Photoelectric smoke detector circuitry
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a photoelectric smoke detector and more particularly to such a detector operable based on the scattered light principle. In accordance with this principle light from a suitable source is not directly impinged upon a sensor device. Instead the sensor is placed at an angle to the light beam and is normally unilluminated. However, when smoke particles enter a smoke sensing chamber the smoke particles or the like interrupt the light beam and result in scattering light onto the sensor. Light scattering intensity is a function of the light wave length from the light source, typically a light emitting diode (LED); it is also a function of the particle density, size and composition. 
     2. Background Information 
     The present invention resides in improvements in the specific context of the electronics or circuitry involved in a photoelectric smoke detector of the type described. This improved circuitry is also found in a detector device described in related application Ser. No. 720,440, filed Apr. 5, 1985, assigned to the assignee of the present invention. 
     In order to provide background material so that the complete context and the various features and advantages of the present invention will be thoroughly appreciated, reference may be made to the following U.S. Pat. Nos.: 4,163,969; 4,186,390; and 4,193,069. 
     All of the above cited references involve optical or photoelectric smoke detectors operating on the scattered light principle and having a flashing or clock-pulsed light source. These referenced patents appear to be concerned with the problem resulting from the presence of transient voltage surges in the power lines, or electromagnetic voltage surges induced in the smoke detector, and other electrical interference or spurious illumination that spuriously simulates the photo cell&#39;s response to smoke scattered light pulses. Each of these patents takes a somewhat different approach to the problem, but they all appear to include as part of the solution a coincident logic scheme which is directed to rendering it unlikely that spurious signals will coincidentally occur with the flashes or pulses of illumination from the light source. 
     SUMMARY OF THE INVENTION 
     Whatever the objects, features and advantages of the references cited hereinabove, such references are not directed to achieving the precise objects that are fulfilled by the present invention. 
     It will be appreciated that the context of the present invention, that is the field of photoelectric smoke detectors, requires that the detector devices be manufactured at a very low cost and at the same time that they be able to provide extremely high amplification because of the very small signals involved. Thus, a light sensor in the form of a photodiode, as used in the detector in which the present invention is incorporated, &#34;sees&#34; only one millionth of the light intensity being put out by the light source in the form of a light emitting diode. 
     Accordingly, it becomes a major object of the present invention to provide a low cost solution to the problem of utilizing only a single operational amplifier to amplify the electrical signal put out by the photodiode; and yet to enable the preclusion or inhibition of transient signals due to noise or the like, such transient signals sometimes being referred to as sliver signals, because they are extremely short in duration (i.e., they have a narrow pulse width). 
     A primary feature of the present invention resides in the provision of an operational amplifier uniquely capable of functioning as a threshold detector, saturation pulse-amplifier and fixed pulse width compensator. Such operational amplifier has a high input impedance, of the order of megohms, which is compatible with the impedance of the photodiode. The photodiode is pulsed on for approximately 100 microseconds and off for a period of 5 seconds by smoke reflection, if present in the chamber. The photodiode is arranged to operate as a photovoltaic-cell that generates voltage wherever an infrared light source is detected. 
     Another primary feature of the invention relates to an arrangement which compensates for the aging process involved with the LED (Light Emitting Diode), which constitutes the light source for the photodetector. The LED is incorporated in a flasher or pulse generator circuit, such circuit comprising a pair of suitably interconnected transistors operating in a free running multi-vibrator mode. A feedback network, constituting the required means for automatically compensating for any increase in impedance of the LED due to aging, is connected such that it functions to increase the pulse width of te output signal of the flasher circuit. Consquently, curent pulses of longer duration are delivered to the LED tomake up or compensate for the ongoing increase in its impdance stemming from the aging process. The advantageous rsult is that substantially the same illumination is always provided by the LED. 
    
    
     Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers. 
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a schematic diagram of the circuitry for the preferred embodiment of a photoelectric smoke detector, in which the features of the present invention have been incorporated. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Before proceeding with a detailed description of the preferred embodiment of the present invention, it should be noted that the circuit aspects herein are related to the invention described and claimed in copending U.S. application Ser. No. 720,440, filed Apr. 5, 1985, assigned to the assignee of the present invention. The circuitry herein described is also found in that related application. The details of the invention disclosure of the copending application are herein included by reference. 
     The photoelectric smoke detector has, as fundamental components, a smoke sensing chamber and a photo-optics system based on the scattered light principle, whereby the photosensor in the form of a diode is not in line with the light beam but is, instead, placed at an angle such that it is normally non-illuminated. Smoke particles entering the smoke-sensing chamber, interrupt the light beam so as to scatter light on to the photosensor. 
     The light source, photodiode, as well as associated lensing, are packaged and selected for comparability in wave length and position. These elements are packaged in a chamber that restricts ambient light entry but readily permits smoke to enter. The photodiode is connected to respond as a photovoltaic device and as such, it is a voltage generator when light is applied onto its surface. The photodiode is connected to the input of an amplifier and is monitored for a threshold smoke-generated voltage. 
     Referring now to the FIGURE, the photoelectric smoke detector circuit shown therein can be divided into four basic sub-circuits or sections: the power supply 10 which includes a bridge and regulator; a flasher section 12, which is a basic multivibrator circuit; a sensor-amplifier section 14; and an alarm section 16. 
     The bridge 18 is used to permit non-polarized voltage inputs to the photodetector terminals seen at the left. The voltage regulator 20 provides regulation for a range of applied input voltages so as to enable a constant voltage to be applied to the flasher section 12 and the sensor-amplifier section 14. Voltage regulation to these sub-circuits is required to maintain a consistent smoke response. Otherwise a slight change in voltage would cause variation in the flasher timing and light intensity, and/or the amplifier setting and, therefore, the resultant amplification. The input to the regulator can be varied between 11 and 26 volts without affecting the required circuit operation. 
     The voltage regulator 20 includes the emitter follower regulator Q1 and a dropping resistor R1, which functions to limit current to a further included element, that is, a Zener diode D3. A varistor V1 is also included as part of the regulator circuit 20 for controlling and reducing high voltage transistors, while diode D2 is used to prevent UHF transmitter frequencies from entering into the circuitry by shortening line terminations. 
     The flasher sub-circuit 12 is designed to flash the light-emitting diode for a period of 100 microseconds repeated once every five seconds. The reason for flashing, or pulsing is to reduce the circuit current that would otherwise be excessive for multi-zone smoke detector operations involving up to fifty detectors connected to one zone. 
     The operational amplifier 22, which forms a part of the sensor-amplifier section 14, is designed as a high-gain, low current integrated circuit operational amplifier which functions to monitor a voltage threshold, which, when exceeded by the voltage generated by photodiode D6, creates regeneration in the detector amplifier circuit so as to initiate an alarm in the alarm section 16. This alarm sub-circuit has a latching device 24 in the form of an SCR, which is capable of maintaining the current requirements for alarming a control panel (not seen) by way of suitable zone wiring. 
     It will be appreciated that in the operation of the circuit of the present invention, the flasher sub-circuit 12 begins to operate as soon as power is applied to the detector. Base current begins to flow through resistors R3 into the base of transistor Q2. The collector current of transistor Q2 is the base current of transistor Q3. The resulting collector current of transistor Q3 divides between (1) the network consisting of the LED flasher designated D5 and its current limiting resistor R7, &#34;porch removal&#34; resistor R6 being connected in parallel therewith; and (2) the feedback path of resistor R4 and capacitor C4 in series. The feedback current adds to the base current of transistor Q2. The resulting positive feedback or regeneration causes saturation of transistor Q3, thereby causing D5 to turn on at full brilliance. 
     It will be understood that the feedback current begins to decay exponentially due to the resistor R4 and the capacitor C4 time constant (turn-on time). The flasher circuit begins to turn off when the current decays to a value that will not sustain saturation of transistor Q3. As transistor Q3 comes out of saturation, the voltage change is fed back to the base of transistor Q2, through the resistor R4 and capacitor C4 path. Regeneration occurs again, causing both transistors Q2 and Q3 to turn off rapidly. The charge on capacitor C4 is such that the base of transistor Q2 is made negative. Capacitor C4 begins to charge toward the regulator positive voltage through resistors R3 and R4 (turn off time). When the base of transistor Q2 reaches the transistor base-emitter &#34;on&#34; voltage, transistor Q2 begins to turn on, starting a new cycle. Capacitor C3 is used to sustain the turn-off on voltage applied to LED D5 and the feedback network at a constant level. When transistor Q3 turns on, capacitor C3 discharges through transistor Q3 and the LED network. Diode D4 is used to block out the discharge path of capacitor C3 through other paths for the remaining circuitry. Resistor R2 controls the charge time of capacitor C3, limiting the in-rush of line and regulator current. 
     The self-compensating feature of the present invention will now be described. It will be recalled that a typical LED such as D5 seen in the flasher section 12 is subject to an increase in impedance as the aging process takes place. However, precisely because the LED network is connected as seen in the FIGURE to the collector output of transistor Q3 and, further, because of the feedback path connection involving the capacitor C4 and resistor R4, the required automatic compensation is effectuated. What happens is that as the LED ages, with an accompanying impedance increase, the pulse width &#34;on&#34; time increases. This is accomplished by reason of the increase in potential at the junction of the anode of LED D5 and the collector of transistor Q3. The higher voltage is applied to the &#34;on&#34; time constant resistor R4 and capacitor C4 (regeneration feedback), causing it to increase the pulse width applied to D5. Thus, as the LED impedance is increased, its light level is decreased. However, the flasher circuit compensates for the light level decrease by increasing the &#34;on&#34; time pulse width for an increase in light duration. The repetition rate of 5 seconds &#34;off&#34; time is due to the time constant of capacitor C4 and resistors R3, R4, R7 and the LED (D5) impedance. 
     Turning now to the sensor-amplifier section 14, it will be seen that the photodiode D6 is connected in shunt across the inputs (minus input and plus input at pins 2 and 3 respectively of the differential operational amplifier 22). The voltage dividing network, consisting of resistor R9 and potentiometers P1 and P2, applies an adjustable positive potential to the input pin 2 of the operational amplifier 22 via the potentiometer P1 wiper. Potentiometer P2 is used to factory calibrate the threshold voltage limit applied to pin 2. Potentiometer P1 is used to field adjust the threshold voltage applied to pin 2 by the user. Resistors R10 and R13 have high resistance values (of the order of 20 megohms) and have a common node or junction 23. The other end of resistor R10 is connected to pin 3 of operational amplifier 22, while the other end of resistor R13 is connected to ground. This arrangement is required to prevent a floating input at pin 3. 
     The aforenoted high resistance values have been chosen for R10 and R13 so as not to load down the photodiode, thereby enabling its maximum detection response. Coupling capacitor C8 is connected between the aforesaid junction 23 of resistors R10 and R13 and pin 6 of operational amplifier 22. This junction 23 has been chosen so as not to load the photodiode, which would have an adverse effect on its sensitivity. Also, capacitor C8 has its value chosen with respect to its time constant with resistors R10 and R13. 
     The capacitor C8 feedback coupling will cause the operational amplifier output pin 6 to saturate whenever the set voltage threshold at pin 2 is exceeded by the operational amplifier pin 3 voltage, which excess generally corresponds with the voltage of the photodiode D6 when it is receiving scattered light. The time constant of capacitor C8 and resistors R10 and R13 are chosen to produce a desired pulse width output at operational amplifier pin 6, such pulse width being selected to be in the order of 4 to 5 milliseconds. 
     The saturated fixed pulse width output is so selected as to pulse the buffer amplifier, that is transistor Q5, into conduction. During the turn-on time of transistor Q5, the time constant of resistor R16 and capacitor C10 must charge capacitor C10 to a voltage level to cause the gate 27 of SCR24 to fire into conduction. This pulse width time constant of resistor R16 and capacitor C10 prevents a &#34;sliver&#34; or transient signal from firing the SCR into false conduction of said voltage source. 
     Referring now to the power supply end of the circuit seen in the FIGURE, it will be noted that power control switch SW1 is provided. This normally closed switch is opened when a smoke detector unit, as described in co-pending application (ED-233), has its grill removed from a cover member supplied as a part of the photodetector. Switch SW1 also affects a circuit, not seen, which causes operation of a trouble indicator at a central location when the grill of any one of a string or series of similar photodetectors, having corresponding switches, is unlatched from its cover, thereby causing opening of its switch SW1. At the same time, as will be appreciated, opening of switch SW1 removes power from the alarm circuit and thus prevents activation of a false alarm signal. 
     In order to provide a man skilled in the art with a detailed set of specifications relating to the components used in the circuitry of the present invention. The following table of values and types of components is included herewith. 
     
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COMPONENT          VALUE OR TYPE                                          
______________________________________                                    
R1                 1.8 megohm                                             
R2                 47 kilohm                                              
R3                 5.6 megohm                                             
R4                 12 ohm                                                 
R5                 100 ohm                                                
R6                 22 kilohm                                              
R7                 4.7 ohm                                                
R8                 5.6 megohm                                             
R9                 1.2 megohm                                             
R10                22 megohm                                              
R11                10 megohm                                              
R12                22 megohm                                              
R13                22 megohm                                              
R14                100 kilohm                                             
R15                33 kilohm                                              
R16                2.4 kilohm                                             
R17                1 kilohm                                               
R18                680 ohm (2 watt)                                       
R19                680 ohm (2 watt)                                       
C1                 .47 microfarads                                        
C2                 6.8 microfarads                                        
C3                 100 microfarads                                        
C4                 1.5 microfarads                                        
C5                 .33 microfarads                                        
C6                 .01 microfarads                                        
C7                 .01 microfarads                                        
C8                 47 picofarads                                          
C9                 .01 microfarads                                        
C10                22 microfarads                                         
D2                 IN4004                                                 
D3                 IN5236                                                 
D4                 IN4004                                                 
D5 (LED)           OP2982                                                 
D6 (photodiode)    22BN18M                                                
D7 (LED)           NSL5053                                                
D8                 IN4004                                                 
V1 Varistor        D39ZA1                                                 
18 Diode Bridge    DIW02M                                                 
Q1                 2N5088                                                 
Q2                 2N5088                                                 
Q3                 2N5366                                                 
Q4                 2N5088                                                 
Q5                 2N5088                                                 
SCR24              2N5064                                                 
22 Operational     LM4250CH                                               
Amplifier                                                                 
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 NOTE:                                                                    
 Unless otherwise specified resistors are 1/4 watt plus or minus 5%.      
 
    
     While there has been shown and described what is considered at present to be the preferred embodiment of the present invention, it will be appreciated by those skilled in the art that modifications of such embodiment may be made. It is therefore desired that the invention not be limited to this embodiment, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.