CA1113570A - Battery powered smoke detector - Google Patents

Abstract

A B S T R A C T
A scatter type of smoke detector wholly supplied by a dry cell battery includes a clock circuit applying energy pulses to an LED light source which directs light pulses on a smoke sensing path. Light pulses scattered by smoke generate detection pulses in a photo diode whose a amplitude is dependent on the smoke density. The clock pulses and detection pulses are applied to a control circuit including a dual, data-type flip-flop logic circuit and a threshold circuit driving an alarm horn. If the smoke density, and hence the detection pulse amplitude, exceeds a predetermined level the control circuit energizes the alarm in a continuous mode. If the battery drops below a preselected level the control circuit is actuated in an inter-mittent mode.

Description

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Smoke detecto~s powe~ed ~rom an ~lternating current line are inherently subject to failure if the line loses power. Battery powered detectors avoid line failure problems, have a naturally simple power supply which is adequate to energize LED light sources, and for these and other reasons are becoming desirable and advantageous. Battery life can be pro-longed by energizing the light source periodically for short times by apply-ing energy pulses to the source as shown in United States patent No. 3,846,773 which issued on November 5, 1974 to Messrs Lintelmann and Frey. However, pulsing the light introduces the problem of distinguishing pulses of light scattered by smoke from pulses induced in the detector circuitry by electric-al surges or noîse spikes in external wiring or the environment. An addition-al problem is that a single induced pulse might trigger a false smoke alarm which cannot thereafter be terminated by the detector circuitry.
Accordingly one purpose of the present invention is to discrimin-ate between noise spikes and smoke detection pulses in a pulsed, battery smoke detector of the scatter type. A further object is to provide control `
circuitry for the alarm which will terminate a false alarm initiated by noise.
Yet another object is to employ a pulse generator both for energizing the light source and sounding an alarm in an intermittent mode distinct from that of the smoke alarm mode.
According to the invention, there is provided an electrical circuit for an optical detector of smoke or other fluid borne light scattering matter comprising: a source of pulsed light directed on a smoke sensing path, photo-electric means responsive to pulsed light to ~enerate electrical detection pulses, clock means periodically producing electrical clock pulses energizing the light source, and a control circuit coupled to the clock means and photo-electric means receiving clock pulses and detection pulses respectively there-from, wherein the photoelectric means responds to increasing scatter of light from the source by matter increasing beyond a predetermined density to gener-ate detection pulses of predetermined amplitude range, wherein the controlmeans includes means responsive to coincident receipt of said clock and detec-tion pulses to convert detection pulses within said predetermined amplitude .

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range into an alarm signal, the control circuit being responsive to sub-sequent clock pulses and a coincident detection pulse outside said predeter-mined amplitude range to terminate the alarm signal~ and wherein the control circuit includes a data-type flip-flop stage having a clock input for the clock pulses, a data input for the detection pulses and an output carrying a voltage level for each clock period corresponding to the amplitude range of the detection pulse at the beginning of the period.
The invention will now be described in greater detail with refer-ence to the accompanying drawing, in which Figure 1 is a schematic diagram of a battery powered, scatter type of smoke detector according to the invention.
Description General~Operation Battery Power Supply 1 Battery Monitor Circuit 8 Clock Circuit 2 Smoke Senser 4 Alarm 7 Logic Circuit 6 General Operation - Figure 1 An example of the invention is shown schematically in the single Figure wherein a clock 2 controls transmission of energy from a power supply 1 to light source 3. The primary object of the invention is to detect a change in the scatter of light from the source 3 to a senser 4 matched to the form of energy of the source. The light produced by the source 3 may be visible light or may extend into the infrared region and the senser 4 is a photoelectric device preferably responsive to the predominant wavelengths of the light source. Smoke scatters light from the source 3 to the senser 4 as fully described in United States patent No. 3,723,747 which issued on March 27, 1973 to Electro Si~nal Lab. Inc. The clock 2 controls transmission of periodic power pulses 11 of energy to the light source 3 and also to a logic circuit 6. When a change in ambient physical condition affects propagation ' -2-~.

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of energy rom the source 3 to the senser 4 the senser will detect the change by generating pulses 12 having the same repetition rate as the clock pulses 11. These condition detector pulses 12 are also applied to the logic circuit 6 in close synchronism with the clock pulses 11. The logic circuit is a dual, ~2a~

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data type flip-10p responsive to the two coincident input pulse trains ll and 12 so as to put out an actuating signal to an alarm 7.
The Figure shows a specific example of the invention embodied in a scatter type of optical smoke detector whose power supply 1 is a battery Bl. The battery supplies power to a clock 2 as well as to a light emitting diode (LED) D2 acting as light source 3, a photodiode D4 of the smoke senser 4, two flip-flops ~UlA and UlB) of a coincidence logic circuit 6, an alarm 7 with a horn Hl and a battery monitoring circuit 8. 120 microsecond duration clock pulses 11 occurring at 10 second periods cause the LED to emit short flashes of light in a dark chamber such as is shown in United States patent No. 3,863,076 which issued on Jan. 28, 1975 to Electro Signal Lab. Inc. In the absence of smoke, and also if the battery monitor 8 senses a predetermined adequate voltage across the battery Bl, the logic circuit transmits no actuating signal to the alarm 7. Smoke in the dark chamber will scatter light -to the photodiode D4 and trar.smit detection pulses to the coincidence logic circuit 6 at the same repetition "rate" as the clock pulses.
In response to such coincident input o clock pulse 11 and detector pulses 12 flip-flop UlA of the logic circuit continuously actuates the alarm circuit. But, if the battery monitor circuit 8 senses a drop in the battery voltage below a predetermined level indicating an impending battery failure, the monitor circuit will enable input of a clock pulse to the second flip-flop UlB of the logic circuit, which input causes the logic cir-cuit to initiate a trouble signal to the alarm 7 sounding the horn. The ~rouble signal is also fed to the flip-flop UlB back through a time delay network R13-C4 ending the trouble signal after a brief (e.g. 50 millisecond) period. The trouble signal thus sounds the horn momentarily every 10 seconds at the clock rate in a mode easily distingulshable audibly from the continuous sounding of the horn when smoke is detected.
~ Battery Power Supply 1 Figure 1 shows schematically one form o$ s~oke detector circuit il~13S`~ ~

suitable for use in a detector structure such as is shown in above mentioned United States patent No. 3,863,076. The power supply 1 comprises a 12 volt battery Bl such as a PoR~ Mallory ~ Co., IncO No. 304116 mercury cell having a positive (+) and negative (-) terminal. A 500 microfarad electrolytic capacitor Cl and a 100 microfarad electrolytic capacitor C2 store energy from the battery for quick release to other circuits. A diode D5 protects the circuits from damage by battery reversal.
Battery Monitor Circuit 8 The voltage supplied by the battery Bl is sensed by a battery monitor circuit. The battery rating is such that it may be expected to supply adequate current to the detector circuits for somewhat over 600 milliampere hours and then decline. A decline to between 10 and 11 volts indicates the beginning of battery end life. The bàttery monitor circuit 3 includes a 10 volt zener diode D3, resistor R8 (nominally 390 kilohms) resistor R9 (470 kilohms) and an NPN transistor Q3 ~2N3414~. The diode and resistors are in series across the battery terminals (I) and (~), the junction of resistor R8 and R9 being connected to the base b of the monitor transistor Q3. Resistor R8 may be adjustable or selected such that above a predetermined significant battery voltage ~e.g. 10.5 volts) the junction voltage holds the transistor Q3 conducting, so that pulses from the clock 2 at its emitter e are shunted to ground rather than being transmitted to the logic circuit 6 through a 470 kilohm resistor R7.
However, when the battery approaches end life by decline to the predetermined significant level (e. g. 10.5 volts), the voltage at the base b of the monitor transistor Q3 drops below cut-off for the transistor which ceases conducting and allows clock pulses to be transmitted to the coincidence logic circuit 6 as will be more fully explained.
Clock Circuit 2 The clock circuit 2 is an astable, asymmetrical multivibrator with two transistor stages, namely PNP transistor Ql (2N2907) and NPN transistor ~3~

Q2 (2N3704) whose period or pulse repetition rate of about 10 seconds is primarily determined by the timing o a 1 microfarad capacitor C3 and an 18 megohm resistor R2. Capacitor C3 is charged from capacitor C2 through the emitter e and base b circuit of transistor Ql (2N2907), diode Dl ~lN4454), resistor R4 (22 ohms) and the collector c to emitter e circuit of transistor Q2. With both clock transistors Ql and Q2 conducting a clock voltage pulse appears at the clock outputs CLl and CL2, and operating current is drawn by the LED light source D2. The 120 microsecond duration of each pulse is approximately determined by the time constant of the above described charging circuit.
Discharge of the clock capacitor C3 begins when its charge approaches the battery voltage, less other voltage drops in the charging path, and current through the transistor Ql drops. Current through resistor R5 (100 ohms) to the base b of transistor Q2 is then reduced and by regenerative action both transistors Ql and Q2 are abruptly cut off. This abrupt transition terminates the clock pulse and illumination of the LED.
The time constant of the 10 second interval between pulses, or its inverse the pulse repetition rate, is determined by the discharge path of the 1 microfarad capacitor C3 and 18 megohm resistor R2, and the small values of resistors R3 (330 ohms), R4 ~22 ohms) and R6 (7O5 ohms).
Smoke Senser 4 The smoke sensing circuit 4 properly includes an infrared LED
light source D2 (RCA Type SG lOlOA) which, however, is shown for simplicity above the clock 2. As is too well known to warrant detailed description, light ~rom the LED D2 is directed by lenses, barrels or masks on a path in a nearly dark chamber with smoke entrances. Smoke in the light path scatters the light to a predominantly infrared sensitive silicon photodiode D4 (Vactec, Inc., Type VTS 4085)o In a scatter type smoke detector, as compared with an obscuration type, light scattered to the photodiode or other cell D4 in-creases as the density of smoke increases. Such density in expressed in per-: . ., ... . : . - . . .. . - . :: ~ . .,: . , . - : .

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centage reduction in light intensity by one oot o~ smoke, or percent smoke, for short. Presently 2% smoke is broadly established as the predetermined density at which a smoke alarm should ~e given in residences, although alarm at somewhat lower levels is acceptableO
At less than alarm density smoke will scatter lower intensities of light. The anode of photodiode D4 is held by voltage divider resistors R21 (6.8 megohms) and R22 ~2.2 megohms) at a level suitable for input to a micro-power operational amplifier U2 which converts the pulsed current output of the photodiode D4 to a voltage pulse each time the LED D2 is pulsed.
Typical types and values of the operational amplifier U2 and its associated circuit components are as follows:

U2 Type CA 3078T
C6,8,9 ~ 11 0O047 microfara C7 0.022 "
C10 100 picofarad R14 100 kilohms R15 ~ 24 1 megohm R17 ~ 19 10 megohms R18 33 kilohms R20 2.2 kilohms R23 33 kilohms A voltage pulse proportional to smoke density and to ~he corres-ponding intensity of light scattered to photodiode D4 appears across a potentiometer R16 C50 kilohms) between the operational amplifier output U2-6 and the base b o voltage inverter transistor Q5 (Type 2N3414). Voltage pulses 12 at the collector c of transistor Q5 are minimal when the smoXe density is low, although shown in exaggerated amplitude in solid line. When the smoke reaches or exceeds predetermined density the voltage pulse 12 rather abruptly approaches its negative maximum amplitude ~broken line). As will be more ~ully explained under the caption Logic Circuit 6 a high amplitude detection pulse results in application by the logic circuit 6 of a constant voltage of corresponding amplitude to the alarm circuit 7. The amplitude of the detection pulse can be adjusted by the potentiometer R16 so that a detection pulse at or above a predetermined amplitude corresponding to smoke of a pre-determined density will cause an alarm.

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Alarm 7 If smoke is absent or at low density the constant voltage trans-ferred by the logic circuit 6 from the smoke senser 4 to the alarm circuit 7 corresponds to a smoke density below predetermined density, the transferred voltage will be below the threshold of the first transistor stage Q4 (2N3414) of the alarm circuit 7. Such a low voltage further divided by resistors Rll (56 kilohms), R12 (56 kilohms), R13 (750 kilohms) and an 0.047 microfarad capacitor C4 holds the transistor Q5 non-conducting, which in turn holds the second alarm transistor Q6 C2N3414) non-conducting. However, when smoke equals or exceeds the predetermined density and the negative going detection pulse 12 correspondingly exceeds the threshold voltage at which the first alarm transistor Q5 conducts, the second alarm transistor completes a circuit through the coil of a horn Hl sounding the horn.
The alarm circuit 7 also includes the following protective or noise suppressing components: -R10 15 kilohms R25 1.5 kilohms C5 0022 microfarads Logic Circuit 6 In the logic circuit the clock pulses 11 are continually applied through texminals Ca and Sb respectively to the Clock inpu~ C of the upper and lower logic sections UlA and ~lB of dual data-type flip-flops (RCA type CD 4013AE). Such dual data flip-flops are described in RCA solid State '74 Data Book SSD-2038, COS/~OS Digital'Integrated'Circ its, at pages 68 and 69.
The upper flip-flop UlA responds to each clock pulse 11 through terminal Ca to its clock input C by transferring the data or voltage level at its data input D in the same polarity to its output Q, or in the inverse polarity to its output Q*. When the smoke scattered light from the LED D2 to the photo-diode D4 is slightly below a predetermined density (e.g. 2% light obscuration per foot) the senser 4 supplies low, negative going pulses 12 dropping slight-ly (solid line) from the 12 volt positive level to the data input D of flip-flop UlAo Then when a clock pulse is concomittantly applied to the flip-flop ~ -_ 7 _ 1~35'Y~

clock input C the inverse, or relatively low negative voltage 13 (solid line) is transferred to the inverse output Q* of flip-flop UlA and maintained at that level until the data input level changes and a clock pulse recurs. Since, as previously explained with respect to the alarm circuit 7, the threshold of alarm transistor Q4 is not reached at low levels, Q4 and Q6 remain non- -conducting and the alarm horn Hl is not energized.
If~ in the case of an incipient fire, the smoke density is at or above the predetermined level, the senser 4 supplies a more negative going voltage detection pulse 12 (broken line) to the data input D of the upper flip-flop UlA. Accordingly at the coincident arrival of the next clock pulse the higher amplitude pulse will be transferred so that the flip-flop inverse output Q* will carry a voltage 13 (broken line) above the threshold of the alarm circuit 7, causing the horn Hl to be sounded continuously until the next clock pulse. If at the occurrence of the next clock pulse the data input level is a low amplitude pulse, a voltage 13 (solid line) below the alarm circuit threshold will be transferred to the output Q* of flip-flop UlA, and the alarm circuit will be shut off. Thus a continuous true alarm sounding of the horn Hl requires repeated coincidence of the clock signal and a detector signal corresponding to alarm threshold.
The above described requirement of repeated detection signals at a greater than alarm threshold level permits discrimination against spurious signals which can be induced in the detector circuit by transitory concen-tration of matter, flashes of ambient light, and particularly voltage surges in building wiring or the atmosphere. With the circuit of the present inven-tion, to produce the distinctive continuous alarm sounding of the horn Hl such spurious pulses must not only occur in the brief (e.g. 10 to 30 microseconds) rise at the beginning of the 120 microsecond interval of the clock pulse, -but they must also be of a polarity equivalent to reduction of light or detection pulse voltage and they must repeat exactly at the clock pulse repetition rate of once each 10 seconds. Even an unlikely single spurious 1~13S'7~
pulse occurring in the correct polarity and amplitude and exactly at the beginning of the clock pulse could actuate the horn for only one clock pulse period. At the subsequent clock pulse the absence of the spurious pulse would return the output Q* of flip-flop UlA to non-alarm level. The likelihood of two suitable spurious pulses occurring exactly during successive clock pulse rises is extremely small. The present detector does not latch in an alarm condition as the result of a spurious pulse. To avoid annoyance of a household user or lack of confidence in the smoke detector it is well worth- ;
while to discriminate against spurious pulses. And yet the present logic circuit does so simply, reliably and in a manner compatible with monitoring the bàttery power supply and indicating loss of battery voltage adequate for sounding a smoke alarm before the battery voltage becomes too low to warn of impending battery failure. Moreover impending battery failure is signalled by a trouble alarm easily distinguishable from the smoke alar~, and also persisting for a long period, over two weeks, after battery end llfe begins.
As previously explained under the caption Battery Monitor Circuit 8 the monitor circuit enables transmission of clock pulses from the base b of the monitor transistor Q3 along a trouble signal path including resistor R7 to the logic circuit 6. Specifically the input through terminal Sb to the set terminal S of the lower flip~flop Ulb sets this flip-flop with the 12 volt ~) level at its output Q. Assuming no smoke alarm is underway, the 12 volt output raises the base b of alarm transistor Q4 above its threshold, thereby causing t~ansistors Q4 and Q6 to conduct and sound the horn Hl. ~ut the rise to the 12 volt level at the output Q~ transmits a voltage rise through resistor R13 of the time delay network 9 whose time constant is primarily determined by the value of resistor R13 (750 kilohms) and capacitor C4 ~0.047 microfarad) at about 25 to 50 milliseconds. After this brief period the voltage rise is fed back to the clock terminal C of flip-flop Ulb, causing transfer of the constant zero or ground voltage at the data terminal D to the output Qb, thereby reducing the base voltage of alarm transistor Q4 below . . . .

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threshold and terminating sounding o the horn. The 25 to 50 millisecond duration of each trou~le pulse 14 is distinctly longer than each 120 micro-second clock pulse, but distinctly shorter than the 10 second clock pulse period. The intermittent sounding of the horn in the case of battery end li~e sounds a trouble signal for 25 to 50 milliseconds each ten second clock period is not only eas~ly distinguishable from the smoke but also uses the same dual flip-flop UlR ~ B alarm circuit 7, and derives its 10 second inter-val from the clock 2 which also energizes the LED D2.

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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electrical circuit for an optical detector of smoke or other fluid borne light scattering matter comprising: a source of pulsed light directed on a smoke sensing path, photoelectric means responsive to pulsed light to generate electrical detection pulses, clock means periodically producing electrical clock pulses energizing the light source, and a control circuit coupled to the clock means and photoelectric means receiving clock pulses and detection pulses respectively therefrom, wherein the photoelectric means responds to increasing scatter of light from the source by matter in-creasing beyond a predetermined density to generate detection pulses of predetermined amplitude range, wherein the control means includes means responsive to coincident receipt of said clock and detection pulses to con-vert detection pulses within said predetermined amplitude range into an alarm signal, the control circuit being responsive to subsequent clock pulses and a coincident detection pulse outside said predetermined amplitude range to terminate the alarm signal, and wherein the control circuit includes a data-type flip-flop stage having a clock input for the clock pulses, a data input for the detection pulses and an output carrying a voltage level for each clock period corresponding to the amplitude range of the detection pulse at the beginning of the period.

2. A circuit according to claim 1 wherein the control circuit includes normally non-conducting electronic switch means connected to the logic circuit and responsive to a voltage level output thereof to above a predetermined amplitude to conduct the alarm signal.

3. A circuit according to claim 1 or claim 2 including an electrical alarm.

4. A circuit according to claim 1 or claim 2 including a battery as the sole power supply for the circuit.

5. A circuit according to claim 1 wherein the alarm signal is con-tinuous so long as smoke of the predetermined density is sensed.

6. A circuit according to claim 1 including a battery supply therefor and a monitor circuit sensing the battery voltage and connected to the clock means to receive clock pulses therefrom, the monitor circuit being responsive to decrease of the battery voltage below a preselected level to cause an intermittent trouble signal at the clock repetition rate.

7. A circuit according to claim 6 including an electrical alarm connected to the control circuit and battery monitor circuit and producing a substantially continuous alarm in response to an alarm signal from the control circuit, and producing an intermittent alarm in response to the trouble signal.

8. A circuit according to claim 6 wherein the battery monitor circuit includes a normally conducting electronic switch normally forming a ground connection for the clock, a trouble signal conductor connected intermediate the clock means and monitor switch, and a voltage divider connected across the battery, the monitor switch having a control connected intermediate the voltage divider and responsive to decrease of battery voltage to cease con-duction and transfer the clock pulses to the trouble signal conductor.

9. A circuit according to claim 6 including a second data-type flip-flop producing trouble pulses at the clock repetition rate.

10. A circuit according to claim 9 wherein the second flip-flop has a set input connected to the trouble signal conductor and a time delay network connected between its output and its clock input, so that after a clock pulse initiates a pulse rise at the output, the pulse is applied with a time delay to the clock terminal thereby terminating the trouble pulse.

11. A circuit according to claim 10 wherein the time constant of the time delay network is of greater duration than each clock pulse and substan-tially shorter than the clock period.