is turned off. The flip-flop output, : . - ' - ~06~17S ;
.
however, stays on until the beginning of the next pulse, at which --time it is turned off by the pulse through the discriminator in -the manner previously described.
As a greater concentration of smoke appears in the view -~
of the cell, more reflected light is received by the cell, and ~-the output voltage of the pulses applied to the amplifier in-; creases, so that the amplifier output increases and reaches the ; required level S slightly sooner in the pulse cycle, as -illustrated by curve A2, providing differential comparator out-put LD2 and flip-flop output FF2.
Although the amplifier may continue to provide an output for a short time after the end of the pulse to the LED and the differential comparator, no output can exist after the end of the pulse, because the differential comparator is de-energized and the signal lead thereof clamped to ground by switch Sl. - ;
A smoke detector utilizing the above described clrcuit has a number of advantages over detectors of the prior art that have utilized a pulsing light source and a pulsed amplifier. By the use of a pulse of very short duration with a slow repetition rate, an amplifier with a response only to very high rates of change of input voltage, and the use of a pulsed level detector after the amplifier, the occurrence of false alarms due to changing light levels or due to electrical transients is almost completely eliminated.
A change of light level that could actuate the alarm must not only occur at an extremely h gh rate, but its occurrence must coincide with the time in which the level detector is on, - which is only 1/100,000 of the total time.
For example, the turning on of an incandescent light cannot cause a false alarm, since the rate of rise of the light output from an incandescent bulb is much too slow to create a voltage pulse that can pass through the capacitor. Although the .
_ g _ .
.
.
.
,. ~'' :-.
resulting increase in ambient light will increase the D.C. voltage at the cell terminals, subsequent pulses of light falling on the cell will cause the cell to generate an output voltage pulse on top of the D. C. voltage (assuming that the ambient light is not so strong as to saturate the cell) which will be detected by the amplifiex.
This effect is illustrated in Figure 3 where curve Va represents the voltage at the cell due to ambient light level and Vp represents the cell voltage during the period that the LED is ... .
illuminated, with 2% smoke present. Since the response of the :-- cell is substantially linear, the sensitivity of the device is not affected by changes in ambient light, since the pulse voltage at 2% smoke remains the same, regardless of the ambient light level, provided that the ambient light level is not so high as to cause saturation of the cell. In the curve of Figure 3, the relative height of the ambient light voltage curve and the height :~ .
of the voltage pulses are necessarily not in proportion, since ~ -the D. C. voltage from ambient light may be on the order of ~1 -volts whereas the additional voltage generated by the pulse of light reflected from smoke particles, at 2% smoke, is only about 600 microvolts.
Although certain light sources, such as lightning, some types of camera flash equipment, and welding apparatus may produce light with a rise time fast enough to be amplified and reach the level detector, such resulting signal not only must be great enough to satisfy the level detector requirements, but also must occur during the 20 micro-seconds that the level detector is on.
The chance of a false alarm from such a source is therefore extremely remote.
; 30 In regard to possible false alarms from random electric-al signals generated in the cell from radio transmitters, tran-sients on the power supply line, and the like, not only must such . .
. ~ - 10 -- 1067~75 :: -~' ;,':
signals occur at the proper instant and generate a signal of adequate magnitude, the signal appearing at the amplifier input must be of the proper polarity.
In the illustrated embodiment of the invention, the signal from the flip-flop is led to an integrator T, comprising a resistor-capacitor network, which integrates pulses received from the flip-flop to provide an output signal to the alarm energizing device R. In one embodiment, the integrator may have a time constant which is at least slightly greater than the total time between pulses, so that two pulses from the flip-flop are required to reach an output level from the integrator to actuate the alarm energizing device.
Although the use of the integrator may not be required in all installations in which the smoke detector is used, it has been found effective in preventing false alarms in locations that are near sources of continuous noige, such as might be produced by arcing electrical apparatus.
Another major advantage of a smoke detector utilizing ,, .
,~ the circuit disclosed herein is its extremely low power consump-tion. Although the pulse to the LED may be of the order of 7 amperes, the short duration of the pulse, and the fact that the level detector is on only during the pulse permits a power consumption of the order of 300 micro-amperes at 6 volts. This power consumption is low enough to allow the device to be operated for over one year on battery power units small enough to be contained within a detector housing, with enough reserve power to ` energize a self-contained alarm.
- Referring to figures 4-6 of the drawing, there is illustrated an electronic circuit for use in a smoke detector of the type operating on the reflected light principle. The circuit includes a light emitting diode LED and a photo-voltaic cell C
positioned out of the direct line of the beam of light from the LED, In a preferred embodiment of the invention the cell C is -positioned to view a portion of the beam in front of the LED at an angle of ab~ut 120 to about 135 to the axis of the beam, to ~ ;
take advantage of the well known "forward scatter" effect.
The cell C is coupled through capacitor F to an amplifier A, the output of which is fed to a level detector L -such as a differential comparator. The level detector output is fed to the set terminal of a flip-flop circuit FF, the output of ;
which is fed to an integrator, which energizes an alarm actuating ; 10 device K.
In a preferred embodiment of the device, the different- -ial comparator is normally off with the signal lead thereof being clamped to ground by an electronic switch Sl.
To energize the LED on an intermittent basis, a pulse generator P is provided which, in addition to providing an energizing pulse to the LED, also simultaneously applies a pulse -to energize the level detector and applies a pulse to the re-set ,' terminal of the flip-flop through differentiator D, which converts the pulse to a spike of voltage applied to the re-set terminal at the beginning of the pulse cycle.
The above described portion of the circuit is similar to that disclosed and described in more detail heretofore in ~ connection with the embodiment of figures 1-3.
- In the modification of the present invention illustrated in Figure 4, one lead Wl is provided from the power source Vl to -,.:; . .
~ the pulse generator, and a second lead W2 is provided from the `
~.... .
~ power source to the amplifier, the level detector, and the flip- -; flop.
The line W2 contains a series connected electronic -switch S2 between the power source and the components energized by said line, and a capacitor Fl is connected between ground and -the line W2 at a point between the switch S2 and the flip-flop.
.
:: .. , .. . . . . . ~ .. . -10~;717S
, .
In the embodiment of Figure 4, the pulse generator, in addition to the functions previously described, also applies a pulse to the switch S2 to open said switch and thereby disconnect the amplifier and associated equipment on lead W2 from the power source during the time that the LED is energized, so that during -~this period, the amplifier and associated equipment operates solely from the charge stored in the capacitor.
The operation of the device may be summarized as follows:
~ le duration of the pulse which energizes the LED is very short, for example, 20 micro-seconds compared to the repet-ition rate, which is 1 or 2 seconds.
, During the time that the pulse generator is off, capacitors Fl and F2 are charged from the power source V. When the pulse generator applies an energizing pulse to the LED, it simultaneously applies a pulse to the switch S2 to open said switch. The amplifier and other equipment on line W2 are there-fore, at the instant that the LED is turned on, disconnected from the power supply, and the energy necessary to operate the ampli-fier comes only from the capacitor Fl.
As described hereinbefore in connection with the - embodiment of Figures 1-3, pulses of light are reflected there-~ from and fall on the photo-cell C, causing a series of voltage -~ pulses at the input of the amplifier, If the amplified pulses are of sufficient magnitude to satisfy the requirements of the level detector, a series of pulses are applied to the "set"
terminal of the flip-flop, which applies a series of pulses (since the flip-flop is re-set at the beginning of each pulse) to the integrator, to actuate the alarm.
At the end of each pulse to the LED, the switch S2 closes so that the capacitor can recover the small amount of ',: .
, ~ 1 0~;7175 charge used in powering the amplifier and associated equipment.
In the particular illustrated embodiment, the current used to power the LED may be of the order of 7 to 10 amperes.
Although this current is drawn for only 20 micro-seconds, it would nevertheless cause a sudden and substantial drop in the voltage at the amplifier (unless the impedence of the voltage source is so low that it would be impractical for a commercial ;
installation). Without the presence of the switch S2 and capacitor Fl, a substantial drop in supply voltage to the amplifier `
would occur. Such drop in voltage would cause a sudden change in bias voltage in the amplifier, which would be interpreted by the amplifier as a signal, which could either cause or negate an ;' alarm (depending on the phase relationship of the amplifier being used), since the change in supply voltage of such a magnitude , . ...
could cause an output signal from the amplifier which is many times the output signal that would be created by a signal result-ing from light reflected from smoke particles.
If the system is being operated from a loop from a power supply at a central control panel, the isolation of the amplifier from the power supply by the opening of switch S2 during the time that the LED is emitting light also prevents transient voltages -occurring on the loop from generating spurious responses in the -; amplifier.
Referring now to Figure 5, there is illustrated a modified form of smoke detector, with the portion of the circuit shown therein enclosed in the dashed line being substituted for the portion of the circuit enclosed in the dashed line of Figure
4. The circuit of Figure 5 includes a choke coil CKl in place of switch S2, and a choke coil CK2 in series with the line to the pulse generator. The choke coils CKl and CK2 have electrical characteristics such that when the pulse generator turns on the LED, the choke coils isolate the amplifier from the high frequency . .
'1()67175 ~
negative pulse caused by the sudden drop in power supply voltage.
Since the power to the amplifier is itself a high frequency pulse ~hich could not pass through the choke coil CK1, the power for the operation of the amplifier is supplied substantially entirely by capacitor Fl.
Referring now to Figure 6, there is illustrated a second modified ~orm of smoke detector, with the portion of the circuit shown therein enclosed in the dashed line being substi-tuted for the portion of the circuit enclosed in the dashed line of Figure 4.
The circuit of Figure 6 includes Zener diodes Zl and Z2 connected between lines W2 and Wl respectively and ground, with resistors Rl and R2 being in series in lines W2 and Wl between 4~
the connection to the Zener diode and the power source V2. In the circuit of Figure 6, the power source V2 is higher than nec-essary to operate the amplifier and the pulse generator, and the voltage is regulated to the correct amount by the Zener diodes.
When the pulse generator turns on the LED, power is drawn from the capacitor F2, and voltage drop at the capacitor input is prevented by the regulating effect of the Zener diode Z2. Similarly, voltage to the amplifier is regulated by the Zener diode Zl so that no residual voltage change at the power source from the turning on of the LED or from other transients on the power line can affect the operation of the amplifier.
It will be understood that various combinations of the illustrated modifications can be used, depending on the parti-cular installation. For example, when operated~ from battery - power in which the same battery also powers the alarm device, the system of Figure 4 may include a choke coil in line W2 to the pulse generator to prevent transients on the line resulting from the energizing of the alarm unit from affecting the operation of the pulse generator, :.
,. . . . . . .
067~75 . :- ,' The switch S2 may, if desired, be incorporated into the modifications of Figures 5 and 6. `` -In any of the modifications of the invention or com-binations thereof, it is essential that during the period that the amplifier is operative to amplify a pulse from the photo-generative device and pass it to the signalling device, the r~
voltage on capacitors Fl and F2 be equal, except for the minute difference in voltage resulting from the fact that during the operative period, the light generating device may draw slightly -~-more power from capacitor F2 than the amplifier and associated ., ,~ ., equipment drawsfrom capacitor Fl, However, the capacitors are sufficiently large in relation to the power needed for the operation of the components that the voltage drop thereof is very small in relation to the supply voltage, and hence the voltage difference between the capacitors at the end of a pulse is in- , , , appreciable.
Although specific embodiments have been illustrated in the akove, this was for the purpose of describing, but not limiting, the invention. Various modifications which will come readily to the mind of one skilled in the art are within ;
the scope of the invention as defined in the appended claims.
. .~ .
This application is a division of Canadian patent application Serial ~o. 207,336, ~iled Auyust 19, 1974' ~ ' -- .
, '''`
'1()67175 ~
negative pulse caused by the sudden drop in power supply voltage.
Since the power to the amplifier is itself a high frequency pulse ~hich could not pass through the choke coil CK1, the power for the operation of the amplifier is supplied substantially entirely by capacitor Fl.
Referring now to Figure 6, there is illustrated a second modified ~orm of smoke detector, with the portion of the circuit shown therein enclosed in the dashed line being substi-tuted for the portion of the circuit enclosed in the dashed line of Figure 4.
The circuit of Figure 6 includes Zener diodes Zl and Z2 connected between lines W2 and Wl respectively and ground, with resistors Rl and R2 being in series in lines W2 and Wl between 4~
the connection to the Zener diode and the power source V2. In the circuit of Figure 6, the power source V2 is higher than nec-essary to operate the amplifier and the pulse generator, and the voltage is regulated to the correct amount by the Zener diodes.
When the pulse generator turns on the LED, power is drawn from the capacitor F2, and voltage drop at the capacitor input is prevented by the regulating effect of the Zener diode Z2. Similarly, voltage to the amplifier is regulated by the Zener diode Zl so that no residual voltage change at the power source from the turning on of the LED or from other transients on the power line can affect the operation of the amplifier.
It will be understood that various combinations of the illustrated modifications can be used, depending on the parti-cular installation. For example, when operated~ from battery - power in which the same battery also powers the alarm device, the system of Figure 4 may include a choke coil in line W2 to the pulse generator to prevent transients on the line resulting from the energizing of the alarm unit from affecting the operation of the pulse generator, :.
,. . . . . . .
067~75 . :- ,' The switch S2 may, if desired, be incorporated into the modifications of Figures 5 and 6. `` -In any of the modifications of the invention or com-binations thereof, it is essential that during the period that the amplifier is operative to amplify a pulse from the photo-generative device and pass it to the signalling device, the r~
voltage on capacitors Fl and F2 be equal, except for the minute difference in voltage resulting from the fact that during the operative period, the light generating device may draw slightly -~-more power from capacitor F2 than the amplifier and associated ., ,~ ., equipment drawsfrom capacitor Fl, However, the capacitors are sufficiently large in relation to the power needed for the operation of the components that the voltage drop thereof is very small in relation to the supply voltage, and hence the voltage difference between the capacitors at the end of a pulse is in- , , , appreciable.
Although specific embodiments have been illustrated in the akove, this was for the purpose of describing, but not limiting, the invention. Various modifications which will come readily to the mind of one skilled in the art are within ;
the scope of the invention as defined in the appended claims.
. .~ .
This application is a division of Canadian patent application Serial ~o. 207,336, ~iled Auyust 19, 1974' ~ ' -- .
, '''`
Claims (8)
1. A smoke detector of the type utilizing photo-electric detection of light reflected from smoke particles, in which a light source is energized intermittently, first means including a photo-responsive device and an amplifier is provided to pro-duce energy pulses in response to the light pulses reflected from smoke particles, and second means utilizing said energy pulses to control a signalling device, the improvement comprising a voltage supply for said first means, a capacitor connected across the voltage supply, and third means between the capacitor and the voltage supply for isolating the first means and the capacitor from voltage pulses occuring at the power supply at least during the time that the light source is emitting light.
2. A smoke detector as set out in claim 1 in which said third means allows current flow when the light source is not producing light, but effectively prevents the passing of voltage pulses in the frequency range to which the amplifier is responsive
3. A smoke detector as set out in claim 1 in which means is provided for allowing the first means to produce an output pulse only while the light source is emitting light, and said third means allows current flow to charge the capacitor while the light source is de-energized but does not allow a voltage pulse to pass therethrough during the period that the light source is energized, whereby the first means draws power substantially only from the capacitor when it is energized.
4. A smoke detector as set out in claim 1 in which said third means is a switch, and means is provided to open said switch when the light source is energized and to close said switch when the light source is de-energized.
5. A smoke detector of the type utilizing photo-electric detection of light reflected from smoke particles, in which a pulse generator pulses a light generating device, a photo-voltaic cell is positioned to produce voltage pulses from the pulsed light reflected from smoke particles, first means for amplifying said voltage pulses, and for producing an output signal when the pulses are above a predetermined magnitude and means for utilizing said output signal to control a signalling device, said pulse generator and said first means being supplied by separate leads from a common power source, second means rendering said amplifier operative substantially only during the time the pulse generator is on, a storage capacitor connected across the leads to the first means and an impedence device disposed between the storage capacitor and the power source, where-by when the pulse generator produces a pulse, the first means draws power principally from the storage capacitor, and the im-pedence means isolates the first means from voltage transients at the power source due to the turning on of the pulse generator and the light generating device.
6. A smoke detector as set out in claim 5 in which said impedence means comprises an inductance having electrical chara-cteristics such that it prevents passage of voltage pulses in the frequency range to which the amplifier is responsive.
7. A smoke detector having a light source, means energizing the light source intermittently, photo-responsive means positioned to receive light reflected from smoke particles illuminated by the light source, and amplifier means for amplifying voltage pulses resulting therefrom to the light source and to the amplifier by first and second power leads, first and second capacitors associated with said first and second power leads, respectively, and means between each capacitor and the power supply for isolating said capacitors from transient voltages occuring at the power supply during the period that the light source is emitting light so that during said period the voltages on said first and second capacitors remain substantially equal.
8. A smoke detector as set out in claim 7 in which said means for isolating from the power supply the capacitor associated with the power lead to the amplifier includes a switch in said lead and means for opening and closing said switch with energiz-ation and de-energization of the light source.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA295,627A CA1067175A (en) | 1973-11-26 | 1978-01-25 | Smoke detector |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US41920673A | 1973-11-26 | 1973-11-26 | |
| US449362A US3917956A (en) | 1974-03-08 | 1974-03-08 | Smoke detector |
| CA207,336A CA1056931A (en) | 1973-11-26 | 1974-08-19 | Smoke detector |
| CA295,627A CA1067175A (en) | 1973-11-26 | 1978-01-25 | Smoke detector |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1067175A true CA1067175A (en) | 1979-11-27 |
Family
ID=27425788
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA295,627A Expired CA1067175A (en) | 1973-11-26 | 1978-01-25 | Smoke detector |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1067175A (en) |
-
1978
- 1978-01-25 CA CA295,627A patent/CA1067175A/en not_active Expired
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