Optical smoke detector circuit
BACKGROUND OF THE INVENTION 
     This invention relates to the field of electronic optical smoke detectors. Prior art optical type techniques for detecting smoke require specially designed chambers for the passage of smoke filled air. These chambers are designed to minimize high ambient lighting situations. Some of these chambers require long airflow pathways to minimize the ambient lighting effects. These long path chambers cause attentuation of the actual smoke signal. Thus, the sensitivity of the smoke chamber is lessened. 
     SUMMARY OF THE INVENTION 
     The smoke detector apparatus of this invention includes a pulsed light source and a phototransistor detector. The detector is incorporated in the feedback path of a band pass filter which controls the bias to the phototransistor. The output of the band pass filter is fed into a synchronous detector combined with a low pass filter to provide a very narrow band pass for excellent frequency selection. The apparatus sensitivity is not affected by the magnitude of ambient light level at the detector. 
     It is an object of the invention to provide an electronic circuit for maintaining the sensitivity of the phototransistor in both high and low (dark) ambient lighting conditions. 
     It is an object of this invention to provide an electronic light sensing circuit and a light source circuit which obviates the need for long airflow path chambers and is impervious to ambient lighting conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a system schematic representation of the smoke detector and is partially in block diagram form. 
     FIG. 2 is a schematic diagram of a preferred embodiment of a portion of the circuit of FIG. 1. 
     FIG. 3 is another embodiment from that shown in FIG. 2. 
    
    
     DESCRIPTION 
     There is described herein an improved smoke detector of the photo-electric type. The smoke detector includes a pulsating light source and a phototransistor light detector. The light detector is connected in the feedback path of a band pass filter. The band pass filter provides not only the band pass frequency but also provides an automatic gain control for the light detector. The automatic gain control is effective to maintain a high and uniform sensitivity of the smoke detector over wide ranges in light intensity from conditions of high intensity ambient lighting to conditions of low (near dark) intensity ambient lighting. It is desirable that the system have a uniform sensitivity to smoke under varying ambient light conditions. In a phototransistor it has been found that with very little emitter current such as normally may tend to exist at a low level of ambient lighting, the transistor has low gain, i.e. low sensitivity. By this improved circuit design to force an increase in the phototransistor current at low ambient light levels, the sensitivity is maintained high. In addition, at high ambient light levels, the phototransistor may tend to saturate, i.e. the emitter current may be high. Again, the improved bias circuit design prevents the phototransistor from saturating, thereby maintaining the sensitivity to smoke. 
     The smoke detector system is broadly shown in FIG. 1, in which a light emitting diode (LED) 10 provides a pulsing light source to a smoke sensing chamber 20. The chamber 20 has adequate openings to allow the ambient air of the area or room being protected to enter the sensing chamber. A phototransistor 30 in the sensing chamber is arranged and positioned to detect the pulsing LED light when it is reflected from smoke which has entered the chamber. As an example, such a chamber in which reflected light reaches the detector is shown in my U.S. Pat. No. 3,185,975. The pulsing light source 10 is controlled by the oscillator 12 of frequency f o  (which may be 1000 hz, for example). LED 10 and the collector electrode of phototransistor 30 are both energized from a positive DC source, the energization of LED 10 being interrupted at the frequency f o  of the oscillator 12 by the switching means 13. When there is smoke in the chamber 20 to reflect the pulsing light towards sensor 30, the sensor will see a signal at the frequency f o . The phototransistor 30 is connected in the feedback loop at the input of an active amplifier in band pass filter 40 which filter also has a secondary function as an integrator. Specifically, a feedback lead 16 of the band pass filter 40 is connected to the control electrode of the phototransistor 30. The emitter of phototransistor 30 is shown as connected by a junction 18 and a resistor 19 to ground, and the phototransistor emitter at the junction 18 is connected to the input of filter 40. The output of the band pass filter 40 is also fed into the synchronous detector or demodulator 50. The synchronous detector is synchronized with the pulsing light source 10 at frequency f o  by the oscillator 12. The generally shown switching points 13 and 15 may be suitable solid state switches. The output of the synchronous detector is fed into a low pass filter 60, the output of which in turn is connected to an alarm circuit. The band pass filter 40 has a relatively larger band pass than that derived from synchronous detector 50 and low pass filter 60 which is very selective (such as f o  ±0.1%) to the frequency of oscillator 12. Signals at frequencies f o  to the input of the synchronous detector translate to DC at the output of the low pass filter 60. 
     A more specific embodiment of the active filter portion of FIG. 1 is shown in the schematic presentation of FIG. 2 which utilizes a conventional active filter with a novel and unique way of coupling a phototransistor into the input. The filter, per se, generally similar to that shown in FIG. 2 is sometimes called a state variable filter or biquad amplifier and is well known in the art. The active band pass filter shown in FIG. 2 comprises operational amplifiers 70, 71 and 72. A dual source, positive and negative, with respect to ground energizes this circuit allowing the outputs of the operational amplifiers to move positive or negative. Junction 18 is connected to the negative input of amplifier 70, the positive input of which is connected through a resistor R 2  to ground. A feedback resistor R 4  from the output of amplifier 70 is connected to the base electrode of phototransistor 30. This portion of the active filter provides a high pass output. The amplifier 70 output is also connected through a resistor R 6  to the negative input of amplifier 71, the other input thereof being connected to ground. A feedback capacitor C 2  is connected from amplifier 71 output 14 to its negative input. Another feedback circuit path connects the output of amplifier 71 through a resistor R 5  to the positive input of amplifier 70. R 5  is selected to provide the proper band width of the band pass filter. Output 14 is also connected through a resistor R 7  to the negative input of an amplifier 72 the positive input of which is connected to ground. A feedback capacitor C 1  connects the output of amplifier 72 to its negative input terminal. The amplifier 72 output is also connected by a resistor R 3  to the base electrode of phototransistor 30. The feedback lead 16 (FIG. 1) is in the embodiment of FIG. 2 made up of signals from feedback resistors R 3  and R 4 . The base current of phototransistor 30 is the sum of the current flowing through R 4  and current flowing through R 3 . The DC currents flowing through R 3  and R 4  maintain the phototransistor&#39;s sensitivity to light to be a constant. In effect, the DC emitter current is kept constant. At the same time, amplifiers 70, 71 and 72 provide the necessary band pass filter operation, and the output 14 provides a band pass output e o  from the active filter. 
     Consider the operation of FIG. 2 when the phototransistor is operating at steady state. That is, phototransistor 30 is receiving a finite amount of ambient light and the currents through R 3  and R 4  are at a constant. Also assume that the output voltage e o  at 14 is at a finite value. If the ambient light that phototransistor 30 receives increases, the phototransistor emitter current (DC) will tend to increase and the resulting DC voltage change into the negative input of amplifier 70 will cause the output of amplifier 70 to decrease. The decrease in output voltage of amplifier 70 will cause the current flowing through R 4  to also decrease. This, in turn, will tend to cause the voltage at 18 to decrease. Thus, a negative feedback circuit is provided by R 4  around the operational amplifier 70 to tend to maintain constant emitter current of the phototransistor when there is either an increase in ambient light or a decrease in ambient light. At the same time as the negative feedback current above is operating, operational amplifier 71 sees the change in output from amplifier 70. The voltage e o   will increase and cause operational amplifier 72 to cause a decrease in voltage. The decrease or falling in voltage at the output of amplifier 72 will tend to lessen the current flowing through resistor R 3  and subsequently cause the current flowing through the base of phototransistor 30 to also tend to decrease. This decrease in base current also causes the voltage at 18 to decrease. Thus, a second negative feedback path for DC changes is provided. Also in very dark ambient lighting, phototransistor (DC) current will be forced by current through R 3  and R 4 . Transistors with very little emitter current have low gain. Forcing this phototransistor current up, in dark conditions, maintains high sensitivity. The net effect of operational amplifiers 71, 72 and 70 tend to cause the voltage at point 18, at the input of operational amplifier 70, to remain near ground potential. Thus, in this improved smoke detector circuit the described feedback to the base of the phototransistor 30 is effective to automatically adjust its base bias to maintain its emitter near ground potential over very large ambient light conditions (i.e. average phototransistor emitter current is a constant). A constant emitter current regardless of ambient lighting maintains the gain or sensitivity of the phototransistor to be nearly constant since sensitivity is a function of emitter current. 
     Because the phototransistor 30 responds to the sum of currents flowing through R 3  and R 4  as in the conventional active filter amplifier circuit of FIG. 1, the circuit of FIG. 2 acts as an active filter when the phototransistor is responding to a pulsating light at the central band pass frequency. Thus, when smoke is present the phototransistor will be responding to the pulsating light at the central band pass frequency f o . 
     A pulsating signal at the band pass frequency f o  will be at the output e o . The active filter will not attentuate the base current at phototransistor 30 at the band pass frequency since the A.C. components of current in R 3  and R 4  cancel each other (i.e. 180° out of phase). The integrator action of each of operational amplifiers 71 and 72 provides the 180° phase shift at the frequency f o . Light pulse signal currents from the phototransistor at the band pass frequency f o  produce the signal that is amplified and supplied to the band pass output e o . 
     Although FIG. 2 shows the preferred circuit embodiment for use in the invention, another embodiment is shown in FIG. 3. In this alternate embodiment, a different form of active filter design is used, however, this general band pass filter type is well known in the art. With the phototransistor in the feedback loop of the amplifier, the circuit of FIG. 3 behaves essentially in the same manner as that of FIG. 2. Here the feedback lead 16 (FIG. 1) is the embodiment of a signal from the bridge T RC feedback network. The two resistors in this network provide the DC component of current to maintain the constant emitter current of phototransistor 30.