IL45331A - Photoelectric smoke detector - Google Patents

Description

Photoelectric emoke detector CHLORIDE BATTERIJEH B.V.

C:-43373 ABSTRACT A smoke detector of the type utilizing photo-electric detection of light reflected from smoke particles, which is almost completely immune to false alarms from changing ambient light and random electrical noise, with a sensitivity that is independent of ambient light, with a power consumption low enough to permit battery operation for a period of over 12 months. The light source is a light emitting diode which is pulsed at a low repetition rate, such as one pulse every two seconds, by an extremely short pulse, such as 20 microseconds. Voltage pulses generated by the photo-generative cell when smoke is present in the housing are amplified and applied to a level detector, the output of which is applied to the "set" terminal of a flip-flop circuit. The amplifier is on continuously, however the level detector is pulsed to the on condition simultaneously with the on pulse to the light emitting diode, and for the same period of time. Simultaneously with the application of the pulse to the light emitting diode and the level detector, a shorter pulse is applied to the 're-set' terminal of the flip-flop circuit. The output of the flip-flop circuit may be applied through an integrator to an alarm energizing switch. The integrator has a time constant that is longer than the pulse time, so that more than a single pulse from the flip-flop must be applied thereto to activate the alarm energizing switch. The photovoltaic cell is capacitor coupled to the amplifier, so that constant or changing light, having a rate of change below that to which the amplifier responds, cannot affect the amplifier to cause a false alarm. Since the level detector is on only about 1/100,000 of the total time, a false alarm can be caused only by an extremely fast change in ambient light or a random noise pulse, that occurs at the exact instant the level detector is on, in two consecutive pulse times.

In one embodiment of the invention, to prevent power supply voltage changes from affecting the operation of the amplifier, a capacitor is provided across the power leads to the amplifier, and means is provided between the capacitor and the power supply for electrically isolating the amplifier and capacitor from the transient voltages from the power supply, which are caused by the turning on of the light generating device, or from other causes.

In one form of the invention said means comprises a choke coil in the line to the capacitor and a second choke coil in the line to the light generating device. In another embodiment of the invention the isolating means may be Zener diodes in place of choke coils. In a third embodiment of the invention the means comprises a switch in the power line between the capacitor and the power supply, with means being provided to open the switch when the light, generating device is energized. In each embodiment, the amplifier is isolated from transient voltage changes during the time the light generating device is energized, so that the pulse of energy required to power the amplifier is provided by the capacitor.

BACKGROUND OF THE INVENTION 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 detector 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 consumption 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 0: e 1 diffusion of air into the housing without allowing ambient light to enter, the 2 electrical circuitry must provide means for compensating for changes in line 3 voltage, and the photo-electric detectors must have a high degree of uniformity 4 and stability. Meeting these requirements adds considerably to the cost of 5 the device. 6 To avoid some of the above disadvantages it has been proposed to 7 utilize a flashing light source, such as a gas filled tube, to reduce the 8 current consumption. It has also been proposed to modulate the pulsed light 9 at a predetermined frequency and provide an amplifier that responds only to 0 said frequency. Such a system is illustrated in U.S. patent 3,316,410 issued 1 April 25, 1967. It has also been proposed that the means amplifying the 2 signal from the light sensitive element should be operative only while the 3 light source is on, so that ambient light changes or electrical disturbances 4 that occur during the period the amplifier is off cannot cause a false alarm. 5 However, in such a system, ambient light changes and electrical disturbances 6 that occur while the amplifier is on can nevertheless cause a false alarm. 7 Examples of ambient light changes that can affect a detector of this type are 8 flashlights, strong sunlight, turning on of room lights, camera flash bulbs, 9 and lightning. Hence the use of a pulsed light source and a pulsed amplifier 0 as shown in the prior art, although having the advantage of lower power 1 consumption, 'does little to reduce the possibility of false alarms, and hence 2 to avoid false alarms from such causes the sensitivity of the device must be 3 reduced. 4 It is often desired for certain installations that such devices be 5 powered from a battery, however one difficulty with a battery power source 6 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 is powered from the same battery, the voltage to the amplifier also drops, which creates a transient signal in the amplifier which may produce 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 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 energization 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 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.

SUMMARY OF THE INVENTION 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 detector, which occurs only when the input signal is above a predetermined 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, simultaneously, 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.

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 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, becaus 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 cycles, and such change would have to occur during the 20 micro-seconds that the pulse is being applied to the level detector. Similarly, random noise that might generate a signal at the amplifier input sufficient to produce an output high 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.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic diagram of an electrical circuit 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 detector; 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 Figure 1 in which Zener diodes are provided in the separate power leads to the amplifier and the light emitting device.

DESCRIPTION OF THE EMBODIMENT OF FIGURES 1-3 Referring to Figure 1, there is illustrated an electronic 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° from 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 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 a alarm actuating device K.

In a preferred embodiment of the device, the differential comparator is normally off with the signal lead thereof clamped to ground by an electronic switch SI.

Light emitting diodes presently commercially available are 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 diodes 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 SI. 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 discriminator D, which converts said pulse to a spike of about 1 micro-second duration, occuring at the beginning of the pulse cycle.

The operation of the circuit can best be understood 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 depending on the type of response, and the magnitude of the various curves on the vertical scale have no relation to each other except as described hereinafter.

Each cycle begins by the application of a pulse from the pulse generator to the LED, the level detector, and the flip-flop reset terminal. The pulse to the LED and the level detector are represented on the diagram by PI, 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 of the flip-flop after passing through the discriminator is represented by P2. The application of the pulse to the LED produces a light output having a duration and relative intensity represented by curve LI.

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 present in the pulsed light beam, a pulsed voltage signal will be produced by the cell, represented by curve VI 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 acceptable amounts of smoke and dust in the atmosphere, the differential 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 comparator 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 output comparator level required to permit the output signal to pass through the differential/ is represented by the horizontal dashed line S.

In a particular embodiment of the invention the differential comparator may have a standby voltage difference between input terminals of about 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 1035 ge 1 1 system so that the alarm point will be at the desired 2%. In the present 2 embodiment of the invention the calibration is accomplished by providing a 3 voltage divider Rl across the power source, with the junction thereof 4 connected to one of the inputs of the differential comparator, and providing 5 a variable resistor R2 across the power supply with the center tap thereof 6 connected to the other input. 7 If the amount of smoke in the view of the cell has reached the 8 specified concentration, the amplifier output will be as shown in curve Al 9 reaching the line S at point Y, thereby producing a differential comparator 10 output represented by line LD1, which applies a signal to the flip-flop set 11 terminal, thereby turning on the flip-flop output (FF1 on Figure 2) to 12 energize the alarm. 13 At the end of the pulse to the LED and the differential comparator 14 both turn off so that the output from the differential comparator to the 15 flip-flop is turned off. The flip-flop output, however, stays on until the 16 beginning of the next pulse, at which time it is turned off by the pulse 17 through the discriminator in the manner previously described. 18 As a greater concentration of smoke appears in the view of the eel 19 more reflected light is received by the cell, and the output voltage of the 20 pulses applied to the amplifier increases, so that the amplifier output 21 increases and reaches the required level S slightly sooner in the pulse cycl 22 as illustrated by curve A2, providing differential comparator output LD2 and 23 flip-flop output FF2. 03! e ' 1 Although the amplifier may continue to provide an output for a short 2 time after^ the end of the pulse to the LED and the differential comparator, 3 no output can exist after the end of the pulse, because the differential 4 comparator is de-energized and the signal lead thereof clamped to ground by 5 switch SI. 6 A smoke detector utilizing the above described circuit has a number 7 of advantages over detectors of the prior art that have utilized a pulsing 8 light source and a pulsed amplifier. By the use of a pulse of very short 9 duration with a slow repetition rate, an amplifier with a response only to 10 very high rates of change of input voltage, and the use of a pulsed level 11 detector after the amplifier, the occurrence of false alarms due to changing 12 light levels or due to electrical transients is almost completely eliminated. 13 A change of light level that could actuate the alarm must not only 14 occur at an extremely high rate, but its occurrence must coincide with the 15 time in which the level detector is on, which is only 1/100,000 of the total 16 time. 17 For example, the turning on of an incandescent light cannot cause 18 a false alarm, since the rate of rise of the light output from an incandescent 19 bulb is much to slow to create a voltage pulse that can pass through the 20 capacitor. Although the resulting increase in ambient light will increase 21 the D.C. voltage at the cell terminals, subsequent pulses of light falling on 22 the cell will cause the cell to generate an output voltage pulse on top of the 23 D.C. voltage (assuming that the ambient light is not so strong as to saturate the cell) which will be detected by the amplifier. ι35 ! 13 1 This effect is illustrated in Figure 3 where curve Va represents 2 the voltage at the cell due to ambient light level and Vp represents the cell 3 voltage during the period that the LED is illuminated, with 2% smoke present. 4 Since the response of the cell is substantially linear, the sensitivity of the 5 device is not affected by changes in ambient light, since the pulse voltage at 6 27o smoke remains the same, regardless of the ambient light leve, provided 7 that the ambient light level is not so high as to cause saturation of the cell. 8 In the curve of Figure 3, the relative height of the ambient light voltage 9 curve and the height of the voltage pulses are necessarily not in proportion, 10 since the D.C. voltage from ambient light may be on the order of .1 volts 11 whereas the additional voltage generated by the pulse of light reflected from 12 smoke particles, at 27» smoke, is only about 600 microvolts. 13 Although certain light sources, such as lightning, some types of 14 camera flash equipment, and welding apparatus may produce light with a rise 15 time fast enough to be amplified and reach the level detector, such resulting 16 signal not only must be great enough to satisfy the level detector requirements 17 but also must occur during the 20 micro-seconds that the level detector is on. 18 The chance of a false alarm from such a source is therefore extremely remote. 19 In regard to possible false alarms from random electrical signals 20 generated in the cell from radio transmit ers, transients on the power supply 21 line, and the like, not only must such signals occur at the proper instant 22 and generate a signal of adequate magnitude, the signal appearing at the 23 amplifier input must be of the proper polarity. 03: e '. 1 In the illustrated embodiment of the invention, the signal from the 2 flip-flop is led to an integrator T, comprising a resistor-capacitor network, 3 which integrates pulses received from the flip-flop to provide an output 4 signal to the alarm energizing device R. In one embodiment, the integrator 5 may have a time constant which is at least slightly greater than the total 6 time between pulses, so that two pulses from the flip-flop are required to 7 reach an output level from the integrator to actuate the alarm energizing 8 device. 9 Although the use of the integrator may not be required in all 10 installations in which the smoke detector is used, it has been found effective 11 in preventing false alarms in locations that are near sources of continuous 12 noise, such as might be produced by arcing electrical apparatus. 13 Another major advantage of a smoke detector utilizing the circuit 14 disclosed herein is its extremely low power consumption. Although the pulse 15 to the LED may be of the order of 7 amperes, the short duration of the pulse, 16 and the fact that the. level detector is on only during the pulse permits a 17 power consumption of the order of 300 micro-amperes at 6 volts. This power 18 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.

DESCRIPTION OF THE EMBODIMENT OF FIGURES 4 - 6 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 about 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 device K.

In a preferred embodiment of the device, the differential comparator is normally off with the signal lead thereof being clamped to ground by an electronic switch SI.

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 discriminator 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 V 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 pulse generator and the power source.

In the embodiment of Figure 3, the pulse generator, in addition to the functions previously described, also applies a pulse to the switch1 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: As in the above identified co-pending application, the duration of the pulse which energizes the LED is very short, for example, 20 micro-seconds compared to the repetition 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 therefore, at the instant that the LED is turned on, disconnected from the power supply, and the energy necessary to operate the amplifier comes only from the capacitor Fl. lo: ge 1 As described hereinbefore in connection with the embodiment of 2 Figures 1-3, pulses of light are reflected therefrom and fall on the photo-cell 3 C, causing a series of voltage pulses at the input of the amplifier. If the 4 amplified pulses are of sufficient magnitude to satisfy the requirements of the 5 level detector, a series of pulses are applied to the "set" terminal of the 6 flip-flop, which applies a series of pulses (since the flip-flop is re-set at 7 the beginning of each pulse) to the integrator, to actuate the alarm. 8 At the end of each pulse to the LED, the switch S2 closes so that 9 the capacitor can recover the small amount of charge used in powering the 10 amplifier and associated equipment. 11 In the particular illustrated embodiment, the current used to power 12 the LED may be of the order of 7 to 10 amperes. Although this current is 13 drawn for only 20 micro-seconds, it would nevertheless cause a sudden and 14 substantial drop in the voltage at the amplifier (unless the impedence of the 15 voltage source is so low that it would be impractical for a commercial 16 installation). Without the presence of the switch S2 and capacitor CI, a 17 substantial drop in supply voltage to the amplifier would occur. Such drop 18 in voltage would cause a sudden change in bias voltage in the amplifier, 19 which, would be interpreted by the amplifier as a signal, which could either 20 cause or negate an alarm (depending on the phase relationship of the amplifier 21 being used), since the change in supply voltage of such a magnitude could 22 cause an output signal from the amplifier which is many times the output 23 signal that would be created by a signal resulting 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 negative pulse caused by the sudden drop in power supply voltage. Since the power to the amplifier is itself a high frequency pulse which could not pass through the choke coil CKl, 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 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 6 includes Zener diodes Zl and Z2 connected between lines Wl and W2 respectively and ground, with resistors Rl and R2 being in series in lines Wl and W2 between the connection to the Zener diode and the power source V2. in the circuit of Figure 6, the power source V2 is higher than necessary to operate the amplifier and the pulse generator, and the \m1 h. no n o v ααι ι 1 nted to t llQ r -nc l" amou n t" hv f h o 7 hen 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 particular 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 2 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.

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 combinations 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 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 draws from 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 inappreciable.

Although in the illustrated embodiment, the amplifier is energized continuously and the level detector is normally de-energized and energized only when the light emitting diode is emitting light, it will be understood that if desired the amplifier may be normally off, and energized only during some part of the period that the light emitting diode is energized. Hence in the claims, when the term amplifier is used, it is intended to include the components of Figure 4, labelled A, L, and FF, unless the context clearly indicates otherwise. 45331-2

Claims (25)

1. A detector comprising a light source arranged to be ·· · · ■. ·. · ' · " intermittently energized, means for producing a signal pulse in response to the light from the source under predetermined conditions, a level detector having an input connected to the output of the pulse producing means and an output for connection to a signalling device, the level detector being arranged to produce an output signal only in response to an input signal pulse above a predetermined value, and means rendering the level detector incapable of producing an output signal when the light source is de-energized and rendering the level detector capable of producing an output signal only when the light source is emitting light.

2. A detector as claimed in Claim 1 having a bi-stable switching device connected to the level detector output and having an output for connection to the signalling device, the bi-stable switching device being normally in a first condition in which the signalling device is not actuated and being responsive to a pulse from the level detector to shift to a second condition in which the signalling device is actuated, and means periodically returning the bi-stable switching device to the first condition.

3. A detector as claimed in Claim 2 having means for applying simultaneously a periodic pulse to the light source to cause it to emit light, to the level detector to render it capable of producing an output signal and to the bi-stable switching device, the periodic pulse to the bi-stable switching device being of substantially shorter duration than the periodic pulses to the light source and the level detector and being so applied to the bi-stable switching device as to Insure that it is in the first condition during an initial portion of each periodic pulse to the light source and the level detector, 45331- to the bi-stable device in response to an input signal pulse will shift the bi-stable switching device to the second condition after the termination of th periodic pulse to the bi-stable device.

4. Ά detector having a light source arranged to be intermittently energized, means responsive to light from the source unde predetermined conditions to produce an energy pulse, a level detector connected to the output of the light-responsive means and arranged to produce an output signal in response only to an input pulse above a predetermined value, a flip-flop circuit having set and reset terminals and an output for connection to a signalling device, the level detector output being connected to the set terminal of the flip-flop circuit, whereby the flip-flo output is turned on when the level detector produces an output signal, and means periodically applying a signal to the reset terminal of the flip-flop, to turn off the flip-flop output.

5. A detector as claimed in Claim 4 in which the level detector is normally incapable of producing an output signal and is arranged to be rendered capable of producing an output signal only when the light source is emitting light.

6. A detector as claimed in Claim 4 in which the level detector is normally de-energized and is arranged to be energized only hen the light source is emitting light.

7. A detector as claimed in Claim 4 in which the signal lead of th level detector is normally connected to earth so as to be incapable of producing an output signal to the flip-flop circuit and is arranged to be disconnected from earth only during the time that the ligh source is emitting .light.

8. A detector as claimed in any one of Claims 4 to 7 ^ having means between the flip-flo circuit and the signalling device for Integrating the voltage received from the flip-flop circuit, the integrating means having electrical parameters such that the duration of the signal applied thereto required to produce an outpu to the signalling device is longer than the time between the initiatio of the consecutive pulses whereby a signal resulting from a single pulse is insufficient to actuate the signalling device.

9. A detector as claimed in any one of Claims 4 to 8 having means for applying a pulse to the flip-flop circuit reset terminal after each pulse of the light source.

10. A detector as claimed in any one of Claims 4 to 8 having a pulse generator for simultaneously applying a pulse to the light source to cause the source to emit light, to the level detector to render it capable of producing an output signal, and to the reset terminal of the flip-flop circuit to turn off the flip-flop output, the pulse to the flip-flop circuit being of shorter duration than the pulse to the light source·

11. A detector as claimed in an preceding claim in which the level detector is a differential comparator and means is provided at the input thereof for adjusting the voltage input required to produce an output therefrom.

12. A detector as claimed in any preceding claim having an amplifier between the light responsive means and the level detector, the amplifier having a frequency response such that it is capable of achieving a steady output signal during each energization of the light source. 45331-2

13. A detector as claimed In any one of Claims 1 to 11 in which an amplifier between the light responsive means and the level detector has a frequency response between 1000 and 100,000cycles, and the pulse duration Is not appreciably longer than required for the amplifier to achieve a constant output in response to an input pulse from the light responsive means*

14. A detector as claimed in any preceding claim having an amplifier for amplifying the energy pulse, a power input for a power 3ource to energize the amplifier, a capacitor connected across the power input, and isolating means between the capacitor and the power input for isolating the amplifier and the capacitor from voltage pulses occuring at the power input at least during energization of the light source.

15. A detector as claimed in any preceding claim in which the light source is a light emitting diode.

16. A detector as claimed in any preceding claim for use as a smoke detector in which the light responsive means comprises a photo-responsive device positioned to receive light from smoke particles illuminated by light from the source.

17. detector according to any of; the previous claims and also comprising a voltage supply for said pulse producing means, a capacitor connected across the voltage supply, and Intermediate means connected between the capacitor and the voltage supply for isolating the amplifier and capacitor from voltage pulses occuring at the voltage supply at least during the time that the light source is emitting light. 45331-2 »

18. A detector as set out in Claim 17 in which said intermediate 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.

19. A detector as set out in Claim 17 in which means are provided for energizing said amplifier only while the light source is emitting light, and said intermediate means allows current flow to charge the capacitor while the amplifier is de-energized but does not allow a voltage pulse to pass therethrough during the period that the amplifier is energized, and whereby the amplifier draws power substantially only from the capacitor when it is energized.

20. A detector as set out in Claim 17 in which said intermediate 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.

21. A detector according to any of the previous claims 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, and comprising means for amplifying said voltage pulses, and means for utilizing said amplified pulses to control a signalling device, said pulse generator and said amplifier being supplied by separate leads from a common power source, means rendering said amplifier operative substantially onl during the time the pulse generator is on,, a stqrage capacitor connected across the leads to the amplifier and an impedence device disposed between the storage capacitor and the power source, whereby when the amplifier is turned on, it draws power principally from the storage capacitor, and the impedence means isolates the amplifier from voltage 45331-2

22. A detector as set out in Claim 21 in which said impedance means comprises an inductance having electrical characteristics such that it prevents passage of voltage pulses in the frequency range to which the amplifier is responsive.

23. A detector according to any of Claims 17-22 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, a power source, means providing power therefrom to the light sourc and to the amplifier by first and second power leads, first and second capacitors associated with each power lead, and wherein said Intermediate means between each capacitor and the power supply are operative 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 secon capacitors remain substantially equal.

24. A smoke detector as set out in Claim 17 in which said means for isolating from the power supply the capacitor associated with the power lead to the ampli ier includes a switch in said lead and means for opening and closing said switch with energization and de-energization of the light source.

25. A smoke detector substantially as herein described with reference to the accompanying drawings. For Hie Applicants DR. REINHOLO COHN AND PARTNERS By : 0 "VI