US3881112A

US3881112A - Smoke and heat detector unit

Info

Publication number: US3881112A
Application number: US361128A
Authority: US
Inventors: Gordon A Roberts
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-09-16
Filing date: 1973-05-17
Publication date: 1975-04-29
Anticipated expiration: 1992-04-29

Abstract

A unit for detecting the presence of smoke in the air and for detecting a high heat condition. The unit comprises a casing containing a light source, a light detecting photocell which receives light deflected from smoke particles in air flowing through the casing and a second compensating photocell which compensates for the effects of variable conditions such as light source voltage and temperature on the detection photocell. A control circuit in the casing includes a noise producing horn and is capable of actuating the horn to provide three readily distinguishable sounds indicating the presence of smoke above a threshold level, a high heat condition, and burnout of the light source, respectively.

Description

[451 Apr. 29, 1975 1 1 SMOKE AND HEAT DETECTOR UNIT 176] Inventor: Gordon A. Roberts, 1509 Kearney Rd.. Ann Arbor. Mich. 48103 22 Filed: May 17. 1973 21 Appl. No.: 361,128

Related U.S. Application Data [63] Continuation of Scr. No. 72,627. Sept. 16. 1970.

abandoned.

[521 U.S. Cl. 250/565; 250/575; 250/210: 340/237 S. 356/207 [51] Int. Cl. G01n 21/26 [58] Field of Search 250/218, 210. 206, 573, 250/574. 575. 564, 565', 356/206. 208. 207; 340/237 S [56] References Cited UNITED STATES PATENTS 2,537,028 1/1951 Cahusac et a], 340/237 S 3.233.781 2/1966 Gruhbs 250/210 X $255,441 6/1966 Goodwin ct a1. 250/218 X 3.319.069 5/1967 Vassil l 1. 250/218 3,421,157 1/1969 Atkins 340/251 3.555.532 1/1971 White et a1. 250/218 X 3.624.629 11/1971 Donaldson 340/251 3.657.737 4/1972 Hamm et a1. 340/237 S Primary E.raminer-Walter Stolwein Attorney, Agent, or FirmOlsen and Stephenson {57] ABSTRACT A unit for detecting the presence of smoke in the air and for detecting a high heat condition. The unit comprises a casing containing a light source, a light detecting photocell which receives light deflected from smoke particles in air flowing through the casing and a second compensating photocell which compensates for the effects of variable conditions such as light source voltage and temperature on the detection photocell. A control circuit in the casing includes a noise producing horn and is capable of actuating the horn to provide three readily distinguishable sounds indicating the presence of smoke above a threshold level. a high heat condition, and burnout of the light source. respectively.

13 Claims, 3 Drawing Figures INVENTOR SHEET 2 BF 2 YENIEDmz 9 ars BY flak ATTORNEYS SMOKE AND HEAT DETECTOR UNIT This is a continuation. of application Ser. No. 72.627 filed Sept. 16. I970. now abandoned.

This invention relates generally to units for detecting and signaling the existence ofsmoke or heat in room air above a predetermined level of the type disclosed in my earlier U.S. patent No. 3.383.670. The unit of this invention is an improvement on the unit shown in the aforementioned patent. The unit of this invention includes the same general arrangement of a light source. a lens containing light transmission conduit. a light interaction region. a light receptacle. a photosensitive cell for receiving light deflected by smoke in the light interaction region and a photocell actuated horn disclosed in the aforementioned patent. In addition. the unit of this invention includes a second photosensitive cell adjustably mounted in the light receptacle and connected in an alarm circuit to the first cell for adjusting the sensitivity of the circuit. The cells receive comparable light intensity at the selected smoke threshhold point. namely. the smoke concentration point which it is desired to sense. and by virtue of this arrangement the cells have similar resistive temperature coefficients and time constants. This cooperative arrangement of the two cells compensates for variable conditions such as line voltage affecting the intensity of the light source and heat conditions affecting the resistivity of the cells which would otherwise adversely affect the sensitivity of the unit. In addition, the unit of this invention includes a control circuit for the signaling noise source which provides for the emission of three readily distinguishable sounds from the noise source to indicate the three conditions which it is desired to sense in the unit. The noise source. a conventional horn. emits intermittent bursts of noise when the light source burns out to indicate to the user of the unit that the light source lamp must be replaced. A second continuous sound at substantial sound level is emitted from the born when the unit senses a high heat condition. A third continuous sound which is readily distinguishable because of its raspy character is emitted from the horn when the unit senses a threshhold smoke condition. Thus, the smoke and heat detector unit of this invention is capable of continuous operation over a prolonged service life to continuously monitor heat and smoke conditions within desired sensitivity limits for fire detection purposes.

Further objects. features and advantages of this invention will become apparent from a consideration of the following description. the appended claims, and the accompanying drawing in which:

FIG. I is a front view of the unit of this invention with the front face of the enclosing casing broken away and other parts shown in section for the purpose of clarity;

FIG. 2 is an enlarged fragmentary sectional view of a portion of the unit of this invention as seen from substantially the line 2-2 in FIG. I; and

FIG. 3 is a diagrammatic illustration of the alarm circuit which forms a part of the detector unit of this invention.

With reference to the drawing. the detector unit of this invention. indicated generally at I0, is illustrated in FIG. I as consisting of a casing 12 having a front wall 14. a back wall 16. a bottom wall I8 and a top wall 20. An inlet opening 22 for room air is formed in the bottom wall I8 and an outlet opening 24 for this air is formed in the top wall 20. Partitions in the casing 12 define a smoke detection chamber of compartment 26 disposed between the openings 22 and 24, a light transmission conduit 28 containing a lens 30. a lower light trap 32 and an upper light trap 34. The casing partitions also cooperate to form a light receiving compartment 36 and a compartment 38 in which a signal horn 40 is located. For a purpose to appear presently. orifice plates 41 and 42, provided with orifices 43 and 44, respectively. are mounted in the light transmission con duit 28 to one side of the smoke detection chamber 26. A light source 46 is positioned in the casing I2 at one end of the light transmission conduit 28. When the light source 46 is energized. it projects a light beam 48 onto the lens 30 and the lens 30 in turn projects this beam of light toward the orifice plate 42 so that an image of the light source 46 is formed at the orifice 44 and a diverging beam of light is projected through the orifice 43 and across the detection chamber 26. This diverging light beam forms a light interaction region 49 in the chamber 26 between the orifice 43 and an inlet open ing 50 for the light receiving compartment 36 and between the upper and lower boundaries 52 of the light beam.

A first photosensitive cell PCI is mounted in the easing I2 at one end ofthe chamber 26 so that some of the light impinging on particles in the interaction region 49 will be reflected onto the cell PCI. A second photosensitive cell PC2 is disposed in the light chamber 36 and is mounted on a pivot 54 which extends through the back wall I6 of the casing I2 as shown in FIG. 2. The pivot 54 terminates in a readily accessible screw head 56 which can be rotated to adjust the position of the cell PC2 in the chamber 36 to in turn adjust the intensity of the light received by the cell PC2 for a purpose to appear presently. The horn 40, the light source 46. and the cells PCI and PC2 are connected in acontrol circuit 60 shown in detail in FIG. 3 and the components of which are housed principally in the compartment 38.

In the signal circuit 60, the detector cell PCl is connected in series with a resistor R1 and the cell PC2, as shown in FIG. 3. The cells PCI and PC2 are such that the resistance of each will vary generally proportionately to the intensity of the light impinging on the cells. Direct current is supplied to the cells through conductors and I03 which are connected to a DC power source I05 which in turn is connected to a conventional I 10 volt AC source via conductors I00 and I02. The horn 40, which is a conventional I I0 volt AC horn is operative at 60 cps and is actuated via the AC lines I00 and 102 in a manner to be hereinafter described.

The circuit 60 includes a four leg bridge defined by the cell PCI and resistor R1. cell PC2. resistor R2 and resistor R3. Cell PCI and resistor R1 and cell PC2 are serially connected with the combination being connected in parallel with serially connected resistors R2 and R3. A resistor R13 is connected in parallel with cell PC2 for a purpose to appear presently. Bridge unbalance is sensed by a field effect transistor amplifier O] which has its gate connected to the juncture between R1 and PC2 and has its drain connected between resistors R2 and R3. in this manner. when a selected threshhold of unbalance occurs in the bridge circuit caused by a predetermined drop in the resistance of PCI relative to the resistance of PC2. the amplifier QI will be rendered conductive. The output circuit of CI is connected to a transistor amplifier 02 such that the source of O1 is connected to the base of Q2. The amplifier O2 is a p-n-p type transistor with its emitter connected to the DC voltage source 105 via line 103 and its base connected to the same line via resistor R4 which acts as a biasing resistor for the base of Q2 and load resistance for the source of 01. Thus, when 01 is rendered conductive. Q2 will also be turned on. The transistor 02 provides a trigger pulse to the gate circuit of a SCR amplifier 03 so as to render amplifier O3 conductive. This is accomplished by a circuit which includes a resistor R8 which is connected to the gate of amplifier O3 and also to the conductor 100 via biasing resistor R5.

The resistor R4 and a parallel capacitor C1 associated with the amplifier Q2 and the resistor R5 and a parallel capacitor C2 associated with the SCR Q3 function as filter subcircuits to minimize the possibility of undesirable actuation of the circuit 60 due to the presence of transient power sources such as start-up power, lightning, and the like.

In order to provide for continuous operation of the horn 40 in response to a sensing by PC1 of a smoke concentration above a selected threshhold level, a latch-up subcircuit is provided of resistors R6 and R7, diode D1, and capacitor C3. The latch-up circuit is connected between the emitter of Q1 and the gate of O2 and operates in response to a voltage rise on capacitor C3 to hold O1 in a turned on condition.

In the operation of the unit 10, the horn 40 is operated in response to a preset differential in resistance be tween PC1 and PCZ. This differential will remain substantially constant irrespective of line voltage changes which affect the intensity of light source 46 and temperature conditions which affect the resistivity of PC]. Consequently, PC1 is capable of sensing a threshhold condition of smoke concentration in interaction region 46 irrespective of variable conditions such as line voltage and temperature because of the compensating or offsetting influence of PCZ. The threshhold condition to be set in readily adjustable to any desired level by ro tating the pivot 54 to adjust the position of cell PC2 to receive more or less light from compartment 36.

When the intensity of light impinging on PC] relative to the intensity of light impinging on PC2 reaches the threshhold condition, sufficient current can flow through Q! to trigger Q2 and in turn provide for triggering of 03. In response to triggering of Q3 approximately half wave drive to the horn 40 is achieved since horn 40 is connected in series with SCR ()3. This results in the emission of a distinguishable raspy noise of high output sound level from the horn 40 and a low temperature rise of the horn. The resistor R1, which is of small magnitude compared to the resistance of PC1 when exposed to light at the threshhold level, provides current limiting protection for O1 in case PC] shorts out. The resistor R13 in parallel with PC2 functions to insure against an unbalance of the bridge toward turnon of ()1 after the cell PC2 has been dark for a period of time, such as after prolonged storage of unit 10, when the circuit 60 is first connected to a power source. The value of resistor R13 is relatively high compared to the resistance of PC2 when exposed to normal light in the unit 10.

In response to the turning on ofOl and 02 the electrical potential at the R7, R6 node is approximately the same potential as that on a capacitor C4. After a short time, as set by the RC time constant of the circuit of C3, the voltage on C3 rises sufficiently to hold 01 on.

thus causing latch-up. The values of R8 and R7 are low enough to maintain the bridge unbalance in the alarm direction even when the resistance of PC] is infinite, i.e. open circuit. Once latchup is achieved, the horn 40 will continue to operate to indicate a dangerous smoke condition until line power to conductors and 102 is removed.

When there is no line power, C4 has a zero charge. For a voltage on C4 less than at the gate of Q1, diode D1 is reverse biased and the latch-up subcircuit is isolated from the bridge and 01. Also, during a no alarm" condition, 02 is off and its collector potential is at or near line 102 potential so that diode D1 is maintained in a reversed biased state. The time constant for the filter subcircuits is chosen so that transients related to connecting power to the circuit 60, lightning, and other typical causes will not turn on Q2.

The circuit 60 also includes a heat responsive circuit which includes a heat responsive switch 106 connected in parallel with the SCR Q3 and hence in series with the horn 40. When the switch 106 is closed by the presence of excessive heat, the horn 40 receives full wave excitation through the switch 106. This produces a sound which is clearly distinguishable from the half wave excitation previously described indicating a threshhold smoke condition.

One of the important advantages of the circuit 60 is its ability to detect burnout of lamp 46. This is achieved by sensing a substantial voltage rise compared with normal operation and producing short bursts of sound from horn 40 in response to this voltage rise. The lamp 46 is connected in series with a resistor R9, a diode D5 and a resistor R10 across the

lines

100 and 102. The peak voltage across lamp 46 and resistor R10 is substantially less than peak AC line voltage which is the input to the series circuit when lamp 46 is not burned out. A diode D6 rectifies the voltage across R10 and cooperates with capacitor C4 to provide a steady DC voltage across the amplifier and bridge circuits thereby defining the DC power source 105. The peak voltage of lamp 46 and resistor R10 is selected so that it is less than the threshhold of the lamp outage detection subcircuit hereinafter described.

The lamp outage detection subcircuit consists of resistors R11 and R12, capacitors C5 and C6 and voltage breakdown device which in the illustrated embodiment of the invention is a neon bulb N connected to the gate for 03. This subcircuit constitutes a neon bulb relaxation oscillator. The bulb N is characterized by the fact that a minimum voltage is required to cause neon breakdown and a resulting flow of current through the bulb N. In the event bulb 46 burns out, nearly full peak line voltage is available through conductor 107, at the bulb N. Diode D5 provides a DC current to charge C6. Each time the voltage on C6 reaches the breakdown voltage of bulb N, C6 is discharged through bulb N and a pulse is coupled to Q3 causing a half wave pulse to the horn 40. This pulse is repeated after a plurality of line frequency passes to achieve the desired intermittent emission of noise from horn 40. The capacitor C5 and the resistor R12 assist in controlling the phase angle of the trigger pulse to 03.

From the above description it is seen that this invention provides a smoke and heat detector 10 in which the single born 40 is employed to provide different distinguishable alarm signals. As a result, alarm conditions such as a dangerous smoke threshhold condition and a dangerous heat condition can be sensed. and a trouble condition. namely. burnout of the lamp 46 can also be sensed. As a consequence of this use of the single horn 40. an operable detector unit [0 can be produced at low cost. in addition. by virtue of the employment of the compensating cell PCZ in cooperation with the detector cell P(l. a smoke threshhold condition which it is desired to detect can be set by adjustment of the posi tion of the cell PCZ. This cooperative relationship of the cells also compensates for variable light levels and temperature conditions which would otherwise adversely affect the sensitivity of the detector cell PCl. The latch-up subcircuit assures continuous operation ofthe horn 40 when an alarm condition is sensed which is advantageous for safety purposes because it requires the building occupant in which the unit I0 is placed to unplug the unit It) in order to shut off horn 40.

What is claimed is:

1. ln a smoke detector unit. a casing having air inlet and outlet openings. means forming a detection chamber in said casing communicating with said openings. a light energy source providing light in said detection chamber. first photosensitive cell means in said casing positioned to receive light energy deflected from airborne particles in said detection chamber. second photosensitive cell means in said casing for receiving light energy from light therein. means adjustably mounting said second cell means on said casing so that the light energy received by said second cell means may be varied by adjusting its position in said casing. said cell means being arranged relative to said light source such that they have a substantially uniform light intensity thereon at all light levels in said casing. said cell means being disposed substantially adjacent one another so that they are subjected to the same environmental con ditions. and signal means operatively associated with said cell means for predetermined actuation in response to a predetermined relationship in the light energies received by said first and second cell means.

2. A smoke detector unit according to claim 1 further including a control circuit operatively connecting said light energy source and said first and second cell means with said signal means. resistor means conductively connected in said circuit so as to form with said cell means a four leg bridge in which the ratio of the electrical resistance of said first cell means to the resistance ofsaid second cell means determines the electrical con dition of said bridge. thereby providing for actuation of said signal means when said predetermined light relationship occurs.

3. A smoke detector unit according to claim 1 further including a control circuit operatively connecting said light energy source and said first and second cell means with said signal means. resistor means conductively connected in said circuit so as to form with said cell means a a four leg bridge in which said first and second cell means form adjacent legs of the bridge. the other adjacent legs ofthe bridge being formed by said resistor means. a detector branch in the circuit having a switch amplifier therein. said branch having a first input connected between said first and second cell means and a second input between said resistor means so that a current will flow through the switch amplifier when the re sistance of said first cell means changes with respect to the resistance of the second cell means in a predetermined manner. and means in said circuit providing for flow ofenergizing current through said signal means in response to flow of current through said switch amplitier.

4. A smoke detector unit according to claim I further including means for actuating said signal means in response to an inoperative condition of said light energy source. said actuating means comprising circuit means connecting said light energy source and said signal means and operable in response to an increase in voltage supply to said circuit means due to an inoperative condition of said light energy source to actuate said signal means.

5. A smoke detector unit according to claim 4 wherein said circuit means constitutes an oscillator circuit providing for intermittent operation of said signal means when said oscillation circuit is operative to thereby distinguish operation of said signal means to indicate an inoperative condition of said light energy source from operation of said signal means to detect smoke.

6. A smoke detector unit according to claim 5 wherein said oscillator circuit includes a neon bulb connected in parallel with capacitor means operable to intermittently pass a voltage to said bulb exceeding the breakdown voltage thereof to in turn provide for intermittent flow of energizing current through said bulb to said signal means.

7. In a unit for detecting a predetermined smoke concentration level in the atmosphere. a casing. a light source in said casing. first circuit means for sensing a predetermined smoke condition and for providing an actuating signal in response thereto. said circuit means including a first photosensitive cell and a second compensating photosensitive cell arranged in said casing so as to have substantially uniform distribution of light thenv on and comparable light intensity at said predetermined smoke level. means adjustably mounting said second cell on said casing so that the light energy received by said second cell may be varied by adjusting its position in said casing. said cells being disposed substantially adjacent one another so that they are sub ected to the same environmental conditions. a switch amplifier for providing said actuating signal when said amplifier is turned on. alarm means responsive to the ocunrence of said actuating signal for providing an alarm signal. and a latch circuit connected to said switch amplifier and operable to maintain said alarm means in operation after discontinuance of said actuating signal. said latch circuit including bridge circuit means and means to unbalance said bridge circuit into an alarm state in response to turning on said switch amplifier.

8. In a smoke detecting unit. a casing having air inlet and outlet openings. means forming a detection chamber in said casing communicating with said openings. at light energy source providing light in said detection chamber, first photosensitive cell means in said casing positioned to receive light energy deflected by airborne particles from said detection chamber. a light receiving chamber positioned opposite said light energy source and adapted to receive light therein. second photosensitive cell means mounted in said light receiving chamber. said second photosensitive cell means being adjustably mounted so that the light energy recciv ed thereby may be varied by adjusting its position within the light receiving chamber. said cell means being disposed substantially adjacent one another so that they are subjected to the same environmental conditions. signal 7 means. a control circuit operativel connecting said light energy source and said first and second cell means with said signal means. and means in said circuit providing for drive of said signal means in response to a predetermined difference in the light energies received by said first and second cell means.

9. A smoke detector unit according to claim 8 wherein said first and second cell means form adjacent legs of a four leg bridge. resistor means forming the other adjacent legs of said bridge. a detector branch in said circuit having a switch amplifier therein. said branch having a first input connected between said first and second cell means and a second input connected between said resistor means so that the current through the switch amplifier will change when the resistance of said first cell means falls below the resistance of said second cell means by a predetermined amount.

[0. A smoke detector unit according to claim 8 further including a fixed resistance resistor connected in parallel with said second cell means and having a resistance less then the resistance of said second cell means when said second cell means is unexposed to light and a resistance substantially greater than the resistance of said second cell means when said second cell means is exposed to light in said compartment from said second light energy source.

I]. A smoke detector unit according to claim 8 further including capacitor means connected to said de tector branch between said switch amplifier and said cell means and connected to said conductor means so that in response to current flow through said switch amplifier the voltage on said capacitor means will reach a value sufficiently high to maintain current flow through said switch amplifier so that said signal means will continue to operate until power to said conductor means is discontinued.

12. A smoke detector unit according to claim 8 wherein said light receiving chamber has a pair of spaced walls that are substantially perpendicular to the non-deflected path of light emanating from said light energy source. means pivotallv supporting said second photosensitive cell means in said light receiving chamber at a position to one side of said path for pivotal movement toward and away from one of said walls to thereby enable adjustment of the intensity of reflected light in said light receiving chamber on said second photosensitive cell means.

13. A smoke detector unit according to claim I wherein said predetermined relationship is a relationship wherein the intensity of light on said second cell means is comparable to the light intensity on said first cell means.

Claims

1. In a smoke detector unit, a casing having air inlet and outlet openings, means forming a detection chamber in said casing communicating with said openings, a light energy source providing light in said detection chamber, first photosensitive cell means in said casing positioned to receive light energy deflected from airborne particles in said detection chamber, second photosensitive cell means in said casing for receiving light energy from light therein, means adjustably mounting said second cell means on said casing so that the light energy received by said second cell means may be varied by adjusting its position in said casing, said cell means being arranged relative to said light source such that they have a substantially uniform light intensity thereon at all light levels in said casing, said cell means being disposed substantially adjacent one another so that they are subjected to the same environmental conditions, and signal means operatively associated with said cell means for predetermined actuation in response to a predetermined relationship in the light energies received by said first and second cell means.

2. A smoke detector unit according to claim 1 further including a control circuit operatively connecting said light energy source and said first and second cell means with said signal means, resistor means conductively connected in said circuit so as to form with said cell means a four leg bridge in which the ratio of the electrical resistance of said first cell means to the resistance of said second cell means determines the electrical condition of said bridge, thereby providing for actuation of said signal means when said predetermined light relationship occurs.

3. A smoke detector unit according to claim 1 further including a control circuit operatively connecting said light energy source and said first and second cell means with said signal means, resistor means conductively connected in said circuit so as to form with said cell means a a four leg bridge in which said first and second cell means form adjacent legs of the bridge, the other adjacent legs of the bridge being formed by said resistor means, a detector branch in the circuit having a switch amplifier therein, said branch having a first input connected between said first and second cell means and a second input between said resistor means so that a current will flow through the switch amplifier when the resistance of said first cell means changes with respect to the resistance of the second cell means in a predetermined manner, and means in said circuit providing for flow of energizing current through said signal means in response to flow of current through said switch amplifier.

4. A smoke detector unit according to claim 1 further including means for actuating said signal means in response to an inoperative condition of said light energy source, said actuating means comprising circuit means connecting said light energy source and said signal means and operable in response to an increase in voltage supply to said circuit means due to an inoperative condition of said light energy source to actuate said signal means.

7. In a unit for detecting a predetermined smoke concentration level in the atmosphere, a casing, a light source in said casing, first circuit means for sensing a predetermined smoke condition and for providing an actuating signal in response thereto, said circuit means including a first photosensitive cell and a second compensating photosensitive cell arranged in said casing so as to have substantially uniform distribution of light thereon and comparable light intensity at said predetermined smoke level, means adjustably mounting said second cell on said casing so that the light energy received by said second cell may be varied by adjusting its position in said casing, said cells being disposed substantially adjacent one another so that they are subjected to the same environmental conditions, a switch amplifier for providing said actuating signal when said amplifier is turned on, alarm means responsive to the occurrence of said actuating signal for providing an alarm signal, and a latch circuit connected to said switch amplifier and operable to maintain said alarm means in operation after discontinuance of said actuating signal, said latch circuit including bridge circuit means and means to unbalance said bridge circuit into an alarm state in response to turning on said switch amplifier.

8. In a smoke detecting unit, a casing having air inlet and outlet openings, means forming a detection chamber in said casing communicating with said openings, a light energy source providing light in said detection chamber, first photosensitive cell means in said casing positioned to receive light energy deflected by airborne particles from said detection chamber, a light receiving chamber positioned opposite said light energy source and adapted to receive light therein, second photosensitive cell means mounted in said light receiving chamber, said second photosensitive cell means being adjustably mounted so that the light energy received thereby may be varied by adjusting its position within the light receiving chamber, said cell means being disposed substantially adjacent one another so that they are subjected to the same environmental conditions, signal means, a control circuit operatively connecting said light energy source and said first and second cell means with said signal means, and means in said circuit providing for drive of said signal means in response to a predetermined difference in the light energies received by said first and second cell means.

9. A smoke detector unit according to claim 8 wherein said first and second cell means form adjacent legs of a four leg bridge, resistor means forming the other adjacent legs of said bridge, a detector branch in said circuit having a switch amplifier therein, said branch having a first input connected between said first and second cell means and a second input connected between said resistor means so that the current through the switch amplifier will change when the resistance of said first cell means falls below the resistance of said second cell means by a predetermined amount.

10. A smoke detector unit according to claim 8 further including a fixed resistance resistor connected in parallel with said second cell means and having a resistance less then the resistance of said second cell means when said second cell means is unexposed to light and a resistance substantially greater than the rEsistance of said second cell means when said second cell means is exposed to light in said compartment from said second light energy source.

11. A smoke detector unit according to claim 8 further including capacitor means connected to said detector branch between said switch amplifier and said cell means and connected to said conductor means so that in response to current flow through said switch amplifier the voltage on said capacitor means will reach a value sufficiently high to maintain current flow through said switch amplifier so that said signal means will continue to operate until power to said conductor means is discontinued.

12. A smoke detector unit according to claim 8 wherein said light receiving chamber has a pair of spaced walls that are substantially perpendicular to the non-deflected path of light emanating from said light energy source, means pivotally supporting said second photosensitive cell means in said light receiving chamber at a position to one side of said path for pivotal movement toward and away from one of said walls to thereby enable adjustment of the intensity of reflected light in said light receiving chamber on said second photosensitive cell means.

13. A smoke detector unit according to claim 1 wherein said predetermined relationship is a relationship wherein the intensity of light on said second cell means is comparable to the light intensity on said first cell means.