Patent Publication Number: US-2005128743-A1

Title: Light apparatus and method for controlling the intensity of a light emitting diode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. provisional application Ser. No. 60/529,777 filed Dec. 16, 2003. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a light apparatus and method for providing a light display using LEDs.  
       2 . Background Art  
      Combining light of one color with light of another color will result in the creation of a third color. For example, red, blue, and green lights can be combined in different proportions or intensities to create almost any color in the visible spectrum. Light emitting diodes (LEDs) of different colors may be used for this purpose. It would be desirable to apply LED lighting technology to an application useful for home sensory therapy. It would further be desirable to have an affordable lighting device based on LED technology that creates a relaxing, stimulating, and entertaining light show for a user.  
      One conventional approach utilizing LEDs powers each of three color LEDs through a transistor biasing scheme, in which a base of a transistor is connected to a respective latch register through biasing resistors. Typically, three latches are all simultaneously connected to the same data lines on a data bus. As such, it is not possible to control all LED transistor biases independently and simultaneously. Biasing of transistors using this approach is inefficient because the power delivered to the LEDs is less than the power dissipated in the biasing network. Therefore, this approach is not well suited to illumination applications requiring any degree of efficiency.  
      In another conventional approach, a pulse width modulated signal is used to provide current to a plurality of LEDs. The pulse width modulation is controlled to create a particular duty cycle. However, most approaches that employ this method make no provision for precise and rapid control over the spectrum of colors emitted.  
      It would be desirable to have a system and method to control the intensity of LEDs that allows for nearly any color in the color spectrum to be emitted at any desired point in time. It would also be desirable to have a high performance, microcontroller-based control for a multi-color LED lighting system that is efficient, highly adaptable to present microcontroller and microprocessor architectures, inexpensive to manufacture, and lends itself to a greater number of physical implementations than pulse width modulation.  
     SUMMARY OF THE INVENTION  
      Accordingly, a light apparatus is provided having a housing, and an array of light emitting units integrally formed within the housing, each light emitting unit containing at least one light emitting diode (LED). The apparatus further includes a processor in communication with the at least one LED in each light emitting unit, and user input controls in communication with the processor for controlling the light emitting units, such that a light color displayed by each light emitting unit can vary with time.  
      In accordance with one aspect of the present invention, each light emitting unit contains three LEDs, with each of the three LEDs emitting a different one of three primary colors. A light diffuser can be included in each light emitting unit for blending the colors provided by each LED. The array can include any number of light emitting units, typically between four and sixty-four units. The light emitting units can be square, rectangular, or any other shape. The housing can also have any shape suitable for the intended application, such as square, rectangular, or wavelike. The light apparatus can be free-standing or arranged to be mounted to a wall. Further, a remote control can be provided that includes one or more user input controls for controlling the operation of the light emitting units.  
      The light apparatus according to the present invention can include various features, such as a speaker and a sound sensor disposed within the housing in communication with the processor. The sound sensor can be configured to provide sound input to the processor, such that operation of the light emitting units is responsive to the sound input. Furthermore, the sensitivity of the sound sensor can be adjustable. The light apparatus can also include a clock in communication with the processor. Additionally, a light sensor can be provided in communication with the processor for operating the light emitting units according to a detected light threshold. The processor may include memory storing at least one algorithm for operation of the light emitting units alone or together with one or more additional features.  
      Various user input controls are contemplated according to the present invention. A program control is provided for selecting a preprogrammed algorithm for operation of the light emitting units. A pause control can be provided for pausing operation of the light emitting units. A timer control can be provided for selecting a period of operation of the light emitting units. A speed control can be provided for selecting a speed at which the light color of each light emitting unit is varied. A color control can be provided for adjusting the light color and intensity of the light emitting units.  
      The light apparatus according to the present invention can include various other components as well. For example, a clock radio can be provided in communication with the processor. The clock can include an alarm function, where operation of the light emitting units is initiated upon transmission of an alarm signal from the clock to the processor. The light apparatus of the present invention can be embodied as a night light, where a connector is provided on the housing and arranged to be received in a wall receptacle for powering the night light. The light apparatus can also be provided in combination with a fountain. In this aspect of the present invention, the housing includes a reservoir arranged to hold a fluid, such as water, and a pump having an inlet in communication with the reservoir and an outlet disposed adjacent to the array of light emitting units.  
      The present invention contemplates several embodiments for controlling the intensity of an LED. One apparatus includes a housing and at least one light emitting unit arranged within the housing and containing at least one LED. A variable frequency signal generator operable to generate a variable frequency signal, such as a square wave or sinusoidal wave, is provided. A low pass filter is provided in communication with the variable frequency generator and the LED, where the low pass filter has a cutoff frequency defining a frequency response characteristic. Control logic is provided in communication with the variable frequency signal generator for controlling a frequency of the variable frequency signal, where the intensity of the LED is varied by changing the frequency of the variable frequency signal in relation to the cutoff frequency.  
      In another embodiment of the present invention, a method for controlling an intensity of an LED includes generating a variable frequency signal, passing the variable frequency signal through a filter having a gain which varies as a function of frequency, and varying the frequency of the signal such that at least one component of the variable frequency signal is attenuated by the variable gain so as to modify the amount of electrical power delivered to the LED.  
      In another embodiment of the present invention, an apparatus for controlling an light intensity of an LED includes a housing and at least one light emitting unit, arranged within the housing, containing the LED. A signal generator generates a variable pulse density signal. A multivibrator generates a pulse of set duration each time a clock edge is detected on the variable pulse density signal. Control logic controls the pulse density of the variable pulse density signal.  
      In another embodiment of the present invention, a method for controlling the intensity of an LED includes generating a variable pulse density signal. A drive signal is generated including a pulse of fixed duration based on at least one edge of each pulse in the variable pulse density signal. The drive signal is supplied to the LED. The pulse density is varied so as to modify an amount of electrical power delivered to the at least one LED.  
      In another embodiment of the present invention, an apparatus for controlling an LED includes a pulse signal generator for generating a first variable pulse density signal and a sample signal generator for generating a sample signal. A flip-flop generates a second variable pulse density signal for driving the LED in response to the first variable pulse density signal and the sample signal. Control logic controls the sample signal and a pulse density of the first variable pulse density signal.  
      In another embodiment of the present invention, a method for controlling the intensity of an LED includes generating a first variable pulse density signal and generating a sample signal. The first variable pulse density signal and the sample signal are supplied to a flip-flop, which generates a second variable pulse density signal for powering the LED.  
      In another embodiment of the present invention, an apparatus for controlling the intensity of an LED includes a signal generator in communication with the LED. The signal generator produces a variable pulse density signal. Control logic controls a pulse density of the variable pulse density signal to vary the intensity of the LED.  
      In another embodiment of the present invention, a method for controlling the intensity of an LED includes generating a variable pulse density signal and supplying the variable pulse density signal to the at least one LED. The pulse density is varied so as to modify an amount of electrical power delivered to the LED.  
      In another embodiment of the present invention, an apparatus for controlling the intensity of a plurality of LEDs includes a signal generator for generating a signal having a continuously variable voltage. A digital number generator generates a digital signal. A decoder receives the digital signal from the digital number generator. A plurality of sample-and-hold circuits are also included. Each sample-and-hold circuit is connected to at least one of the plurality of LEDs. The intensity of a different subset of the LEDs is varied by changing the continuously variable voltage and by setting the appropriate output from the decoder.  
      In another embodiment of the present invention, a method for controlling the intensity of an LED includes generating an analog control signal and generating a digital signal. At least one sample signal is generated from the digital signal. The analog control signal and the sample signal are supplied to a sample-and-hold circuit, which generates a second analog control signal for supplying the LED. The analog control signal is varied so as to modify an amount of electrical power delivered to the at least one LED.  
      In another embodiment of the present invention, an apparatus for controlling the intensity of an LED includes a PWM signal generator operable to generate a first pulse width modulated (PWM) signal. A sample signal generator generates a sample signal. A flip-flop generates a second PWM signal for driving the at least on LED in response to the first PWM signal and the sample signal. Control logic controls the sample signal and a duty cycle of pulses of the first PWM signal.  
      In another embodiment of the present invention, a method for controlling the intensity of an LED includes generating a first pulse width modulated (PWM) signal and generating a sample signal. The first PWM signal and the sample signal are supplied to a storage device which generates a second PWM signal. The LED is driven with a signal based on the second PWM signal.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of a light apparatus according to the present invention;  
       FIG. 2  is a top plan view of the light apparatus of  FIG. 1  depicting one embodiment of a control panel;  
       FIG. 3  is a top plan view of the light apparatus of  FIG. 1  depicting another embodiment of a control panel;  
       FIG. 4  is a block diagram showing components of the light apparatus according to the present invention;  
       FIG. 5  is a perspective view of a light apparatus according to another aspect of the present invention, the light apparatus configured with a horizontal display;  
       FIG. 6  is a front elevational view of a light apparatus according to another aspect of the present invention, the light apparatus having a clock and an AM/FM radio;  
       FIG. 7  is a perspective view of a night light apparatus according to another aspect of the present invention;  
       FIG. 8  is a perspective view of a light apparatus according to another aspect of the present invention, the light apparatus arranged to be mounted to a wall;  
       FIG. 9  is a front elevational view of a remote control for use with any embodiment of the light apparatus according to the present invention;  
       FIG. 10  is a perspective view a light apparatus according to another aspect of the present invention having a housing with a wave configuration;  
       FIG. 11  is a perspective view of a light apparatus according to another aspect of the present invention, the light apparatus including a fountain;  
       FIG. 12  is an exploded view of the light apparatus of  FIG. 7  illustrating the assembly of diffuser and shade components;  
       FIG. 13  is a cross sectional view of the diffuser and shade assembly shown in  FIG. 12 ;  
       FIG. 14  is a block diagram illustrating variable frequency control for controlling the intensity of an LED according to an aspect of the present invention;  
       FIG. 15  is a frequency plot of a low pass filter for controlling the intensity of an LED;  
       FIG. 16  is a plot of a square wave that can be used as an input to a low pass filter;  
       FIGS. 17 and 18  are plots illustrating the effect of passing a square wave of differing fundamental frequencies through a low pass filter for controlling light intensity;  
       FIG. 19  is a plot illustrating pulse density control according to an aspect of the present invention;  
       FIG. 20  is a block diagram illustrating a circuit that employs pulse density control to control the intensity of an LED according to an aspect of the present invention;  
       FIG. 21  is a block diagram illustrating another circuit that employs pulse density control to control the intensity of an LED according to an aspect of the present invention;  
       FIG. 22  is a block diagram illustrating yet another circuit that employs pulse density control to control the intensity of an LED according to an aspect of the present invention;  
       FIG. 23  is a block diagram illustrating a circuit that can be used to implement multiplexed analog control to control the intensity of an LED according to an aspect of the present invention;  
       FIG. 24  is a block diagram illustrating a circuit that employs pulse width modulation to control the intensity of an LED according to an aspect of the present invention; and  
       FIG. 25  is a circuit diagram illustrating one example of a transistor driver circuit that may be used in combination with any of the previous circuits to provide power to an LED according to an aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE PRESENT INVENTION  
      Referring first to  FIG. 1 , a light apparatus  100  is illustrated according to an aspect of the present invention. Light apparatus  100  has a housing  111  that generally comprises a front side  112 , a back side  114 , a right side  116 , a left side  118 , a top side  120 , and a bottom side  122 . As depicted in  FIG. 1 , front side  112 , back side  114 , top side  120  and bottom side  122  may be essentially flat, while right side  116  and left side  118  may be rounded for aesthetic reasons. Of course, it is understood that other shapes of housing  111  are fully contemplated according to the present invention. Light apparatus  100  is designed to stand vertically on bottom side  122 , and can include a standard power cord (not shown) for plugging into a wall outlet or alternatively be battery-operated.  
      Front side  112  generally comprises a display area  123  having a plurality of light emitting units  124 . In the example shown in  FIG. 1 , a total of sixteen light emitting units  124  are provided in four rows and four columns. Of course, light apparatus  100  according to the present invention can have fewer or greater numbers of light emitting units  124 , and the plurality of light emitting units  124  need not be arranged as an equal number of rows and columns. In accordance with the present invention, it is fully contemplated that any number of light emitting units  124  may be implemented to meet the design criteria of a particular application. Furthermore, while light emitting units  124  are depicted herein as being square in shape, it is understood that light emitting units  124  can be of any shape, such as rectangles, circles, octagons, hexagons, and others.  
      Each of the light emitting units  124  contains at least one light emitting diode (LED)  16 , as best shown in  FIG. 13 . According to one aspect of the present invention, each light emitting unit  124  contains three LEDs, each LED emitting one of the three primary colors. This configuration allows any of the light emitting units  124  to emit any color in the visible spectrum. Light apparatus  100  according to the present invention is operable to display a light show that is visually stimulating and/or relaxing to a viewer. The light show can comprise patterns of changing colors in a horizontal direction, vertical direction, or combinations thereof, such as various colors chasing themselves around display area  123  or fading in and out at different rates. The light show may be controlled by preprogrammed algorithms representing a number of modes, such that a user can select which show he/she would like to view by selecting the appropriate mode, as described further below.  
      Referring now to  FIG. 2 , a control panel  127  of light apparatus  100  is illustrated. Control panel  127  can be disposed on any part of housing  111 , such as top side  120  as illustrated herein. Control panel  127  typically comprises a number of user input controls, such as buttons  128   a - c . As shown, there may be a POWER button  128   a  for turning light apparatus  100  on and off, a TIMER button  128   b  for setting a timer for the operation of light apparatus  100 , and a SPEED button  128   c  for controlling the speed at which the light emitting units  124  change colors. TIMER button  128   b  may be used to select among a number of preprogrammed timer modes, such as a fifteen-minute mode, a thirty-minute mode, or a sixty-minute mode. SPEED button  128   c  may be used to select between a number of preprogrammed speed modes, such as a slow mode, a medium mode, or a fast mode. There also may be a number of color display modes preprogrammed into light apparatus  100  as described in greater detail below. The user may select among these preprogrammed color display modes, such as by depressing POWER button  128   a  to cycle through the modes. While three buttons  128   a - c  have been described, any number of buttons may be employed to meet the design criteria of a particular application.  
      Another control panel  127  is illustrated in  FIG. 3 . In this example, top side  120  may comprise a number of buttons  128   a - g  that may include a PROGRAM button  128   d , a SOUND button  128   e,  a COLOR button  128   f , and a PAUSE button  128   g  in addition to buttons  128   a - c  described above. PROGRAM button  128   d  may be used to select among a number of lighting modes, such as a GEOMETRICS mode, a NATURALISTICS mode, a RANDOM mode, and a MIX mode. These various lighting modes represent preprogrammed algorithms that control what patterns, pseudo-random patterns, or random patterns light apparatus  100  uses for the light display. SOUND button  128   e  may be used to select from a number of sound modes, such as a RAINFOREST mode, a THUNDER mode, a SUMMER NIGHT mode, and a SUNRISE mode. These sound modes represent preprogrammed algorithms or prerecorded sounds which are emitted by light apparatus  100 . COLOR button  128   f  may be used to select the color palette of the light emitting units  124  as well as the intensity of the light, and PAUSE button  128   g  may be used to pause operation of the light display after the lighting mode is selected. While a number of control buttons and modes have been described herein, it is understood that any number and type of buttons and modes may be implemented in accordance with the present invention.  
      Referring next to  FIG. 4 , a block diagram of the components of light apparatus  100  according to the present invention is illustrated. As shown, each light emitting unit  124  is in communication with a processor  130  provided in housing  111 . Processor  130  includes a memory  132  for storing at least one preprogrammed lighting algorithm as described above. User input controls  128  are also in communication with processor  130  for controlling the operation of the light emitting units  124 . Light apparatus  100  can further include a speaker  134  in communication with processor  130  for emitting sound in accordance with a user-selected mode, and a sound sensor  136  in communication with processor  130  for detecting ambient sound in the area of light apparatus  100 . In accordance with one aspect of the present invention, the operation of the light emitting units  124  can be responsive to the sounds detected by sound sensor  136 , such that the light display can be coordinated with music or other sounds emanating from speaker  134  as well as other sounds having a source external to light apparatus  100 . Furthermore, sound sensor  136  can have an adjustable sensitivity or sound threshold, with the resulting responsiveness of the light display varied according to the threshold. A clock  138  is provided in communication with processor  130  for providing the timer functionality described above. Still further, a light sensor  140  can be provided in communication with processor  130  for controlling the operation of the light emitting units  124  in accordance with a light threshold, which can also be adjustable. For example, light emitting units  124  can be activated when the room in which it is contained reaches a certain darkness, such as for a night light as described below with reference to  FIG. 7 . Although the above components have been described with reference to light apparatus  100 , it is understood that this description is equally applicable to the additional embodiments described below.  
      With reference to  FIG. 5 , an alternative light apparatus  200  is illustrated. Light apparatus  200  is similar to light apparatus  100  shown in  FIG. 1  and can include all the features described above, wherein like components have like reference numerals except for the substitution of a “2” prefix. Light apparatus  200  is designed to lie on back side  214  on a table or other horizontal support surface such that display area  223  is parallel to the table. Light apparatus  200  preferably includes control buttons  228   a - 228   c  on front side  212  for easy user access, although it is understood that buttons  228   a - 228   c  could alternatively be disposed on another side of light apparatus  200 .  
      Referring now to  FIG. 6 , another light apparatus  300  according to the present invention is illustrated, wherein reference numerals correspond to like elements from light apparatus  100  except for the substitution of a “3” prefix. Light apparatus  300  can include all the features of light apparatus  100  and light apparatus  200 , and further includes an AM/FM clock radio  342  and associated control buttons  344  as described below. As with light apparatus  100 , the array of light emitting units  324  is preferably located on front side  312  of housing  311 , and light apparatus  300  is designed to stand vertically on bottom side  322 . It is understood, however, that light apparatus  300  could alternatively be configured to lie horizontally, such as with light apparatus  200 . Information display  330  and control buttons  332  are also depicted herein as disposed on front side  312 , but could of course be disposed on another side of housing  311  in accordance with the present invention.  
      With continuing reference to  FIG. 6 , clock radio  342  may be an LED or liquid crystal display (LCD) designed to display time and other information. In addition to the functions described above with reference to light apparatus  100  and  200 , control buttons  344  are provided to control clock radio functions as is known in the art. Referring again to  FIG. 4 , clock  138  can provide an alarm signal to processor  130  such that light apparatus  300  may awaken a user by providing a light show with light emitting units  324 . The light show could begin with a very dim lights that slowly increase in intensity until the light show is bright enough to wake up the user. Likewise, the light show could begin with slowly moving lights which gradually increase in speed in order to awaken the user. The light show may be accompanied by music that increases in volume with the increase in light intensity or speed.  
      Turning next to  FIG. 7 , a night light apparatus  400  is shown in further accordance with the present invention. Once again, night light apparatus  400  can include all the features of light apparatus  100 - 300 , where like components are designated with like reference numerals except for the substitution of a “4” prefix. Night light apparatus  400  further includes a plug  446  (see  FIG. 13 ) arranged to be received in a standard wall socket for supplying power to night light apparatus  400 . Housing  411  can include a number of control buttons  428  as described above, such as on front side  412 , for controlling the function of night light apparatus  400 . Night light apparatus  400  preferably contains light sensor  140  described with reference to  FIG. 4  for controlling the operation of light emitting units  424 .  
      Referring now to  FIG. 8 , another light apparatus  500  is illustrated in accordance with the present invention. Light apparatus  500  can include all the features of light apparatus  100 - 400 , where like components are designated with like reference numerals except for the substitution of a “5” prefix. Housing  511  of light apparatus  500  preferably has a thinner profile than the light apparatus embodiments described above, and is arranged to be mountable on a wall or other vertical surface. Alternatively, light apparatus  500  can lie on a table or other horizontal support surface. The array of light emitting elements  524  is provided on front side  512 , and control buttons  528  are preferably provided on a side of housing  511 , such as side  516  depicted herein, so as to be accessible to a user but not otherwise noticeable.  
      A remote control  550  is illustrated in  FIG. 9  that can be used in combination any of the aforementioned light apparatus embodiments, in particular light apparatus  500  shown in  FIG. 8 . As is known in the art, remote control  550  includes a transmitter (not shown) for sending signals, such as infrared signals, to a receiver (not shown) on light apparatus  100 - 500  for controlling the operation thereof. Remote control  550  may comprise a number of control buttons  552   a - c  including, but not limited to, a PROGRAM button  552   a , a TIMER button  552   b , and a SPEED button  552   c  as described above. While three control buttons  552   a - c  have described, any number of buttons may be implemented to meet the design criteria of a particular application.  
      A light apparatus  600  having a wave configuration is illustrated in  FIG. 10  in accordance with the present invention. As before, light apparatus  600  can include all the features of light apparatus  100 - 500 , where like components are designated with like reference numerals except for the substitution of a “6” prefix. Light apparatus  600  comprises a housing  611  having a wavelike construction, wherein light apparatus  600  is preferably designed to stand vertically on bottom side  622 . Light apparatus  600  comprises a plurality of vertical, generally rectangular light emitting units  624  that function similar to the light emitting units described above for other light apparatus embodiments  100 - 500 . Of course, the rectangular shape of light emitting units  624  is merely exemplary, and other shapes are fully contemplated in accordance with the present invention.  
      With reference to  FIG. 11 , a combination light/fountain apparatus  700  is illustrated in accordance with another aspect of the present invention. Light/fountain apparatus  700  can include all the features of light apparatus  100 - 600 , where like components are designated with like reference numerals except for the substitution of a “7” prefix. In this embodiment, light emitting units  724  are disposed within a recessed area  756  of housing  711 , and a fluid reservoir  758  and pump  760  are disposed within housing  711  below light emitting units  724 . Pump  760  includes an inlet  762  in fluid communication with reservoir  758 , and an outlet  764  disposed above light emitting units  724 . In operation, fluid F, such as water, is pumped out of reservoir  758  by pump  760 , through outlet  764 , and flows past light emitting units  724  providing a pleasing visual effect. The fluid F is returned to reservoir  758  via a drain  766  provided at the bottom of recessed area  756 . Of course, other configurations wherein a fountain is provided in combination with light emitting units  724  are also fully contemplated.  
      In accordance with one aspect of the present invention, each of the foregoing light apparatus embodiments  100 - 700  can include light emitting units  124 - 724  each containing three LEDs, with each LED emitting a different primary color. When the light exits the light emitting unit  124 - 724 , it is desirable that the light of the three LEDs is blended to produce the desired resultant color. As illustrated in  FIGS. 12-13  for night light apparatus  400 , this effect may be aided with the use of a diffuser lens  470  and shade  472 . As depicted herein, diffuser lenses  470  can be hemispherical in shape and enclose the LEDs, although other shapes are also contemplated. Diffuser lenses  470  can be semi-transparent and have properties that do not permit light of the multiple LEDs contained therein to exit the lens  470  separately without blending to create a resultant color. Each diffuser lens  470  may also be designed such that the light emitted from the LEDs is diffracted so as to emit from the entire area of the lens  470 . Shades  472  can be utilized to make the emitted LED light appear more tender and even. Square-shaped shades  472  are illustrated, but it is understood that any shape can be used to meet the design criteria of a particular application. Of course, diffuser lenses  470  and shades  472  can be implemented in any of the light apparatus embodiments  100 - 700  described herein.  
      Each light apparatus  100 - 700  described above functions to produce a light display by controlling a plurality of LEDs disposed therein. The following figures and description disclose various systems and methods that can be employed to control the intensity of the LEDs, such as within any foregoing light apparatus  100 - 700 .  
      Referring to  FIG. 14 , a block diagram illustrating variable frequency control for controlling the intensity of an LED according to an aspect of the present invention is shown. The circuit in  FIG. 14  generally comprises a microcontroller or microprocessor  10  having one or more outputs  12   a - n . Each output  12   a - n  is connected to an amplifier  13 , a low pass filter (LPF)  14 , an LED  16 , and a current limiting resistor  18 . The low pass filter  14  attenuates high frequency components appearing on output  12   a  based on the frequency of each component. Thus, by changing the frequency of one or more components of a signal on output  12   a , the intensity of LED  16  is varied. The signal on output  12   a  may be sinusoidal, rectangular, triangular or any other periodic shape as well as aperiodic shapes.  
      In any of the described embodiments, the microprocessor or microcontroller  10  may be any microprocessor or microcontroller or any electronic circuit that is capable of producing a variable frequency output. The microcontroller  10  may generate any type of wave form. In one example, the microcontroller  10  may directly generate a signal on outputs  12   a - n  for filtering. In another example, microcontroller  10  may generate a signal for controlling an external signal generator as is known in the art.  
      Referring to  FIG. 15 , a frequency plot is shown illustrating the frequency response of a typical low pass filter  14 . This plot illustrates the effect filter  14  has on input signals as a function of frequency. The spectrum for most low pass filters can be divided into two or more regions based on how they perform in frequency. In the simple case illustrated in  FIG. 15 , two regions can be defined. For frequencies less than the cutoff frequency, f c , an input signal or signal component suffers little or no relative attenuation. This region is known as the pass band. For frequencies greater than the cutoff frequency, an input signal or signal component suffers an attenuation that increases as the frequency increases. This region is known as the reject band.  
      For example, a sinusoidal input signal having a frequency f 1 , in the pass band may experience almost no relative attenuation when passed through low pass filter  14 , as shown in the plot of  FIG. 15 . In contrast, a sinusoidal signal having a frequency f 2  outside of the pass band may experience considerable relative attenuation when passed through filter  14 . The amount of relative attenuation is a function of the order and construction of filter  14  as well as the frequency of the signal component under consideration. For example, the low pass filter  14  may be a passive first order low pass filter. Such a filter can be constructed with a capacitor in series with a resistor if the output is taken across the capacitor. However, any order low pass filter may be used to meet the design criteria of a particular application. The construction of both active and passive low pass filters is well known in the art.  
      Referring to  FIG. 16 , a plot of a square wave is shown where the square wave has a period inversely proportional to f s , the square wave fundamental frequency. Any periodic waveform, including the square wave of  FIG. 16 , may be represented mathematically by sunning sinusoids at proper amplitudes that are integer multiples of the fundamental frequency. Such a square wave passed through low pass filter  14  will experience varying modifications depending on the relationship between the fundamental frequency of the square wave and the characteristics of low pass filter  14 , predominantly the cutoff frequency. The fundamental frequency of the square wave shown in  FIG. 16  may be varied by microcontroller  10 , thereby controlling the amount of light emitted by LED  16 .  
      As will be recognized by one of ordinary skill in the art, the filter need not be a low pass filter. Any filter with variable attenuation over a range of frequencies of interest may be used to control the amount of power delivered to and, thereby, the intensity of light generated by, an LED.  
      Referring to  FIGS. 17 and 18 , the effect of passing a square wave of differing fundamental frequencies through a low pass filter for controlling light intensity is shown. If the fundamental frequency of the square wave falls well within the pass band of low pass filter  14 , the resulting signal will look substantially like the input signal, as in  FIG. 17 . In this case, most of the power seen at the input of low pass filter  14  is passed to the output of low pass filter  14  to LED  16 . If the fundamental frequency of the square wave falls in the reject band, the resulting signal will be highly distorted, as in  FIG. 18 . In this case, much of the power seen at the input of low pass filter  14  is dissipated as heat by low pass filter  14  and is therefore not supplied to LED  16 . Thus, by varying the frequency of a generated signal, microcontroller  10  can vary the intensity of light emitted by LED  16 .  
      A plot illustrating pulse density control according to an aspect of the present invention is shown in  FIG. 19 . The signal shown includes a number of uniform pulses each having a width, d. Each of the pulses is shown occurring at different times t 1 , t 2 , . . . . A property of the human eye known as persistence of vision averages out the pulsed light emitted by an LED. If these pulses were used to drive an LED, the average intensity of light emitted by the LED would be determined by the density of the pulses. So, for the purpose of illustration, light emitted during a span of time including pulses at times t 1 , t 2 , t 3 , and t 4  would appear dimmer than light emitted during a span of time including pulses at times t 5 , t 6 , t 7  and t 8 . Thus, by controlling the average density of pulses over a period of time, the intensity of light emitted by an LED, as perceived by a human, can be varied.  
      Referring to  FIG. 20 , a block diagram illustrating a circuit that employs pulse density control to control the intensity of an LED according to an aspect of the present invention is shown. The circuit shown in  FIG. 20  includes a microcontroller  20  having one or more outputs  22   a - n . Each output controls a monostable multivibrator, or one-shot,  24 , an LED  16 , and a current limiting resistor  28 . One-shot  24  generates a fixed width pulse at its output when an appropriate edge, rising or falling, is received at its input. The microcontroller  20  may supply a control signal from the output  22   a  over a control line to one-shot  24  having an appropriate edge at each time a pulse is desired. Since LED  16  is coupled to the output of one-shot  24 , each of the pulses generated by one-shot  24  is supplied to the LED  16 . If the period between any two pulses is short enough, the intensity of LED  16  can be varied without causing noticeable flicker.  
      Referring next to  FIG. 21 , a block diagram illustrating another circuit that employs pulse density control to control the intensity of an LED according to an aspect of the present invention is shown. The circuit illustrated in  FIG. 21  includes a microcontroller  30  having one or more data outputs  32   a - n  and one or more sample outputs  33   a - m . One data output  32   a - n  is connected to a D input and one sample output  33   a - m  is connected to a clock input of at least one D flip-flop  34 . Each flip-flop  34  has an output, Q, which drives one or more sets of LEDs  16  and current limiting resistors  38 .  
      Typically, a plurality of LEDs  16  would be implemented with a plurality of D flip-flops  34  such that the microcontroller  30  controls a plurality of LEDs. The output  32   a - n  of the microcontroller  30  may be an n-bit output. In one example, n-bit output  32   a - n  may be an 8-bit data output. The data output  32   a - n  may be 8 bits wide and may be coupled to eight D flip-flops  34 . The eight D flip-flops  34  may be controlled by a common clock signal coupled to one of the sample outputs  33   a - m . In one example, a plurality of multi-bit flip-flop IC packages or cells may be applied to this design. The sets of data inputs of the multi-bit D flip-flop IC packages may be coupled together in parallel and a separate bit of the second output  33   a - m  of the microcontroller  30  may supply each package with a sample signal. Using this method, a multiplexed data implementation may be achieved that may allow n×m LEDs  16  or sets of LEDs  16  to be controlled using the circuit shown in  FIG. 21 . Each flip-flop  24  may be used to sample and store a data signal, such as the pulse density signal illustrated in  FIG. 19 .  
      A block diagram illustrating another circuit that employs pulse density control to control the intensity of an LED according to an aspect of the present invention is shown in  FIG. 22 . The circuit includes microcontroller  40 , which has one or more outputs  42   a - n . The output  42   a - n  typically comprises a multi-bit output having n bits. The output  42   a - n  may be configured to implement a direct data method where the output  42   a - n  directly supplies a variable pulse density control signal, as illustrated in  FIG. 19 . In one example, the output  42   a - n  may be coupled to n sets of LEDs  16  and current limiting resistors  48  through a driver circuit, not shown.  
      Referring to  FIG. 23 , a block diagram is shown illustrating a multiplexed analog control method. The circuit shown in  FIG. 23  includes a microcontroller  50  having one or more analog outputs  52   a - m  and a plurality of selecting outputs  54   a - n , a decoder  56 , a plurality of sample-and-hold circuits  58   a - i , a plurality of amplifiers  60   a - i , a plurality of LEDs  16   a - i , and a plurality of current limiting resistors  64   a - i . Each analog output  52   a - m  of microcontroller  50  generates an analog signal indicative of a desired LED intensity level. Alternatively, microcontroller  50  may generate digital outputs which are supplied to one or more external digital-to-analog converters. Select outputs  54   a - n  of microcontroller  50  are coupled to inputs of the decoder  56 . The decoder  56  asserts one of its outputs based on the value received from select outputs  54   a - n.    
      Each sample-and-hold circuit  58   a - i  has a signal input connected to an analog signal such as one of analog outputs  52   a - m . Each sample-and-hold circuit  58   a - i  also has a sample input connected to one output from decoder  56 . When this input is asserted, the sample-and-hold circuit capacitively stores the voltage on its input and presents this voltage to an LED  16  through an output amplifier  60 .  
      In operation, microcontroller  50  generates an analog voltage for a particular LED  16  on an output  52   a - m  associated with LED  16   a - i . Microcontroller  50  then outputs to decoder  56  the appropriate number on outputs  54   a - n  to select sample-and-hold  58   a - i  associated with the desired LED  16   a - i . Microcontroller  50  can then set analog output  52   a - m  for the next desired LED  16   a - i . If a large number of LEDs  16   a - i  are to be controlled, microcontroller  50  may control a plurality of analog outputs  52   a - m . This has the advantage of not basing the scan rate on the voltage change rate of an individual digital-to-analog converter.  
      A block diagram illustrating a circuit that employs pulse width modulation to control the intensity of an LED according to an embodiment of the present invention is shown in  FIG. 24 . This circuit includes microcontroller  70  having one or more data outputs  72   a - n  and one or more sample outputs  74   a - m . One data output  72   a - n  is connected to a D input and one sample output  74   a - m  is connected to a clock input of at least one D flip-flop  76 . Each flip-flop  76  has an output, Q, which drives one or more sets of LEDs  16  and current limiting resistors  80 .  
      Typically, a plurality of LEDs  16  would be implemented with a plurality of D flip-flops  76  such that the microcontroller  70  controls a plurality of LEDs. The output  72   a - n  of the microcontroller  70  may be an n-bit output. In one example, n-bit output  72   a - n  may be an 8-bit data output. The data output  72   a - n  may be 8 bits wide and may be coupled to eight D flip-flops  76 . The eight D flip-flops  76  may be controlled by a common clock signal coupled to one of the sample outputs  74   a - m . In one example, a plurality of multi-bit flip-flop IC packages or cells may be applied to this design. The sets of data inputs of the multi-bit D flip-flop IC packages may be coupled together in parallel and a separate bit of the second output  74   a - m  of the microcontroller  70  may supply each package with a sample signal. Using this method, a multiplexed data implementation may be achieved that may allow n×m LEDs  16  or sets of LEDs  16  to be controlled using the circuit shown in  FIG. 24 . Each flip-flop  76  may be used to sample and store a data signal, such as a pulse width modulated signal.  
      Referring lastly to  FIG. 25 , a circuit diagram is shown illustrating one example of a transistor driver circuit that may be used in combination with any of the previous circuits to provide power to an LED.  
      While each of the circuit diagrams discussed above illustrates, in one example, a defined number of inputs and outputs for each component, it will be understood by those skilled in the art that the number of inputs and outputs described can be increased or decreased by using the appropriate microcontroller and supporting structure, thus shrinking or enlarging the scopes of the circuits and allowing the invention to control a greater or lesser number of LEDs.  
      In the foregoing description, certain detailed aspects of the circuits described that are well known to those skilled in the art have been omitted, such as power and ground connections for the microcontrollers and other circuits, transistor driver circuits that may be necessary to supply the power to LEDs, and other electronic circuits that facilitate the implementation of the present invention. In addition, multiple LEDs may be driven in parallel by any of the embodiments illustrated such that language referring to a single LED applies equally well to sets of LEDs.  
      While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.