Abstract:
A circuit and method for controlling one or more light emitting diodes (LEDs) includes one or more LEDs connected to the emitter a transistor and controlling the brightness of the LEDs over a large range based on a voltage range at a base of the transistor controlled by an automatic adjusting mechanism.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present application relates generally to a circuit for controlling the brightness of a light emitting diode (LED) and more specifically to a circuit for extending the range over which the LED can be illuminated. 
     Description of Related Art 
     With the rising cost of energy, the search continues for lighting solutions that consume less power and operate at a lower overall cost. For a time, compact fluorescent light bulbs, or CFLs as they are commonly known, were believed to be a viable energy efficient solution. One problem with CFLs, however, is that they contain a small amount of mercury (Hg), a potentially dangerous substance, making disposal of the spent CFL bulbs difficult because they cannot simply be thrown in the garbage. Additionally, the mercury from broken CFLs can present a health hazard if not promptly and properly cleaned-up. In response, the Environmental Protection Agency (EPA) has issued guidelines for cleaning up and disposing of CFLs. Because of these issues, an energy efficient alternative to CFLs has been pursued. 
     Light emitting diodes (LEDs) are small light sources that become illuminated by the movement of electrons through a semiconductor material. Most LEDs belong to one of two categories, low power or high power. LEDs are also increasing in popularity and can be integrated into all sorts of products to provide white and colored light, such as indicator lights, flashlights, light bulbs, and integrated light fixtures. Significant advances have been made in LED technology to produce higher power at lower initial cost to the consumer. Also, LEDs last longer, are more efficient, and produce less heat than traditional incandescent light bulbs. LEDs also contain no mercury. 
     Circuits for controlling the ON/OFF nature and, to a degree, the brightness of LEDs are known. Conventional circuits for controlling the luminescence, or brightness, of an LED in devices such as lighted GFCIs, electrical receptacles with a nightlight feature, stand-alone nightlights and lighted switches, just to name a few, however, provide limited dimming range. For example, such conventional devices include placing the LED(s) in the collector circuit of a bipolar junction transistor (BJT) and attempting to control the brightness of the LED by controlling the voltage, or current, at the base of the transistor. 
     Referring to  FIG. 3 , a conventional circuit  300  for controlling an LED  310  is shown. Specifically, BJT  320  has a base terminal (B), a collector terminal (C) and an emitter terminal (E). Collector terminal (C) of the BJT is connected to the line, or positive, side of a power source at terminal  350  through LED  310 , resistor  330  and diode  340 . Emitter terminal (E) of the transistor is connected to the neutral, or negative, side of the power source at terminal  360 , and base terminal (B) of the transistor is tied to a voltage divider circuit comprising resistor  370  and photo resistor  380 . 
     When the base (B) of transistor  320  is biased with a voltage greater than the base-emitter junction voltage (V BE ), current flows through the collector circuit, that is, through resistor  330  and LED  310 , to the emitter (E) and ultimately to ground. If the current flowing through the collector circuit exceeds the value necessary to turn ON the LED, LED  310  will illuminate. Because the LED is in the collector circuit and a typical value for the base-emitter voltage, V BE , of a BJT is only 0.6 volts, however, the dimming range of LED  310  in the arrangement shown in  FIG. 3  or, in other words, the range by which the brightness of the LED can be controlled, is very narrow. In particular, in accordance with this arrangement, when the base voltage of the transistor is less than 0.6 volts, as compared to the voltage at the emitter (E), which is zero because it is tied to ground, the LED will remain OFF and when the base voltage is equal to or greater than 0.6 volts, the LED is ON. Thus, the brightness of LED  310  is controlled to be either dark or bright, with very little, or no, range in-between. 
     Thus, according to conventional circuits for controlling an LED lamp such as the circuit shown in  FIG. 3 , a wide range of brightness is unachievable and such circuits would not be ideal for use in certain devices. For example, certain devices may be used to provide light for people to see in a room where the amount of ambient light varies over the course of the day. Circuits such as the one in  FIG. 3  would not be ideal because the LED would either be OFF, when a certain amount of ambient light is present, or ON, when the ambient light drops below that threshold. Accordingly, at times light, or a certain brightness of light, is provided when it is not necessarily needed or desired, and at other times light, or an increased brightness of light, is desired but not provided. 
     To overcome the problems described above in connection with the conventional circuit shown in  FIG. 3 , it is has been known to add components to the collector circuit to regulate, or vary, the current flowing in the collector and, thus, in the LED. This technique adds more range of brightness for the LED as determined by the additional circuitry. For example, referring to  FIG. 4A  a schematic is shown in accordance with this revised conventional approach. Specifically, the schematic shown in  FIG. 4A  includes a circuit  400  to drive LEDs  410  which are used, for example, to light the area in the vicinity of a conventional electrical receptacle or GFCI device  480  through lens  490 , as shown in  FIG. 4B . When the ambient light is above a certain level, light sensor  420  reacts to the ambient light level and diode  425  begins to conduct. Sensor  420  is implemented by a light sensing diode and the amount of current conducted by sensor  420  is related to the amount of incident ambient light sensed by the sensor. 
     As the ambient light increases beyond a predetermined level, a level adjustable by potentiometer  430 , the Darlington transistor pair (Q1, Q2) is turned OFF. Specifically, the current flowing through diode  425  pulls down the base of transistor Q1 and transistor Q1, in turn, pulls down the base of transistor Q2. When the ambient light begins to decrease, e.g., as night begins to fall, the current flowing through sensor  420  begins to decrease, accordingly. At some predetermined ambient light level, the current flowing through sensor  420  diminishes to the point where current begins to flow through diode  425  and resistor  427 . As a result, transistors Q1 and Q2 are turned ON and collector/emitter current in Q2 flows, thus, energizing LEDs  410 . 
     In the schematic shown in  FIG. 4A , a dimmer potentiometer  415  is provided to allow the user to adjust the brightness of the LEDs  410 . Sensor  420  and variable resistor  430  function as a voltage divider. Therefore, the voltage presented to diode  425  changes in accordance with the variable resistance of sensor  420 . 
     Although this approach provides additional range in the brightness of the illuminated LED, it also adds complexity and cost to the circuitry and may not be desirable in many applications. 
     Accordingly, it is desirable to provide a circuit for controlling the brightness of one or more LED lamps over a relatively wide range where the circuit is simple and inexpensive and can be used in a variety of electrical devices. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention address at least the above problems and/or disadvantages and provide at least the advantages described below. 
     According to embodiments of the present invention, LED(s) are placed in the emitter circuit of the transistor, as opposed to placing them in the collector circuit like in the conventional circuits described above. Configuring the circuit in this manner takes advantage of the additional voltage range afforded for controlling the brightness of the LED(s). More particularly, by placing the LED(s) in the emitter circuit and varying the base voltage over a range of voltages, for example, by using a photocell resistor in the base circuit, the brightness of the LED(s) can be controlled over a larger range corresponding to the range of voltages at the base of the transistor. 
     For example, if two high intensity LEDs, each having a forward bias voltage of 1.2 volts, are placed in series between the emitter of the transistor and ground, and a voltage divider including a photocell resistor is placed in the base circuit of the transistor, the effective sensitivity of the voltage divider circuit is increased to approximately 3.0 volts, i.e., approximately 0.6 volts for the V BE  of the transistor plus 2.4 volts for the two LEDs. Accordingly, the LEDs can be dimmed over a wider range, i.e., from full brightness to OFF and the continuous range in-between. 
     Accordingly, an aspect of exemplary embodiments of the present invention is to provide a circuit for dimming the LED over a wider range using a photocell to generate a varying bias voltage on a transistor. Such a dimmable LED can be used, for example, in a wall switch, a regular or GFCI receptacle, a nightlight or some other illumination device. LED type lights are becoming increasingly popular due to the high energy efficiency and brightness of LEDs. Additionally, previously used neon lamps are becoming increasingly unavailable, leading to even more demand for LED lamps. 
     According to one exemplary embodiment of the invention a circuit for controlling the brightness of one or more light emitting diode (LED) is provided. A circuit according to this embodiment provides a wide range of dimming and comprises a transistor having a base, an emitter and a collector. A voltage source generates a voltage at the base of the transistor and an LED is disposed between the emitter of the transistor and electrical ground. 
     According to a further aspect of this embodiment an automatic adjusting circuit is provided for controlling the voltage at the base of said transistor. The automatic adjusting circuit can include a sensor for measuring an amount of ambient light present and increasing the brightness of the LED when the amount of ambient light decreases. The automatic adjusting circuit can also include a voltage divider circuit having two or more resistive elements. 
     According to a further aspect of this embodiment the range of brightness of the one or more LEDs is related to a voltage drop across the LEDs. Also, according to a further embodiment the voltage at the base of the transistor is controlled by the automatic adjusting circuit to continuously vary between about 1.80 volts when said LED is OFF and not illuminated, and about 3.12 volts when said LED is ON and maximally illuminated. 
     Also, the brightness of the light generated by the LED according to this embodiment varies within a continuous range from zero millicandelas when the ambient light is greater than a predetermined first threshold to a value greater than 50,000 millicandelas when the ambient light is less than a predetermined second threshold. 
     According to a further embodiment an electrical wiring device comprises an LED, a housing including a plurality of line terminals and a lens from which light from the LED emanates. A circuit for controlling the brightness of the LED is also provided that includes a transistor having a base, an emitter and a collector, and an automatic adjusting circuit for controlling the voltage at the base of the transistor. The LED is disposed between the emitter of the transistor and electrical ground. 
     A further aspect of this embodiment includes a light sensor, where the automatic adjusting circuit includes a control circuit configured to regulate the intensity of light emitted by the lens in response to the amount of ambient light detected by the light sensor. The intensity of the light emitted by the lens increases as the intensity of the ambient light decreases, or the intensity of the light emitted by the lens decreases as the intensity of the ambient light increases. 
     According to yet another embodiment of the invention the electrical device having the LED(s) and the dimming circuit is an electrical GFCI receptacle with a lens disposed on a front surface of the housing. 
     A method is also provided in accordance with an embodiment of the present invention where the method includes disposing one or more LEDs in an emitter circuit of a transistor, varying a voltage at the base of the transistor over a range of voltages and varying an electrical current flowing in the one or more LEDs in direct relation to varying the voltage at the base of the transistor. According to a further aspect of this embodiment an AC voltage is rectified and provided to a voltage divider circuit for varying the voltage, where the voltage divider circuit includes a photocell device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other exemplary features and advantages of certain exemplary embodiments of the present invention will become more apparent from the following description of certain exemplary embodiments thereof when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is an electrical schematic of a circuit for controlling the brightness of an LED in accordance with an embodiment of the present invention; 
         FIG. 2  is an electrical schematic for a circuit used in a wiring device that includes the circuit of  FIG. 1  in accordance with an embodiment of the present invention; 
         FIG. 3  is an electrical schematic of a conventional circuit for controlling the brightness of an LED; 
         FIG. 4A  is an electrical schematic of a further conventional circuit for controlling the brightness of an LED; 
         FIG. 4B  is a perspective view of a conventional electrical wiring device that includes the circuit shown in  FIG. 4A ; 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The matters exemplified in this description are provided to assist in a comprehensive understanding of exemplary embodiments of the present invention disclosed with reference to the accompanying figures. Those of ordinary skill in the art will recognize that various changes and modifications of the exemplary embodiments described herein can be made without departing from the scope and spirit of the claimed invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
       FIG. 1  is an exemplary circuit in accordance with one embodiment of the present invention. Circuit  100  includes BJT  120  which has respective base (B), collector (C) and emitter (E) terminals. Collector terminal (C) is connected to one end of a series resistor pair ( 130 ,  131 ) each of which, according to this particular embodiment, has a value of 4.7 k-Ohms. The other end of the resistor pair is connected to the positive, or line, side of a power source (not shown) via diode  140 . In this embodiment, the power source is an AC source and, thus, diode  140  is provided as a half-wave rectifier to reduce the voltage delivered by the AC source to the collector circuit of BJT  120 . 
     Base terminal (B) of BJT  120  is connected to the middle of a voltage divider circuit made up of resistor  170  and resistor  180 . According to this embodiment, resistor  170  has a value of 470 k-Ohms and resistive element  180  is a photocell device having a variable resistance value that changes based on an amount of ambient light sensed by the device. For example, according to this embodiment resistive element  180  is a photocell PDV-P9200 device by Advanced Photonix, Inc. (API) of Camarillo, Calif., which provides a resistance that varies from about 50 k-Ohms, when in the presence of maximum ambient light, or light having a wavelength of approximately 700 nm, to about 5 M-Ohms, when there is no, or virtually no, ambient light sensed by the device, that is, when the wavelength of any ambient light is below 400 nm. 
     The emitter terminal (E) of BJT  120 , according to this embodiment, is connected to two LEDs,  110 ,  111 , connected in series which, in turn, are connected to the neutral rail of the AC power source. For example, LEDs  110  and  111  may be white high-intensity LEDs with part number OVLEW1CB9 by OPTEK Technology Inc. of Carrollton, Tex. The relative brightness of such LEDs increases from zero, or OFF, when there is no forward current flowing through the LEDs, to about 60,000 millicandela (mcd) when the forward current is about 8.5 mA. Accordingly, to take advantage of the full range of brightness of the LED(s), a forward current, i.e., the value of the current in the emitter circuit of BJT  120 , is controlled to be from 0 mA, when there is full ambient light present and no output is desired from the LEDs, to about 8.5 mA when there is little or no ambient light present. 
     According to the embodiment shown in  FIG. 1 , to achieve a wide range of brightness for the LEDs  110 ,  111 , a variable input voltage is established at the base terminal (B) of BJT  120 . For example, the input voltage is varied from a low value of about 1.80 volts, e.g., when the LEDs are OFF, to about 3.12 volts, e.g., when the LEDs are at their maximum brightness. Thus, the full range of brightness is achieved using an input voltage that varies by about 1.32 volts. Of course, a circuit consistent with this embodiment could be designed where the range of the input voltage is controlled to be even greater than 1.32 volts but such a circuit would not necessarily provide additional range in brightness. For example, the input voltage could be controlled to vary from a value less than 1.80 volts to a value greater than 3.12 volts. But no further range in brightness would be achieved according to this specific embodiment because the LED would not turn ON until the input voltage is 1.80 volts and the maximum brightness from the LED is achieved when the input voltage is 3.12 volts. 
     In comparison, when the LEDs are placed in the collector circuit, as in the conventional example shown in  FIG. 3 , the BJT acts like a switch and there is little or no range of forward current through the LEDs. As a result, there is little or no range in brightness illuminated from the LEDs. For example, in accordance with one test conducted on a circuit arranged in accordance with  FIG. 3 , the LEDs were at full brightness when the input voltage at the base (B) of the BJT was 0.43 volts. The input voltage was then reduced slowly and the LEDs went completely OFF when the input voltage reached about 0.25 volts. Accordingly, the full range of brightness was achieved over an input voltage that only varied by about 0.18 volts. Because the circuit shown in  FIG. 3  acts as a switch, that is, either enough current is drawn through the collector circuit to drive the LEDs ON, or enough current is not drawn through collector circuit and the LEDs are OFF, the circuit shown in  FIG. 3  does not, and cannot, take advantage of the full range of brightness of the LEDs. 
     When a wider dimming, or brightness, range is available for illumination devices, certain benefits are realized. For example, instead of the light suddenly turning ON, i.e., at full brightness, after previously being completely OFF, and, thus, potentially causing a distraction, a lamp in accordance with the present invention will gradually become brighter, for example, as ambient light diminishes when current is controlled by a light sensor. Further, according to the present embodiment current flowing in the device is controlled to gradually increase over a period of time, e.g., hours, as opposed to being controlled to switch from zero current to maximum current all at once. This potentially results in an energy savings over time. 
       FIG. 2  is an electrical schematic in accordance with a further embodiment of the circuit in accordance with the present invention. The schematic of  FIG. 2  includes a circuit  200  which is used in a ground fault circuit interrupting device (GFCI) having a nightlight feature. As shown, dimmer circuit  250 , which is part of the overall circuit  200 , is structurally and functionally, as described in more detail below, similar to circuit  100  described in regard to  FIG. 1 . 
     Referring to  FIG. 2 , hot and neutral terminals  210 ,  220 , respectively, are connected to the hot and neutral rails of an AC power source (not shown). The AC waveform input on terminals  210  and  220  is then full-wave rectified by rectifier circuit  230  which provides power to GFCI chip  240 . GFCI chip  240  can be an RV4145 device by Fairchild Semiconductor, Inc. or any other suitable GFCI device. When latch assembly  260  is closed and a load is connected to the GFCI device, current is permitted to flow from the hot input terminal,  210 , through the sense and grounded neutral transformers,  215 ,  217 , respectively, on hot line conductor  219  to the hot face and hot load terminals  270 ,  272 , respectively, to deliver power to the connected load(s). In the absence of a ground fault or a grounded neutral fault, the current flowing on hot line conductor  219  returns from the load(s) on neutral conductor  221  via one or both of neutral face terminal  275  and neutral load terminal  277  through grounded neutral transformer  217  and sense transformer  215  back to the neutral rail of the power source. Under certain conditions, a small amount of the current on conductor  219  is also diverted to dimming circuit  250  and end-of life circuit  280  on hot conductor  218 . Dimmer circuit  250  and end-of-life circuit  280  are described in more detail below. 
     As mentioned above, under normal operation, i.e., with no faults present and latch assembly  260  closed, power is delivered to any load(s) connected to the face terminals,  270 ,  275 , and/or the load terminals,  272 ,  277 , via hot and neutral conductors  219 ,  221 . Under these conditions the amount of current flowing on hot conductor  219  from the line terminals to the load and face terminals is equal to the amount of current flowing on neutral conductor  221  from the face and load terminals to the line terminals. 
     When a fault occurs, e.g., a ground fault and/or a grounded neutral fault, and the amount of current flowing on hot conductor  219  is not equal to the amount of current flowing on neutral conductor  221 , a “fault” is detected by GFCI chip  240  and a gate signal is output from GFCI chip  240  on conductor  242 . The gate signal is delivered to the gate terminal of SCR  264  to turn the SCR ON, thus, enabling it to conduct current. When the SCR is ON it draws current through solenoid  262  to trip the device. That is, when a fault occurs and current is controlled to flow through solenoid  262 , latch assembly  260  is opened to prevent current from flowing in conductors  219 ,  221 , and power is no longer delivered to the load(s). After the fault condition has cleared, reset button  266  can be pressed by the user which mechanically closes latch assembly  260  to once again place the device in condition for normal operation. 
     Further, it is recommended by the industry that devices such as the one described in accordance with the present embodiment be tested periodically to ensure the device will trip if and when an actual fault occurs. Accordingly, when test button  268  is pressed, some of the current flowing on hot conductor  219  is diverted onto conductor  218  and back to the neutral line terminal  220  via bypass conductor  212  through resistor  211 . As a result, the current flowing on hot conductor  219  is not equal to the amount of current flowing on neutral conductor  221  and a fault is, thus, simulated. Under proper operating conditions this simulated fault condition is detected by GFCI chip  240  in similar fashion to a real fault, as described previously, and the device trips, opening latch  260 . Similar to the situation when there is a real fault, after a simulated fault condition is generated and the device properly trips, reset button  266  can be pressed by the user which mechanically closes latch assembly  260  to once again place the device in condition for normal operation. 
     The configuration and operation of dimming circuit  250  is now described in accordance with the present embodiment. Similar to the circuit described in regard to  FIG. 1 , dimming circuit  250  of GFCI circuit  200  includes an input diode  251  for half-wave rectifying the AC waveform resident on conductor  219 . The collector of BJT  255  is connected to two resistors,  256 ,  257 , which are arranged in-series between the transistor collector and the rectified power signal. The emitter of BJT  255  is connected to one end of two series-connected LEDs,  258 ,  259 , and the other end of the series LED combination is connected to one end of a photocell device  262 . The opposite end of the photocell device  262  is connected to the base of BJT  255  and also to one end of resistor  264 . The opposite end of resistor  264  is connected to the rectified power signal. 
     When a sufficient amount of ambient light is available in the vicinity of GFCI device, photocell device  262  has a very low resistance value. Accordingly, a small amount of current is permitted to flow from hot conductor  219  to neutral conductor  221  through a branch circuit which includes conductor  218 , diode  251 , resistor  264  and photocell  262 . Under this condition no current, or very little current, flows into the base of transistor  255  and, thus, no voltage is present on the base of transistor  255 , and LEDs  258 ,  259  remain OFF, or non-illuminated. 
     As the amount of ambient light diminishes, for example as nightfall approaches or the lighting in the room where the GFCI device is installed is dimmed or completely extinguished, the resistance value of photocell  262  begins to increase. As a result, an increasing amount of current is permitted to flow into the base of transistor  255 , as a diminishing amount of current flows through photocell  262 , and an increasing voltage is created on the base of the transistor. 
     End-of-life circuit  280  operates as follows. When test button  268  is pressed, a simulated ground fault is generated, as described previously, and if the device is operating properly, the device trips. If, however, the device does not trip when the test button is pressed, for example, because the GFCI chip  240  failed to detect the fault condition or the latch assembly contacts were stuck in the closed, or reset, state, and end-of-life condition (EOL) is indicated. Specifically, if the device does not trip when contacts 1 and 2 of test button  268  engage, contacts 1, 2 and 3 of test button  268  engage and current is permitted to flow from the hot conductor  219  through the branch circuit including conductor  218 , test button  268  and fuse  286  before returning to neutral conductor  221 . As a result of this continued current flow, fuse  286  opens and current is permitted to flow from the hot conductor  219  through the branch circuit including conductor  218 , resistors  284 ,  282 , capacitor  294 , diode pair  292 , resistor  290  and LED  288 . As capacitor  294  charges and discharges current is drawn through the branch circuit and the LED  288  blinks, indicating the end-of-life condition. 
     While the present invention has been shown and described with reference to particular illustrative embodiments, it is not to be restricted by the exemplary embodiments but only by the appended claims and their equivalent. It is to be appreciated that those skilled in the art can change or modify the exemplary embodiments without departing from and the scope and spirit of the present invention.