Abstract:
An auto darkening eye protection device including a shutter assembly, a light sensing circuit, a control circuit and a power source. The shutter assembly is adjustable to a plurality of shade levels. The light sensing circuit senses light from a welding arc and providing an output indicative of the shade level at which the shutter assembly should be operated, the light sensing circuit includes a phototransistor operative to receive the light from the welding arc and produce a phototransistor output representative thereof. The phototransistor output from the phototransistor is used to form the output of the light sensing circuit. The phototransistor is configured for surface mount and has an external base connection connected to the base of the phototransistor. The control circuit is configured to receive the output from the light sensing circuit and provide a drive signal to the shutter assembly responsive to said output. The drive signal is operative to drive the shutter assembly to one of said plurality of shade levels. The power source is coupled to the light sensing circuit and control circuit for powering same. The present invention provides reduced power consumption, improved attenuation of low intensity light signals, a sharp rise time from the phototransistor in response to high intensity light, and allows implementation into a smaller sleeker eye protection device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present patent application is a continuation of U.S. Ser. No. 10/827,014, (U.S. Pat. No. 6,855,922), filed on Apr. 19, 2004, which is a divisional patent application of U.S. Ser. No. 09/659,100, (U.S. Pat. No. 6,815,652) filed on Sep. 11, 2000, the entire content of both applications are hereby expressly incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to the field of auto-darkening eye protection devices, such as welding helmets having a shutter (or lens) assembly that automatically darkens upon the detection of a welding arc. A photosensitive device such as a photodiode or a phototransistor may be used to sense the intensity of light incident on the area of the shutter assembly so as to provide an indication to the circuitry controlling the shutter assembly that the shutter assembly needs to be driven to either a dark state or a clear state. If a welding arc is present, the welding helmet protects the eyes of the welder from any danger caused by the intensity of the welding arc by driving the shutter assembly to a dark state, thereby decreasing the amount of energy passing through the lens to the welder&#39;s eyes. U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, 5,248,880, 5,252,817, 5,347,383, 5,533,206, 5,751,258, 5,959,705, 6,067,129, and 6,070,264 each disclose various shutter assemblies and liquid crystal driver electronics that can be used in conjunction with the present invention. The disclosures of these above-mentioned patents are hereby incorporated in their entireties by reference.  
         [0003]     Commonly-owned U.S. Pat. No. 5,347,383 discloses a driving circuit for a liquid crystal shutter. The sensor circuitry of this invention utilizes a photodiode to detect the occurrence of welding. This sensor circuitry also utilizes a comparator to compare the sensed light signal with a threshold value to determine whether the shutter assembly should be driven to a dark or clear state. Additionally, the &#39;383 patent discloses the use of a 9 V supply.  
         [0004]     While the invention disclosed in this patent functioned for its intended purpose, a need was felt for an improvement in the power consumption by the sensor circuit. As incident light increases on a photodiode, the voltage across the photodiode will begin to saturate. To prevent the photodiode from saturating, a steadily increasing load must be put on the photodiode which leads to excessive power consumption.  
         [0005]     To alleviate the excessive power consumption inherent in a photodiode-based sensor circuit, a phototransistor has been utilized as a weld sensor. The use of a phototransistor allows the use of feedback to bias the phototransistor so that less current is needed to keep the phototransistor in its operational mode. Commonly-owned U.S. Pat. Nos. 5,252,817, 5,248,880, 5,751,258, and 6,070,264 are illustrative of sensor circuits using phototransistors as weld sensors. Each of these patents discloses a sensor circuit wherein the output of the phototransistor is fed into a comparator. The comparator compares the phototransistor output with a threshold level. If the phototransistor output exceeds the threshold level, the drive circuitry is activated to darken the shutter assembly. If the phototransistor output does not exceed the threshold level, the drive circuitry operates the shutter assembly in a clear state. While the circuits disclosed in these patents utilize feedback to bias the phototransistor and avoid the excessive drawing of current, heavy loads were still needed. The circuits implementing such designs used voltage supplies ranging from 5.6 V to 9 V. Therefore, a need was still felt for a sensor circuit having improved power consumption characteristics.  
         [0006]     Moreover, the phototransistors used in the prior art designs were metal can phototransistors. Metal can phototransistors are relatively big and bulky. Their size, height and relative difficulty in mounting serves as a limiting factor in the ability of designers to reduce the size of the units in which the sensor circuit is implemented. Thus, a need was felt to use a smaller and more compact phototransistor that is more easily mountable to a circuit board to produce a smaller, sleeker unit while still having the ability to maintain a constant signal level without excessive loading or the drawing of excessive current.  
         [0007]     Additionally, the sensor circuits of the prior art produced an output voltage from the phototransistor in response to incident light intensity as seen in FIG. 3 of the &#39;880 and &#39;817 patents. As can be seen, low light intensities produce a steep rise in output of the phototransistor. Because of the power drain caused by the response of the phototransistor to low intensity incident light, it is desirable that the phototransistor be configured to minimize the phototransistor output signal when the sensor circuit is exposed to low intensity incident light. Thus, a need existed for a sensor circuit that provided greater attenuation in the response of the phototransistor to low intensity incident light.  
         [0008]     While it is desirable to minimize the phototransistor output when the sensor circuit is exposed to low intensity incident light, the phototransistor still must be able to quickly increase its output in response to a transition from low intensity light to high intensity light, such as the light provided by a welding arc. Thus, an ever present need exists within the art to sharpen the rise provided by the phototransistor in response to sharp increases in light intensity.  
         [0009]     Also, the sensor circuitry of the prior art used a comparator to correlate the sensed light signal with the desired shade level. The comparator compared the output of the phototransistor with a threshold voltage signal to determine whether the shutter assembly should be driven to a dark state or a clear state. This design required additional circuitry to set the threshold voltage level. This additional circuitry not only complicated circuit design, but also increased the drain on the power supply. Thus, a need was felt to simplify the sensor circuitry to provide a more power-efficient way of correlating the phototransistor output to the proper shade setting of the shutter assembly.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     In order to solve these and other problems in the prior art, the inventor herein has succeeded in designing and developing an improved welding detection circuit utilizing a novel phototransistor-based sensor circuit. This sensor circuit comprises a phototransistor biased via a feedback circuit and having an output connected to an amplifier. The sensor circuit can be connected to a power supply and a control circuit to drive a shutter assembly to either a dark state or a clear state depending upon the intensity of incident light.  
         [0011]     One feature of the present invention is the use of a resistor coupled between the base and emitter of the phototransistor. This resistor helps reduce the current produced by the sensor during low ambient light conditions, thereby attenuating the phototransistor output in response to low intensity light signals, and helps produce a sharply rising voltage from the phototransistor in response to high intensity light signals. Preferably, the feedback circuit also includes a second resistor coupled between the emitter of a feedback transistor and ground to further attenuate phototransistor output in the presence of low intensity ambient light.  
         [0012]     Another feature of the present invention is its use of a planar phototransistor. Because of the planar phototransistor&#39;s small size, as compared to the metal can phototransistors used in the prior art, and because of the planar transistor&#39;s ability to maintain a constant signal level without excessive loading or the drawing of excessive current, the use of a planar phototransistor not only performs as well as metal can phototransistors, but also allows a reduction in the size of the unit in which the circuit is implemented. Preferably, the planar phototransistor is configured for a surface mount to further simplify construction of the circuit.  
         [0013]     Another feature of the present invention is its use of a closed loop noninverting amplifier to provide a gain for the phototransistor output. The gain of the amplifier is preferably set so that a sufficient output voltage will be generated to activate the shutter assembly when the phototransistor produces an output indicative of the presence of a welding arc. Preferably, a capacitor is coupled between the phototransistor output and noninverting input of the amplifier to block the DC portion of the phototransistor output.  
         [0014]     Another feature of the present invention is its use of the energy saved by an improved and efficient circuit design to recharge a rechargeable battery. By recharging the battery, the present invention extends the battery life of the invention&#39;s power supply.  
         [0015]     Another feature of the present invention is its use of a solar cell to reduce the circuit&#39;s power consumption. By using a solar cell to power various components of the circuit, the present invention prevent those components from acting as a drain on the power supply when the invention is left unexposed to light. Often, while not in use, a welding helmet will be left in a dark room or left face down on a table. When in these conditions, it is undesirable for the circuit to operate as a drain on the power supply. When, the welding helmet is in use, it will be either outdoors, in a lighted room, or in a dark environment with the presence of welding arc. In such conditions, it is desirable to use the light incident on the welding helmet to power the circuitry therewithin.  
         [0016]     The present invention uses the solar cell to power the phototransistor and the amplifier that is coupled to the output of the phototransistor, thus preventing those two components from draining the power supply when the welding helmet is left unexposed to light.  
         [0017]     The present invention also uses the solar cell to power an activation circuit, the activation circuit functioning to activate a signal generator. The signal generator, once activated, generates the voltage level and frequency signal to be used to drive the shutter assembly to a dark state. The generation of this signal acts as a drain on the power supply. By using the solar cell to power the activation circuit, the present invention improves the circuit&#39;s power consumption by triggering the signal generator when light is incident on the welding helmet.  
         [0018]     Yet another feature of the present invention is its use of a selector circuit for selecting the drive signal that will be delivered to the shutter assembly. If the sensor circuit indicates to the selector circuit that a welding arc is present, the selector circuit will cause a dark state drive signal to be delivered to the shutter assembly. If the sensor circuit indicates to the selector circuit that no welding arc is present, the selector circuit will cause a “clear state” drive signal to be delivered to the shutter assembly. The selector circuit uses a transistor as a switch to control the selection of the drive signal. An RC circuit is part of the selector circuit. The RC circuit utilizes its RC time constant to delay the transition of the “dark state” drive signal to the “clear state” drive signal, thus preventing the shutter assembly from switching to a clear state during brief “off” periods in the weld pulsations that exist with various weld types.  
         [0019]     While the principal advantages and features employed are explained above, a fuller understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a block diagram of the circuit of the present invention;  
         [0021]      FIG. 2  is a schematic diagram of the circuit of the present invention;  
         [0022]      FIG. 3   a  is a schematic diagram of an equivalent circuit for the sensor circuit when both the phototransistor and feedback transistor are “off.” 
         [0023]      FIG. 3   b  is a schematic diagram of an equivalent circuit for the sensor circuit when the phototransistor is “on” and the feedback transistor is “off.” 
         [0024]      FIG. 3   c  is a schematic diagram of an equivalent circuit for the sensor circuit when both the phototransistor and the feedback transistor are “on.” 
         [0025]      FIG. 4  is a graph depicting the voltage from the phototransistor as a function of incident light intensity.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]     A block diagram of the circuit of the present invention is depicted in  FIG. 1 . As can be seen, a power supply  250  is connected via power lines  251 ,  252 ,  253 ,  254 , and  255  to the sensor circuit  200 , activation circuit  326 , selector circuit  316 , signal generator  325 , and delivery circuit  315 . The power supply  250  furnishes the circuit with the power necessary for operation. Activation circuit  326 , selector circuit  316 , signal generator  325 , and delivery circuit  315  function together to control the shutter assembly  400  depending upon the signals received from power supply  250  and sensor circuit  200 .  
         [0027]     Activation circuit  326  receives power from the power supply  250  and sends an activation signal to the signal generator  325 . Upon activation by the activation circuit, the signal generator  325  generates a frequency signal  102  and a voltage signal  104  and sends the two signals  102  and  104  to the delivery circuit  315 . The delivery circuit  315  uses the frequency signal  102  and the voltage signal  104  to assemble a “dark state” drive signal for the shutter assembly  400 .  
         [0028]     The sensor circuit  200  senses incident light  256  and produces an output signal representative of the amount of incident light sensed. This output signal  211  is sent to selector circuit  316 . Depending upon the sensor circuit output, the selector circuit delivers a selection signal  107  to the delivery circuit  315 . If the sensor circuit  200  produces an output representative of the presence of high intensity light, such as the light produced by a welding arc, the selector circuit will send a selection signal  107  to the delivery circuit indicating that a “dark state” drive signal should be delivered to the shutter assembly  400 . If the sensor circuit  200  produces an output representative of the presence of low intensity light (i.e., no welding arc is present), the selector circuit will send a selection signal  107  to the delivery circuit indicating that a “clear state” drive signal should be delivered to the shutter assembly  400 . The delivery circuit  315  uses the selection signal  107  to determine the voltage level for the drive signals. If the selection signal  315  indicates that a “dark state” drive signal is needed, the delivery circuit will assemble a drive signal having a frequency set by the frequency signal  102  and voltage levels transitioning between the voltage signal  104  and the voltage of the power signal  253 . If the selection signal  315  indicates that a “clear state” drive signal is needed, the delivery circuit will assemble a drive signal having a constant voltage level set by the voltage of the power signal  253 . The delivery circuit  315  delivers this drive signal to the shutter assembly  400  via drive signal lines  110  and  112 . If the drive signal is a “dark state” drive signal, the shutter assembly  400  will be driven to a dark state. If the drive signal is a “clear state” drive signal, the shutter assembly  400  will be driven to a clear state.  
         [0029]     Referring to  FIG. 2 , a detailed schematic of the circuit depicted in the block diagram of  FIG. 1  is shown. Preferably, power supply  250  includes a rechargeable 3 V supply and a solar cell  257 . However, a single power source, either a battery or a solar cell, can be used without dramatically altering the operation of the circuit. It is also preferable that the solar cell serve as the circuit&#39;s primary power source, with the 3 V supply functioning to provide additional power to various circuit components when the solar cell voltage falls below the battery voltage. The solar cell power supply supplemented by the 3 V rechargeable supply is seen on line  100  (the signal on this line will be referred to as the 3 V signal). The solar cell power supply that is not supplemented by the 3 V rechargeable supply is seen on line  150 . Because of the improved power efficiency of the present invention, the circuit can utilize unused energy to recharge the 3 V supply.  
         [0030]     To implement these preferences, the 3 V supply is coupled to diode D 5  as shown, with the output of D 5  fed back to its input. Also, diode D 6  is coupled between the output of D 5  and V SOL+  as shown. Capacitor C 11  is coupled between V SOL+  and ground as shown. C 11 , preferably 0.1 μF, serves as a filter for V SOL+ . Capacitors C 10  and C 13 , coupled between the 3 V supply and ground, function to filter and smoothen the 3 V supply on line  100 . Preferably, C 10  and C 13  are each 3.3 μF. When light  256  reaches the solar cell, the voltage V SOL+  will increase until the solar cell reaches approximately 3.3 V (depending on the amount of incident light). The solar cell also functions to recharge the 3 V supply.  
         [0031]     The power supply  250  delivers a 3 V signal along line  100  to the V CC  pin of the 4060 chip  310 , to the V CC  pin of the 4053 chip  320 , to the X 1  pin of the  320  (through R 16 ), to the Y 1  pin of  320 , to the Z 0  pin of  320  and to the collector of transistor Q 3  (through resistor R 15 ). The power supply delivers V SOL+  along line  150  to the collector of the phototransistor S 1 , to the supply for amplifier  210 , and to the base of transistor Q 1  (through resistor R 5 ).  
         [0032]     When light incident on the solar cell increases, thereby causing an increase in voltage on line  150 , the voltage at the base of control transistor Q 1  will increase. Once the voltage at the base of Q 1  reaches approximately 0.6 V, Q 1  will turn “on” to activate the signal generator  325 . By only activating the signal generator when there is light incident on the solar cell, the power drain on the circuit is reduced because the signal generator will only be active when the welding helmet is likely to be in use. When the welding helmet is not in use, it is typically left in either a dark room or with its solar cell face down, which in either case would prevent the solar cell from triggering Q 1 . With Q 1  “off,” the signal generator will not drain the power supply. When the welding helmet is in use, it will be exposed to outdoor light, indoor light, or weld light. In these situations, the solar cell will trigger Q 1  to activate the signal generator.  
         [0033]     The voltage at the base of Q 1  is set by the resistor divider circuit formed by the junction  101  of R 5  and R 7  as shown. Preferably, R 5  is 2 MΩ and R 7  is 1 MΩ. The emitter of Q 1  is grounded. The collector of Q 1  is connected to the 3 V supply through R 12 . The collector of Q 1  is also connected to the RESET pin of  310  via line  105 . The signal on line  105  serves to activate the signal generator  325 . Once Q 1  is turned “on,” the voltage at the collector of Q 1  will change from 3 V to substantially 0 V as a path to ground is created. This transition causes the signal on line  105  to go from high to low, which removes the reset signal from  310 . With the reset signal low,  310  begins toggling C IN  and C OUT . The two C OUT  pins and the C IN  pin of  310  are connected to a charge pump  324  as shown. Charge pump  324  comprises capacitors C 2 , C 3 , C 4 , C 5 , C 6 , and C 7  and diodes D 1 , D 2 , D 3 , and D 4  as shown. The charge pump  324  functions to generate the signal used to set the voltage level of the “dark state” drive signal. This voltage level is sent to the delivery circuit  315  via line  104 . Preferably, this voltage level is set to be approximately −15 V using charge pump capacitors C 2  through C 7  of 0.1 μF apiece. The −15 V signal on line  104  is stored in capacitor C 8 , which is preferably 6.8 μF. RC circuit  327  sets the frequency of the charge pump. Preferably, the RC circuit is designed to produce a frequency of approximately 550 Hz using an R 1  of 2 MΩ, an R 2  of 1 MΩ, and a C 1  of 680 μF. This frequency can be passed through a binary counter in chip  310  to divide the frequency to about 0.5 Hz. The 0.5 Hz signal exits  310  through line  102  at pin Q 10  as shown. Line  102  delivers this frequency signal to pins B and C of the 4053 chip  320 . This frequency serves as the frequency for the “dark state” drive signal.  
         [0034]     The output of the sensor circuit on line  211  is coupled to the base of control transistor Q 3 . The collector of Q 3  is connected to a 3 V power supply through R 15 . The collector of Q 13  is also coupled to the ground through capacitor C 14 . The collector of Q 3  is also coupled to pin A of the 4053 chip  320  via line  107 .  
         [0035]     Pins B and C of the chip  320  receive the frequency signal  102  from signal generator  325 . Thus, pins B and C toggle at the frequency of frequency signal  102 , which is preferably 0.5 Hz. Pin A controls the selection of pins X 0  and X 1 . When pin A is “high,” pin X 1  is selected. When pin A is “low,” pin X 0  is selected. Pin B controls the selection of pins Y 0  and Y 1 . When pin B is “high,” pin Y 1  is selected. When pin B is “low,” pin Y 0  is selected. Pin C controls the selection of pins Z 0  and Z 1 . When pin C is “high,” pin Z 1  is selected. When pin C is “low,” pin Z 0  is selected. The selection of a particular pin means that the signal on the selected pin will be passed on to the output associated with the pair. For example, when pin A is “high,” pin X 1  is selected and the signal at pin X 1  is passed through to output pin X. When pin B is“low,” pin Y 0  is selected and the signal at pin Y 0  is passed through to output pin Y.  
         [0036]     Pin X 0  is connected to the B 15  V voltage signal supplied on line  104  by signal generator  325 . Pin X 1  is connected to the 3 V power supply via R 16 . Thus, the status of the signal at pin A determines whether a B 15  V signal or a 3 V signal is passed through to output pin X. Pin Y 0  is connected to output pin X via line  106 . Thus, whatever signal is passed to X will be received at Y 0 . Pin Y 1  is connected to the 3 V supply. Pin Z 0  is also connected to the 3 V supply. Pin Z 1 , like pin Y 0 , is connected to output pin X via line  106 . Output pins Y and Z are connected to the shutter assembly  400 .  
         [0037]     The signal on line  211  at the output of the sensor circuit  200  controls whether Q 3  is turned “off” or “on.” Q 3  needs a signal on line  211  of about 0.6 V to turn “on.” The sensor circuit  200  is configured to produce an output of at least 0.6 V when a welding arc is present. If no welding arc is present, Q 3  will not receive a sufficient voltage on line  211  to turn “on.” 
         [0038]     When in the “off” state, the voltage on line  107  will be “high,” that is, substantially equal to the 3V supply. While pin A is “high,” the signal at pin X 1  is passed through to output pin X. Since pin X 1  is substantially 3 V, this signal will be passed to input pins Y 0  and Z 1 . Thus, when pin A is “high” (which corresponds to no welding arc being present), pins Y 0 , Y 1 , Z 0 , and Z 1  will all receive a substantially 3 V signal. Thus, as pins B and C alternate from “high” to “low” at 0.5 Hz (the frequency of signal  102 ) and pins Y 0  and Z 0 , and pins Y 1  and Z 1  are alternately passed through to output pins Y and Z, the resultant signal on lines  110  and  112  will be a substantially steady 3 V signal. This steady 3 V signal on lines  110  and  112  corresponds to a “clear state” drive signal, that is, the drive signal which will transition the shutter assembly to a clear state.  
         [0039]     When the output of the sensor circuit  200  is sufficient to turn “on” Q 3  (indicating the presence of a welding arc), the signal on line  107  will quickly go from 3 V to 0 V as a path to ground is created through Q 3 . Thus, pin A will go “low.” When pin A is “low,” the signal at pin X 0  is passed through to output pin X. Since pin X 0  is B 15  V, this B 15  V signal will be received at pins Y 0  and Z 1 . As pins B and C alternate from “high” to “low” at 0.5 Hz (the frequency of signal  102 ), the value of Y will be 3 V when the value of Z is B 15  V and vice versa. The signal on line  110  will alternate between 3 V and B 15  V at 0.5 Hz. The signal on line  112  will alternate between 3 V and B 15  V at 0.5 Hz out of phase with the signal on line  110 . Thus, the resultant signal delivered to the shutter assembly  400  will be an 18 V square wave having a 0.5 Hz frequency. This 18 V, 0.5 Hz, square wave corresponds to a “dark state” drive signal, that is, the drive signal which will transition the shutter assembly to a dark state.  
         [0040]     When the welding arc ceases, the voltage on line  211  will be insufficient to maintain Q 3  in an “on” state. Once Q 3  turns “off,” the signal on line  107  will return to a “high” state. However, this transition will not be instantaneous due to the RC circuit formed by R 15  and C 14 . The transition of  107  from “low” to “high” will be delayed as C 14  charges. By selecting the RC time constant for R 15  and C 14 , the delay can be set to accommodate brief “off” periods in the “on/off” pulsating light of various welding conditions. Before C 14  recharges, the light pulse of the weld arc will pass through the sensor circuit  200  and reactivate Q 3  to cause a quick transition on line  107  back to “low.” Preferably R 15  is 2 MΩ and C 14  is 0.1 μF. The “low-to-high” transition on line  107  will be about 0.25 seconds in a circuit with those parameters.  
         [0041]     Sensor circuit  200  includes a phototransistor S 1  coupled to a feedback circuit  249 . Additionally, resistor R 13  is coupled between the base and emitter of the phototransistor. The output of phototransistor S 1  is sent to line  208 . A load resistor R 6  is connected between line  208  and ground. Additionally, a capacitor C 9  couples line  208  to line  209 . Resistor R 4  is connected between line  209  and ground. Line  209  is also connected to the noninverting input of amplifier  210 . Amplifier  210  is preferably configured as closed loop noninverting amplifier wherein the R 9  and R 3  feedback loop is connected to the inverting input of amplifier  210  as shown. The output of amplifier  210  on line  211  serves as the sensor circuit output. Line  211  is connected to the input of selector circuit  316 .  
         [0042]     The solar cell  257  powers phototransistor S 1  and amplifier  210  via line  150 . Thus, if the solar cell is left unexposed to incident light, phototransistor S 1  and amplifier  210  will not receive power, thus preventing the phototransistor and amplifier from draining the power supply when the welding helmet is not in use (when not in use, the welding helmet is typically not exposed to light).  
         [0043]     The feedback circuit  249  for the phototransistor S 1  comprises a resistor capacitor circuit  248  connected between the emitter of the phototransistor and ground, and a feedback transistor Q 2  having a base coupled to line  205  of the resistor capacitor circuit  248 , a collector coupled to the base of the phototransistor, and an emitter coupled to the ground via resistor R 10 .  
         [0044]     Phototransistor S 1  serves as the weld sensor. It receives an input of incident light  256  and produces an output on line  208  representative of the intensity of the incident light. The phototransistor S 1  used in the present invention is preferably a planar phototransistor configured for a surface mount. The planar phototransistor is smaller than conventional metal can phototransistors, thus allowing a reduction in size of the unit in which the sensor circuit is implemented. While the metal can phototransistors used in the sensor circuits of the prior art had a thickness of about ½ inch, the planar phototransistors with a surface mount used in the present invention have a thickness of only about ¼ inch. This reduction is thickness allows the sensor circuit to be implemented into a smaller and sleeker unit. Further, the surface mount configuration of the phototransistor S 1  allows the phototransistor to be easily affixed to a circuit board. The inventor herein has found that the TEMT4700 silicon npn phototransistor manufactured by Vishay-Telefunken is an excellent phototransistor for the present invention as it has a smaller size than conventional metal can phototransistors and allows the sensor circuit to maintain a constant signal level without excessive loading or the drawing of excessive current.  
         [0045]     The resistor capacitor circuit  248  and the feedback transistor Q 2  in the phototransistor feedback circuit  249  function to adjust the sensitivity of the phototransistor S 1 . The resistors R 8  and R 11  and capacitor C 12  are chosen to be of a size to provide a relatively large time constant, and therefore a relatively slow response to changes in voltage on line  208 . The delay exists because of the time it takes for the voltage on line  205  to charge to an amount sufficiently large to activate Q 2 . Exemplary values for R 8  and R 11  are 1 MΩ and 2 MΩ respectively. An exemplary value for C 12  is 0.1 μF. A detailed description of the operation of the resistor capacitor circuit  248  and feedback transistor Q 2  can be found in prior U.S. Pat. Nos. 5,248,880 and 5,252,817, the disclosures of which have been incorporated by reference.  
         [0046]     R 13  functions to attenuate phototransistor output in response to low intensity incident light by essentially shutting down the phototransistor when low intensity light is present. R 13  further aids the response of the phototransistor by enabling the phototransistor to sharply increase its output when high intensity light is detected. R 10 , connected between the emitter of Q 2  and ground further improves the sensor circuit by attenuating phototransistor output in response to low intensity light signals. Load resistor R 6  is coupled between phototransistor output  208  and ground helps to further attenuate phototransistor output when low intensity light is incident upon the phototransistor. An exemplary value for R 10  is 20 kΩ. An exemplary value for R 6  is 1 MΩ.  
         [0047]     Referring to  FIGS. 3   a ,  3   b , and  3   c , the operation of the sensor circuit  200  will be described. First, the phototransistor has operational characteristics similar to a photodiode whose output is fed into the base of a conventional npn transistor. The equivalent circuit for a phototransistor is depicted in  FIGS. 3   a ,  3   b , and  3   c . Photodiode  221  is connected between the base and collector of npn transistor  222 . Incident light will produce a photocurrent, I PHOTO , from the photodiode  221 . I PHOTO  serves to feed the base of the transistor  222 . However, in the sensor circuit of the present invention, resistor R 13  is also coupled between the base and emitter of the phototransistor. Thus, in the equivalent circuit model, R 13  is connected between the base and emitter of transistor  222  as shown.  
         [0048]     When light  256  first reaches the phototransistor, the phototransistor S 1  is in the “off” state. Additionally, feedback transistor Q 2  is in the “off” state.  FIG. 3   a  depicts the equivalent circuit model for the sensor circuit  200  in this mode of operation. In the equivalent circuit model, the photocurrent, I PHOTO , sees an essentially open circuit in the path to the base of transistor  222  because transistor  222  is “off.” Thus, I PHOTO  passes through R 13  as shown in  FIG. 3   a . The voltage drop across R 13  caused by I PHOTO  will be equal to the base-emitter voltage drop across transistor  222  because R 13  is coupled between the base and emitter of  222 . To turn “on” the transistor  222 , the voltage drop across the base and emitter of transistor  222  needs to be about 0.47 V to 0.53 V. By selecting a value of R 13  that will keep the voltage drop across R 13  below 0.47 V to 0.53 V in response to a photocurrent that corresponds with low intensity incident light, R 13  can attenuate the phototransistor&#39;s output in response to low intensity incident light. An exemplary value for R 13  is 10 MΩ. Because the phototransistor is not turned “on,” the photocurrent is kept away from the base, preventing amplification of the photocurrent (the base of the transistor  222  drives the gain of the phototransistor S 1 ). When transistor  222  is turned “on,” photocurrent will feed the base of transistor  222 , and the output of the phototransistor will be amplified accordingly. Once on line  208 , the photocurrent will be further diverted to ground through R 6 , through the resistor capacitor circuit  248 , and through R 4  (via C 9 ). The current passing through the resistor capacitor circuit  248  will begin the charging of capacitor C 12  at line  205 .  
         [0049]     As more light reaches the phototransistor, I PHOTO  will increase. When I PHOTO  is sufficiently large to create a voltage drop across R 13  of about 0.47 V to 0.53 V, the transistor  222  will turn “on.” Also, if intense incident light, such as light from a welding arc, reaches the phototransistor, a large photocurrent will be produced. The large photocurrent passing through R 13  will quickly create a voltage drop across R 13  that is sufficient to turn “on” transistor  222 , thus achieving a sharp increase in phototransistor output in response to high intensity light. While in the preferred embodiment R 13  is a resistor, it is conceivable that any nonreactive element providing a quick voltage drop in response to a current may be used in the invention.  
         [0050]     When transistor  222  first activates, the feedback transistor Q 2  will still be in the “off” state while it waits for the voltage on line  205  to charge through capacitor C 12 .  FIG. 3   b  depicts the equivalent circuit model for the sensor circuit in this mode of operation. Part of I PHOTO  will be fed into the base of transistor  222  and part of I PHOTO  will be diverted through R 13 . The current fed into the base of  222  will drive the gain for the phototransistor. The output of the phototransistor on line  208  will be the sum of the emitter current of transistor  222  and the current diverted through R 13 . Once on line  208 , the current will be further diverted to ground through R 6 , through the resistor capacitor circuit  248 , and through R 4  (via C 9 ).  
         [0051]     As previously explained, the current passing through the resistor capacitor circuit  248  will cause the capacitor C 12  to charge. As C 12  charges, the voltage on line  205  will begin to increase toward 0.6 V. Once the voltage on line  205  reaches about 0.6 V, the feedback transistor Q 2  is turned “on.” Once Q 2  is activated, it drains some of the photocurrent away from the base of the transistor  222  as shown in  FIG. 3   c . By diverting photocurrent from the base of the phototransistor, the feedback transistor Q 2  decreases the gain provided by the phototransistor, thereby causing a drop in the phototransistor output despite an incident light level that remains essentially constant. This biasing operation allows the phototransistor to maintain a constant signal level for a steady light intensity.  
         [0052]     The signal on line  208  if fed into an amplifier  210 . The signal is first passed through a capacitor C 9  to block the DC component of the detected signal. Line  209  contains the DC blocked detected signal. The current on line  209  is diverted to ground via resistor R 4 .  
         [0053]     The sensor circuit operates in the presence of both AC welds and DC welds. In an AC weld (also known as a MIG weld), the welding light is pulsating. Thus, the phototransistor will detect a pulsating light signal. The frequency of the pulsations is often 120 Hz. In a DC weld (also known as a TIG weld), the welding light is substantially continuous, with the exception of a small AC component. When an AC weld is present, the phototransistor will produce a pulsating output on line  208 . The variations in the voltage signal due to the pulses will be passed through the capacitor to line  209  and fed into the amplifier. The amplifier will then provide gain for the signal on line  209  which is sufficient to trigger the delivery of a “dark state” drive signal to the shutter assembly  400 . The slow charge time of capacitor C 14  in selector circuit  316  will prevent the transition from a dark state to a clear state during brief interruptions in the AC weld pulses. Before C 14  recharges, the next AC pulse will cause the capacitor to discharge before a “clear state” drive signal is triggered.  
         [0054]     When a DC weld is present, the phototransistor will quickly produce an output on line  208  catching the rising edge of the DC weld. This sudden rise in voltage on line  208  will be passed through to the amplifier  210  causing a signal on line  211  sufficient to trigger the delivery of a “dark state” drive signal to the shutter assembly  400 . Thereafter, capacitor C 9  will block the DC component of the DC weld, allowing only the AC variations in the DC weld to pass through to the amplifier. The amplifier  210  must have a gain sufficient to keep the shutter assembly in the dark state when the AC variations in the DC weld reach the amplifier.  
         [0055]     The amplifier  210  is a closed loop, noninverting amplifier as described above. The output of the amplifier is fed into a selector circuit  316 . The selector circuit  316  uses a phototransistor to send a selection signal to the delivery circuit  315  via line  107 . As previously explained, for the selector circuit  316  to send a signal indicating that a “dark state” drive signal should be delivered to the shutter assembly, a 0.6 V signal needs to be applied to the base of control transistor Q 3  on line  211 . Thus, it can be seen that amplifier  210  must produce a signal of about 0.6 V on line  211  when the phototransistor produces a signal on line  208  indicative of the presence of a welding arc. The gain of amplifier  210  must therefore be set such that it will sufficiently amplify its input voltage to produce an output voltage of about 0.6 V when the input signal on line  209  indicates the presence of a welding arc. The gain of the amplifier  210  is set by resistors R 9  and R 3  in the feedback loop. The gain of the amplifier having this configuration is: 
 
Gain=( R   9 / R   3 )+1 
 
         [0056]     The inventor herein has noted that a gain of about 3.67 will be sufficient for the amplifier to trigger the “dark state” drive signal when a welding arc is lit. Exemplary values for R 9  and R 3  would be 1 MΩ and 374 kΩ respectively.  
         [0057]     Referring to  FIG. 4 , the output of the phototransistor will be described in relation to the amount of incident light. The curve of  FIG. 4  depicts the output of the phototransistor on line  208  (on the vertical axis) as a function of the intensity of incident light (on the horizontal axis). The curve has a relatively steep portion  241  for lower intensity incident light and a less steep portion  242  for higher intensity incident light. The operation of the phototransistor in these portions of the curve are discussed in detail in prior U.S. Pat. Nos. 5,248,880 and 5,252,817, the disclosures of which have been incorporated by reference. Of note for the present invention is curve portion  243  which represents an extremely low voltage response from the phototransistor when the incident light has low intensity. This gap in the voltage response of the phototransistor is due to the effect of R 13  whereby it prevents the activation of the phototransistor in the presence of low intensity light. However, the invention still provides a sharp increase in phototransistor output when the light intensity increases as can be seen by the steep slope of curve portion  241 .  
         [0058]     The invention has been disclosed herein in the context of the inventor&#39;s preferred embodiment. However, changes and modifications thereto as would be apparent to one of ordinary skill in the art are intended to be included by the inventor and the invention should be limited only by the scope of the claims appended hereto, and their equivalents.