Smoke and heat detector incorporating an improved smoke chamber

Abstract

A novel smoke and heat detector incorporating an improved smoke chamber. The smoke detector includes a radiation detector mounted orthogonally to a radiation source emitting a beam of non-visible radiation through the smoke chamber. The radiation detector measures the radiation scattered by smoke particles in the chamber, initiating an alarm whenever a dangerous level of smoke is detected. A self-test circuit is provided for electronically simulating a predetermined concentration of smoke to test the radiation detector circuitry and the alarm. The alarm is also enabled by a fault detection network if the radiation source becomes non-operative. The smoke chamber includes a novel deflector apparatus for channeling air through the chamber in a substantially vertical direction and through the point where the optimum detection angle of the radiation detector intersects the incident radiation beam. Moreover, a high-temperature, low-heat source is positioned near the top of the chamber to establish a large temperature differential between the ambient input air and air leaving the chamber, maximizing air flow through the chamber.

Description

This invention relates to smoke and heat detector apparatus and in particular to such apparatus where smoke warning or alarm is provided after detecting dangerous levels of smoke particles.

Reference may be made to the following U.S. Pat. Nos.: 3,504,184; 3,505,529; 3,534,351 3,430,220; 3,555,532; 3,579,216; 3,585,621; and 3,659,278.

A variety of smoke detecting devices have been used for many years in which visible light emitting means are situated so as to direct visible light through a smoke chamber. If there are smoke particles present in the chamber the radiation is scattered by the smoke particles and detected by a photocell for initiating an alarm. Such smoke detection systems using visible radiation have encountered several problems in use. In particular, the incandescent filament lamps generally used as the source of visable light, radiate heat which often causes problems with various heat sensitive components in the system. Moreover, incandescent lamps are themselves prone to failure as a result of heat build-up with in the lamp or due to mechanical shock such as that resulting from careless handling of the device. In addition, as the incandescent lamps ages, its efficiency and therefore its output radiation level drops off so that the sensitivity of the system is always changing and finally becomes too low for reliable operation. Another often annoying problem is due to the attractiveness of the visible light from incandescent lamps to various bugs which collect in the smoke chamber, scatter the light and set off numerous false alarms. Many of the above problems can be partially solved with increased maintenance, however, this is economically prohibitive in systems involving hundreds of such detectors. Even in single unit usage, such as in homes, the improbability of the required maintinance being performed often makes these units impractical.

It has therefore been suggested to use light emitting diodes (LED's) instead of incandescent lamps in smoke detector systems. It is currently possible using available semiconductor light emitting diodes to obtain a long lasting, reliable unit with minimum maintinance. However, it is not easy to determine whether the radiation source is operating properly when such detectors are normally mounted in locations which are not readily accessible. Standard fail-safe techniques used for incandescent filament type lamps require several additional components, are expensive, and do not lend any assistance in solving this problem since only an open circuit lamp condition needs detection. With the LED, the diode may be opened or shorted, and in either case some indication must be given to prevent erroneous and possibly dangerous reliance on an inoperative smoke detector unit. Moreover, prior art self-test arrangments have been limited to testing the power supply and the alarm without checking the critical detection circuits.

Prior art smoke detectors have commonly included a smoke chamber for continuously channeling room air past a smoke detection arrangement to detect dangerous concentrations of smoke within the room. To improve air flow through the chamber, several prior art systems have utilized heat sources, such as incandescent lamps or power resistors, to warm the air so that a convection draft is created. However, as heat is radiated throughout the chamber, the air at the input is warmed, and the temperature differential between air entering the chamber and air leaving the chamber decreases. This, in turn, actually reduces air flow through the chamber. Moreover, the room air may be heated by a fire, reducing air flow through the chamber, before a sufficient level of smoke is developed to trigger an alarm. In fact, it is possible that the first alarm may be initiated only after the heat sensor is enabled.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, there is provided a smoke particle detector utilizing a radiation emitting semiconductor device as a radiation source to transmit a non-visible radiation beam and a radiation detector to receive scattered radiation and trigger an alarm when dangerous levels of smoke particles are present. The radiation source and detector have matching spectral radition characteristics so as to greatly increase the detection efficiency. The complementary detection circuit further enables overall system efficincy to be maintained fairly constant with a minimum of maintenance required. Following detection of scattered radiation, a comparator circuit is triggered to lock up an alarm thereby indicating the presence of dangerous smoke levels.

Relatively inexpensive fault detection apparatus also is included to detect either an open circuit or a short circuit condition of the semiconductor light emitting diode and to indicate an alarm. The fault detection apparatus operates directly into the system comparator circuit and alarm lock-up apparatus and does not require an additional power sources for operation.

Self-test apparatus is further included to provide a self-contained test capability for electronically simulating the effect of dangerous levels of smoke. The entire smoke detection portion of the apparatus can be checked, resulting in alarm activation if the components are in proper operating condition.

The smoke particle detector also includes a novel smoke chamber having a high-temperature low-heat source for maximizing air flow through the smoke chamber over a wide range of temperatures by maintaining a large temperature differential between incoming and outgoing air. The smoke chamber is further effective to channel the air flow through the chamber at the point where the incident radiation beam is optimumly scattered to the radiation detector.

The resulting smoke detector constructed in accordance with the present invention thus utilizes desirable infrared radiation and a novel smoke chamber for smoke particle detection, and includes inexpensive components in providing required fail-safe and self-testing features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic block diagram of smoke and heat detector constructed in accordance with the principles of the present invention;

FIG. 2 illustrates a schematic circuit diagram of the electronic components in a smoke and heat detector according to FIG. 1;

FIG. 3 illustrates an alternative embodiment of a fail-safe feature;

FIG. 4 is a perspective view of a novel smoke chamber used in conjunction with the smoke and heat detector shown in FIGS. 1 and 2;

FIG. 5 is a top plan view of the smoke chamber shown in FIG. 4;

FIG. 6 is a partially cut-away front view of the smoke chamber shown in FIG. 4 illustrating the internal features thereof; and

FIG. 7 is a sectional view of the smoke chamber taken along lines 7--7 in FIG. 6.

DETAILED DESCRIPTION

Referring now to FIGS. 1 and 2, there is illustrated a smoke and heat detector 10 in which non-visiable infrared radiation is scattered by smoke particles, detected and amplified to energize an alarm. The smoke and heat detector 10 includes an infrared radiation source 12 emitting non-visible radiation directly towards infrared radiation absorber 14 at one end of the smoke chamber 16. If there are smoke particles present in the chamber 16, the non-visible radiation is deflected and scattered by such smoke particles as shown by the dashed lines 18 to infrared radiation detector 20 which is mounted transversely to the direct non-visible infrared beam.

Infrared detector 20 provides a detected signal to a comparator 22 which compares the signal level from detector 20 with the incoming signal level from a voltage reference 24. The comparator 22 is set at a threshold level related to voltage reference 24 so that the detector will be sensitive to smoke caused by burning materials common to homes and offices, but will not be accutely sensitive to usual levels of cigarette, cigar or pipe smoke. Thus the system will only be activated by dangerous levels of smoke to provide a signal through lock-up network 25 to the alarm 26. A reset switch 28 is provided for overriding the lock-up network 25 and shutting off the alarm 26 as desired. In addition to smoke detection, the device includes an indepenedent heat sensor 30 for detecting dangerous levels of heat above 135.degree. F. so as to actuate the alarm 26.

A fail-safe feature is included through fault detection network 34 for indicating a non-operating radiation source 12 without requiring additional expensive power sources and components. Provision is also included in the detector 10 for initiating a self-test of several components in the detector. As is shown in FIG. 1, a test switch 32 when operated places a signal into infrared radiation detector 20 of an amplitude just sufficient to inititate alarm 26. This self-testing procedure thus checks the radiation detector, comparator, lock-up network, alarm and AC power source.

Referring now to FIG. 2, there is illustrated a schematic circuit diagram of a preferred embodiment of the smoke and heat detector 10. The infrared radiation source 12 comprises a radiation emitting device such as a light emitting semiconductor device emitting short wavelength infrared radiation (IR-LED) in the smoke chamber towards the radiation absorber 14. Short wavelength infrared radiation is utilized rather than long wavelength infrared because it does not generate any appreciable heat. Moreover, because bugs are not attracted by infrared radiaiton, the smoke detector system of the present invention is not as susceptible to false alarms. It should be understood, however, that light emitting diodes producing radiation at wavelengths other than in the infrared region may also be used.

The radiaiton absorber 14 absorbs substantially all of the direct radiation from the IR-LED 12, so that any infrared radiation 18 at detector 20 is due to scattering from smoke particles in chamber 16. Thus, in the presence of smoke particles, some of the direct infrared radiation is scattered and detected by infrared detector 20 mounted in the smoke chamber at right angles to the IR-LED 12. The detector 20 comprises a photoconductive field effect transistor (FET) having gate, drain and source electrodes labeled G, D, and S as indicated on FIG. 2.

Comparator transistor 40 includes an emitter element coupled to reference point 42 at one end of Zener diode 44. The other end of Zener diode 44 is connected back to back with a silicon diode 46 in a temperature compensated arrangement. Zener diode 44 and diode 46 may be replaced by a tapped temperature compensated diode or a bipolar transistor wherein the base-emitter junctions serves as a Zener diode and the base-collector junction serves as a silicon diode. Reference point 42 is maintained at about 6.2 volts by a 10 volt Zener diode 48 maintaining junction point 50 at 10 volts supplied from the rectified AC power source 52. Dropping resistior 54 drops the voltage from 10 volts at junction point 50 to the reference point 50 to the referecne 6.2 volts at reference point 42.

Resistors 58 and 60 comprise a voltage divider network across the 6.2 volt reference to properly bias the photodector FET 20. The photodetector FET 20 typically needs from 1 to 4 volts bias and this is provided by coupling its gate element G through resistor 56 to the voltage divider. Resistor 99 is coupled between the source element S and said reference point 42 to provide source degeneration thereby improving the linearity of the photodetector FET 20. Further, the drain current is adjusted to provide an essentially zero temperature coefficient. If there is no scattered infrared radiation impressed on photodector FET 20, the drain element D is at about 13 volts.

Alternatively, the photoconductive FET 20 can be replaced by a bipolar transitor if the biasing arrangement is appropriately revised. Of course, the self-temperature compensation feature of the photoconductive FET 20 would not then be available.

The drain element D is coupled to the base of a common emitter DC amplifier 62, DC coupled with source degeneration through Zener diode 64. The source degeneration decreases the gain of amplifier 62 and linearizes it at the same time. The pupose of the 5.6 volt Zener diode 64 is to give a voltage level shift such that the DC amplifier can be DC realistically. In addition Zener diode 64 is a positive temperature coefficient device of approximately 0.035% per.degree.C. Bipolar transistor 62 exhibits a negative temperature coefficient at the base-emitter junction of approximately the same value. Thus diode 64 and the bipolar PNP transistor 62 provide a DC amplifier with very linear gain characteristics and almost a zero temperature coefficient. Zener diode 64 and bipolar transistor 62 may be thermally bonded to each other if a high degree of compensation is desired over large temperature ranges.

The collector of the transistor 62 is connected to a series combination of two resistors 66 and 68 such that when there is no smoke in the chamber 16 there is approximately a four volt drop across these two resistors at point 70. Connection point 70 is also tied to the base of comparator transistor 40. When smoke is present in the chamber and some of the energy from the infrared light emitting diode 12 is scattered in the direction of the FET 20 the voltage at point 70 is changed from about 4 volts towards 6.7 volts, which is enough to trigger on transistor 40. The comparator transistor 40 is turned on, thus also turning on transistor 72 and drawing operating current through a relay solenoid coil 74. Normally open relay contacts 76 then close to operate buzzer alarm 78. Other alarm, warning or indicating devices may instead be operated as desired.

It may be noted that once smoke is detected and the alarm 78 energized, continued smoke need not be present in the chamber to keep the alarm energized. A locking circuit is provided by diode 80 which conducts and connects a positive voltage to point 70 at the input to the comparator transitor 40, forcing this transistor to remain on. The collector electrode of transistor 40 is coupled to the anode of diode 80 through the collector circuit of transitor 72 thereby providing a positive feedback path. If there is no longer any smoke in the chamber, the system can be reset by depressing momentary reset switch 82 to short out the input to the comparator and thereby turn off the alarm. However, alarm turn off will only be momentary if there is still smoke present in chamber 16, since upon release of the reset switch the comparator transitor 40 will again be triggered to energize the alarm.

An automatic resetting heat sensor 84 is also provided to operate at approximately 135.degree. F. and directly connect buzzer alarm 78 to an operating voltage. Thus, whenever the temperature of the surrounding atmosphere is greater than 135.degree. F., an alarm will be given, regardless of the smoke detector components. The alarm 78 will automatically be shut off when the ambient temperature drops below about 130.degree. F.

A distinct feature of the present invention is the initiation of a fail-safe procedure in the event that the light emitting diode 12 is no longer operating. In the event of a failure of the diode 12, a fault detection network 34 operates to activate the comparator transistor 40 so as to turn on the alarm. It must be realized of course that the use of an extremely reliable light emitting diode 12 as opposed to the usual incandescent, visible light emitting devices of the prior art makes it extremely remote that the infrared radiation source (i.e., LED 12) would fail. However, the radition source is such a very important component of the system that it is extremely desirable to provide an alarm should this component fail. As opposed to prior art techniques utilizing separate expensive power sources and extra components for the fail-safe procedure, the present invention instead utilizes diodes 90 and 92 as a fault detection network between the infrared light emitting diode 12 and the comparator 22.

Referring now to FIG. 2, it can be seen that the diode 90 has its anode element connected to one end of the infrared light emitting diode 12 at reference point 94. The cathode end of diode 90 is connected to reference point 70 at the input to the base element of comparator transistor 40. Assuming that the infrared light emitting diode 12 has become open circuited, reference point 94 will rise to about 9 volts. This forward biases diode 90 and being coupled into reference point 70 drives comparator transistor 40 on. When transistor 40 is triggered, transistor 72 is triggered, and the alarm is given in the same manner as if smoke had been detected in the smoke chamber 16.

If the infrared light emitting diode 12 becomes shorted, diode 92, having its cathode connected to reference point 94 and its anode connected to reference point 95 intermediate silicon diode 46 and Zener diode 44, is forward biased to drive the photo FET 20 and activate the alarm circuit as if smoke had been detected. This operation is provided as follows. Under normal circumstances, about 6.2 volts are present at reference point 42. In addition, silicon diode 46 is normally forward biased so that there is about 0.6 volt at reference point 95 which results from the normal forward bias voltage drop for silicon diodes. On the other hand, diode 92 is a germanium diode which normally exhibits a much lower forward bias voltage drop of about 0.2-0.3 volts as compared to the silicon diode 46. Thus, if we assume that the infrared light emitting diode 12 is shorted, this forward biases germanium diode 92 and drops the votage at reference point 95 from about 0.6 volts to about 0.3 volts. This reduces the voltage at point 42 from the normal 6.2 volts to about 5.9 volts, which represents about a 5% change in voltage at reference point 42. Since the gate bias voltage for the photo FET 20 is supplied through resistors 56, 58, 60, this same percentage of voltage change is reflected back to the gate bias voltage to turn on the photo FET 20. Transistor 62 then turns on raising the voltage at reference point 70 to trigger on the comparator transistor 40 and turn on the alarm through transistor 72 as in normal smoke detection.

In either case, if the light emitting diode 12 is not operating because of being opened or shorted, comparator transistor 40 is constantly turned on so that the alarm will always be activated. Thus, even if the operator momentarily depressed reset switch 82 to momentarily short out the input of transistor 40 and cut off the alarm, the alarm would go back on again when the momentary push button switch 82 was released. The constant activation of alarm 78 reminds the user and thus encourages him to disconnect the smoke detector from the main power line so that it will not be relied upon to preform its normal functions until the infrared diode 12 is replaced and the units can thereafter be put back into service.

The aforementioned fail-safe technique is particularly useful for home installations of the smoke detector 10. The normal home user is not technically oriented and therefor a continuous sounding buzzer or other alarm safely discourages him from attempting to rely on a defective smoke detector unit, i.e., one in which the IR-LED fails to operate.

In large, commercial or industrial plants incorporating a large quanity of smoke detectors 10 in a smoke warning system, it would be annoying and unnecessary to continuously sound an audio alarm upon failure of the IR-LED in one unit. Industrial and commerical smoke warning systems normally provide a degree of overlapping between individual detector units as a certain measure of fail-safe protection. In such systems it is only necessary to indicate visually at a central alarm office the failure of an IR-LED in a particular unit. Thus, diodes 90 and 92 may be removed, and the voltage developed at junction 94 can be coupled to a main control panel where any deviation from the standard voltage is to be interpreted as a radiation source failure. Accordingly, the alarm 26 may consist of a light operating or not operating at the central office. Alternatively, such visual indication may be provided on each individual unit, as shown for instance in FIG. 3. A visible LED 96 normally drawing about 20 milliampers is placed in parallel across a LED 97 which is normally drawing about 250-300 milliamperes. If LED 97 becomes an open circuit and fails to operate, the visible low power LED 96 is required to draw in excess of 100 milliamperes and soon burns out. In the event LED 97 shorts out, the voltage across the visible LED 96 is reduced to such a level that it cannot be put into conduction. Thus, in either event a visual indication is given denoting a failure in the LED 97.

Testing of all of the remaining components of the smoke detector is accomplished by momentarily depressing test switch 98 which shorts out the source resistor 99 in series with the FET source element. This changes the bias level supplied to the FET 20 and further drops the voltage level at the drain element, D. If the comparator and alarm components of the smoke detector are operating normally, the transistor 62 is turned on so as to trigger comparator transistor 40 and transistor 72 so as to activate the alarm 78. Failure of the alarm 78 to activate indicates there is some component failure in the detector, comparator, lock on network or alarm sections of the smoke detector. If an AC coupled system is utilized, this test feature may be incorporated therein. However, instead of shorting resistor 99, a small portion of the sinusoidal or pulsed signal from the radiation source 12 must be coupled to the source element S of photo FET 20 via a switch. Again, the photo FET 20 interprets this signal as indicative of a dangerous concentration of smoke, and the alarm 78 is enabled.

Referring now to FIGS. 4 and 5, there is shown a novel smoke chamber, identified generally at 100, which is useful for optimumly channeling a continuous air sample through the smoke and heat detector 10 previously described. The smoke detector 100 comprises an open-ended main chamber 102 having a cross slotted tube 104 traversing one wall of the chamber for mounting a radiation source 12 (e.g., a light emitting diode) therein. Consequently, a beam of incident radiation (I) is transmitted across the main chamber 102.

A radiation detector 20 is mounted in a tube 106 traversing the back wall of the chamber, orthogonally to the incident radiation beam (I) emitted by the radiation source 12. Consequently, an alarm is sounded whenever a predetermined level of radiation is deflected from the incident beam and detected by the radiation detector 20.

A radiation absorber 14 is mounted opposite the radiation source 12 and external to the chamber 102 for absorbing any of the incident radiation (I) that is not deflected by the smoke particles so that it will not be reflected to the detector 20, triggering a false alarm. Further, all the interior surfaces of the smoke chamber 100 have a flat black finish to prevent reflection of the incident radiation beam from the chamber walls to the radiation detector 20. In addition, the detector 20 is "set back" in tube 106 to reduce the reflected radiation which it "sees". Thus, the radiation detector 20 will detect only that radiation scattered within its radiation detection envelope where it intersects the incident radiation beam (I). Accordingly, to effect maximum sensitivity, the air flow should be channeled to the point where the incident radiation beam (I) and the optimum detection angle of detector 20 intersect.

As may be more clearly seen in FIGS. 6 and 7, a bottom cover 108 and a bottom deflector plate 110 positioned at the bottom of main chamber 102 combine to direct a continuous flow of air to the back of the chamber. An oppositely disposed top deflector 112 affixed to the back wall of chamber 102 near its top end forces the air toward the front of the chamber so that it is substantially channeled through the center of the main chamber 102. Having passed out the top end of chamber 102, the air is directed out of the smoke chamber 100 by an output deflector 114 and the top cover 116. As shown in FIG. 7, the top deflector 112 and the output deflector 114 may be combined into an integrated deflection unit.

The bottom cover 108 and the top cover 116 combine with the bottom deflector 110, the top deflector 112, and the output deflector 114 to prevent external radiation from entering the smoke chamber 100 thereby minimizing any possibility that the calibration of a smoke detection system will be affected.

To insure that the air flow is centered at the intersection of the incident radiation beam (I) and the optimum detection angle of detector 20 with respect to the side walls of the chamber 102 as well as the front and back walls, the top deflector 112 and the top cover 116 have notches 112a and 116a, respectively, centered on their edges to facilitate air flow in a substantially vertical direction through the center of the main chamber 102. It should be noted that the direction of air flow is substantially vertical which is the optimum direction of air flow since there is minimum drag effect on the chamber walls and there is less resistance due to gravity.

To maximize air flow through the main chamber 102, a high-temperature source, such as the high-temperature resistor 118 shown in the present embodiment, is positioned in the space between the top deflector 112 and the top cover 116 to maintain a substantial temperature differential between air exiting the smoke chamber 100 from the top cover 116 and the ambient air entering through the bottom cover 108. More particularly, in the present embodiment, a pair of ceramic insulator sleeves 120 located in opposite side walls of the top cover 116 facilitate connection of the high-temperature resistor 114 to the external smoke detector circuitry (FIG. 2) while properly positioning the resistor 118 to maximize air flow.

In order to maximize the air flow, it is important to maintain a substantial temperature differential between incoming ambient air and air leaving the smoke chamber over a wide range of ambient air temperatures. That is, the volume of a gas, such as air, is directly proportional to the absolute temperature of the gas irregardless of heat transfer. As the air near the output of the smoke chamber 100 (i.e., high temperature resistor 118) expands, the weight per unit volume decreases because there are fewer molecules found therein. Accordingly, the gravitational pull on the air decreases proportionately, causing the less dense air to rise and be replaced by the cooler ambient air from the input. Thus, a continuous air sample is effectively drawn through the smoke chamber 100. If, on the other hand, the air at the input is at very nearly the same temperature, the air entering the smoke chamber 100 is for all practical purposes as dense as the air leaving the chamber so that the amount of air flowing through the chamber is decreased.

Because the high-temperature resistor 118 is capable of continuous reliable operation at temperatures above 600.degree. F. while dissipating only a small amount of power, typically less than 3 watts, it is especially well-suited to maintain a large temperature differential between the input and output of the smoke chamber 100 over a wide range of ambient air temperatures. Typical power resistors, however, which are primarily designed to dissipate heat do not operate at high temperatures. On the other hand, any heat transferred from the high temperature resistor 118 to the surrounding air in smoke chamber 100 is rapidly removed from the upper portion of the chamber by the updraft of air.

Accordingly, the smoke chamber of the present invention is effective to maintain a large temperature differential (e.g., in the order of 500.degree. F.) between incoming ambient air and air leaving the chamber even when the ambient air approaches temperatures at which prior art systems using heat transfer to create convection currents would be ineffective.

As an alternative embodiment, the radiation absorber 20 may be replaced by a substantially parabolic reflector which is highly reflective at wave lengths corresponding to the incident radiation beam (I) frequency. By properly focusing the reflective beam to be incident at the intersection of the incident beam and the optimum detection angle of radiation detector 20, the radiation source 12 can be operated at a reduced power level while retaining the same system sensitivity. Or, the radiation source power may be maintained to further increase the detector sensitivity since more photons are then passed through the scattering region. Another parabolic reflector having an aperture to pass the incident radiation beam (I) may be positioned at the radiation source 12 for subsequently reflecting the reflected radiation. It may also be desirable to position a third reflector on the chamber wall opposite radiation detector 20 to focus radiation scattered away from the radiation detector 20 back towards the radiation detector thereby increasing the sensitivity of the smoke detector system.

Thus, it can be seen that there has been provided a very reliable smoke detector unit utilizing an infrared light emitting diode and a novel smoke chamber with significant advantages obtainable over prior art visible light smoke detectors. In addition, there is also provided economical means for achieving a fail-safe feature and providing self-testing with this desirable infrared smoke detecting technique.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom as modifications will be obvious to those skilled in the art.

Claims

I claim:

1. A smoke detector for detecting the presence of smoke particles within a smoke chamber, said smoke detector comprising:

radiation source means including a radiation emitting semiconductor device for transmitting a beam of radiation within said smoke chamber;

radiation detector means for receiving radiation scattered from said beam by said smoke particles, said radiation detector means providing a detected signal representative of the concentration of said smoke particles in said smoke chamber;

alarm means for generating a warning signal whenever said detected signal indicates that a dangerous concentration of said smoke particles is present within said smoke chamber; and

self-test means for electronically simulating a preselected concentration of said smoke particles just sufficient to cause said radiation detector means to enable said alarm means.

2. A smoke detector for detecting the presence of smoke particles within a smoke chamber, said smoke detector comprising:

radiation source means including a radiation emitting semiconductor device for transmitting a beam of radiation within said smoke chamber;

radiation detector means for receiving radiation scattered from said beam by said smoke particles, said radiation detector means providing a detected signal representative of the concentration of said smoke particles in said smoke chamber;

alarm means for generating a warning signal whenever said detected signal indicates that a dangerous concentration of said smoke particles is present within said smoke chamber;

fault detection means for enabling said alarm means responsive to said radiation source being disabled as a result of said light emitting semiconductor device being short-circuited or open circuited; and

self-test means for electronically simulating a pre-selected concentration of said smoke particles just sufficient to cause said radiation detector means to enable said alarm means.

3. A smoke detector for detecting the presence of smoke particles within a smoke chamber, said smoke detector comprising:

radiation source means including a radiation emitting device for transmitting a beam of radiation within said smoke chamber;

radiation detector means for receiving radiation scattered from said beam by said smoke particles, said radiation detector means providing a detected output signal representative of the concentration of said smoke particles in said smoke chamber;

voltage reference means developing a substantially constant reference voltage;

comparator means for generating a control signal whenever said detected output signal exceeds said threshold level,

said comparator means including a transistor having base, emitter and collector electrodes, said reference voltage being coupled to said emitter electrode for establishing a conduction threshold level for said comparator transistor corresponding to a dangerous concentration of said smoke particles, said detected output signal being coupled from said radiation detection means to said base electrode, said comparator transistor being turned on to generate a control signal at said collector electrode when said base electrode is biased to exceed said conduction threshold responsive to a dangerous concentration of said smoke particles; and

alarm means coupled to said collector electrode for generating a warning signal whenever said comparator means generates said control signal.

4. A smoke detector in accordance with claim 3 wherein said radiation detector means includes a photo-conductive field effect transistor (FET) having an output electrode coupled to said comparator base electrode, said voltage reference means being coupled to said FET for biasing said FET to develop a detected output signal voltage at said output electrode proportional to the amount of radiation scattered by said smoke particles and impinging on said FET, said FET detected output signal voltage biasing said comparator base electrode to turn on said comparator transistor if said conduction threshold of said comparator transistor is exceeded.

5. A smoke detector in accordance with claim 4 including fault detection means comprising first and second electronic switch means for enabling said alarm means responsive to said radiation emitting device being short-circuited or open-circuited, said first electronic switch means being coupled between said radiation source means and said voltage reference means to reduce said reference voltage for simultaneously reducing said comparator conduction threshold below the level at which said comparator base electrode is biased and increasing the bias applied to said FET to increase said detected FET output voltage to bias said comparator transistor to conduction responsive to said radiation emitting semiconductor device being short-circuited, said second electronic switch means being coupled between said radiation source means and said comparator base electrode to bias said comparator transistor to conduction responsive to said light emitting semiconductor device being open-circuited, said comparator transistor being turned on responsive to either condition to enable said alarm means despite the absence of a dangerous concentration of said smoke particles.

6. A smoke detector in accordance with claim 5 wherein said voltage reference means includes a Zener diode and a semiconductor device serially coupled between said conductor emitter electrode and two planes of differing reference potential for establishing said constant voltage threshold level, said FET being coupled to the junction of said Zener diode and said semiconductor device for said bias, said first electronic switch means comprising a diode coupled between said radiation emitting device and the junction of said Zener diode and said semiconductor device, said diode being conductive to reduce said reference voltage applied to said comparator emitter electrode and lower said conduction threshold thereof and to bias said FET to turn-on said comparator transistor if said radiation emitting device is short-circuited.

7. A smoke detector in accordance with claim 5 wherein said second switch means comprises a diode coupled between said radiation emitting device and said comparator base electrode.

8. A smoke detector in accordance with claim 4 including test switch means for increasing the bias applied to said FET to electronically simulate a preselected concentration of said smoke particles and cause said FET to develop said detected output voltage signal just sufficient to exceed said comparator threshold level and enable said alarm means, said test switch means being effective to check said radiation detector means, said comparator and said alarm means for proper operation.

9. A smoke detector in accordance with claim 3 including fault detection means comprising a visible light emitting semiconductor device coupled in parallel with said radiation emitting device, said visible light emitting semiconductor device producing a visual light as long as said radiation emitting device is operative.

10. A smoke detector in accordance with claim 3 including switch means coupled to said comparator collector electrode for enabling said alarm means responsive to said comparator transistor being turned on.

11. A smoke detector in accordance with claim 10 including lock-up means for biasing said comparator base electrode to maintain said comparator transistor in a conductive state thereby enabling said alarm means even though the detected signal applied to said base electrode has subsequently dropped below said conduction threshold.

12. A smoke detector in accordance with claim 11 including a reset switch for overriding said lock-up means and disabling said alarm means after said detected signal has dropped below said conduction threshold.

13. A smoke detector in accordance with claim 11 wherein said lock-up means comprises a semiconductor switching device coupled between said comparator collector electrode and said base electrode, said semiconductor switching device being conductive to apply a portion of the output voltage developed at said collector electrode to said base electrode when said comparator transistor is turned on.

14. A smoke detector in accordance with claim 13 wherein said semiconductor switching device comprises a diode.

15. A smoke detector in accordance with claim 3 wherein said radiation detector means includes a photoconductive semiconductor device positioned substantially orthogonal to said radiation emitting device for detecting said scattered radiation and developing output signal representative of the concentration of said smoke particles.

16. A smoke detector in accordance with claim 3 including radiation absorber means positioned on the opposite side of said smoke chamber for absorbing said radiation beam to prevent said beam from being reflected to said radiation detector means, said radiation absorber means insuring that any of said radiation detected by said radiation detector means is a result of scattering said beam by said smoke particles.

17. A smoke detector in accordance with claim 3 including heat sensor means for detecting dangerous levels of heat and enabling said alarm means, said heat sensor means being independent of said radiation source means and said radiation detector means.