Display device with variable color background
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to variable color display devices. 
     2. Description of the Prior Art 
     A display device that can change color and selectively exhibit characters is described in my U.S. Pat. No. 4,086,514 entitled Variable Color Display Device and issued on Apr. 25, 1978. This display device includes display areas arranged in a suitable display font, such as well known 7-segment font, which may be selectively energized in groups to exhibit all known characters. Each display area includes three light emitting diodes for emitting light signals of respectively different primary colors which are blended within the display area to form a composite light signal. The color of the composite light signal can be controlled by varying the portions of the primary light signals. 
     The capability of a variable color display device to vary the color of its display areas over the entire spectrum has a drawback in that certain colors of the display areas may be similar to the color of the display background, thereby rendering the exhibited character difficult to recognize, particularly in imperfect viewing conditions. From an aesthetic aspect, small color differences between the display and background areas are likely to be displeasing and give an impression of instability. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is the principal object of this invention to provide an improved variable color display device capable of illuminating its display areas in substantially any color of the spectrum and its background area in a complementary color for providing better recognition of exhibited characters. 
     It is another object of the invention to provide a variable color display device that exhibits characters in an aesthetically pleasing and harmonious manner. 
     In summary, variable color display device of this invention includes a plurality of variable color display segments arranged in a pattern and surrounded by a variable color background area. The displayed character may be illuminated in a desired color, and its background area may be illuminated in a color substantially complementary to more effectively exhibit the character. 
     Further objects of the invention will become obvious from the accompanying drawings and their description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings in which is shown the preferred embodiment of the invention, 
     FIG. 1a is a plan view of a variable color display device of the present invention on which numeral `1` is illuminated in green color on purple background. 
     FIG. 1b is a similar view of a variable color display device on which numeral `3` is illuminated in red color on blue-green background. 
     FIG. 1c is a similar view of a variable color display device on which numeral `7` is illuminated in blue color on yellow background. 
     FIG. 2 is a cross-sectional view, taken along the line 2--2 in FIG. 1b, revealing internal structure of a variable color display device. 
     FIG. 3 is a block diagram showing the activation of a variable color display device. 
     FIG. 4 is a simplified schematic diagram of a variable color display device. 
     Throughout the drawings, like characters indicate like parts. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now, more particularly, to the drawings, in FIG. 1a is shown a variable color display device 11 of the present invention accommodated in a housing 30 and including a variable color display area consisting of seven segments 31 arranged in a well known 7-segment font on which digits and selected characters may be exhibited in variable color. The invention resides in the addition of a variable color background area 32, substantially surrounding the display area, which may be illuminated in a color definitely different from, and preferably complementary to, the color of the display area to exhibit the characters more effectively. It will be recalled that complementary colors are colors that produce a neutral color when additively mixed in suitable proportions. Generally, red colors are complementary to blue-green colors, green colors are complementary to purple colors, and blue colors are complementary to yellow colors. By referring to several illustrated examples, in FIG. 1a is shown numeral `1` illuminated in green color on purple background, in FIG. 1b is shown numeral `3` illuminated in red color on blue-green background, and in FIG. 1 c is shown numeral `7` illuminated in blue color on yellow background. It is readily apparent that the maximum color contrast between the display and background areas facilitates recognition of displayed characters and is aesthetically pleasing and harmonious. 
     As will be revealed more clearly subsequently, each display segment and background region includes a triad of light emitting diodes (LEDs) adapted for emitting light signals of respectively different primary colors. An important consideration has been given to physical arrangement of the light emitting diodes in the display and background areas, as illustrated in FIG. 2. In the display segment 31f, red LED 1f, green LED 2f, and blue LED 3f are disposed on a support 10 in a display light blending cavity 8f and completely surrounded by transparent light scattering material 34. When forwardly biased, the LEDs 1f, 2f, and 3f emit light signals of red, green, and blue colors, respectively, which are blended by passing through light scattering material 34, acting to disperse the light signals, to form a composite light signal that emerges at the upper surface 35f of the display segment 31f. The color of the composite light signal may be controlled by varying the portions of red, green, and blue light signals. In the display segment 31b, the red LED 1b, green LED 2b, and blue LED 3b are similarly disposed in a display light blending cavity 8b and may be similarly activated. 
     In a similar fashion, red LED 4g, green LED 5g, and blue LED 6g in the background region 32g are disposed on the support 10 in a background light blending cavity 9g and surrounded by transparent light scattering material 34. When forwardly biased, the LEDs 4g, 5g, and 6g emit light signals of red, green, and blue colors, respectively, which are blended by passing through the light scattering material 34 to form a composite light signal of a composite color that emerges at the upper surface 36g of the background region 32g. In the background region 32f, the red LED 4f, green LED 5f, and blue LED 6f are similarly disposed in a background light blending cavity and may be similarly activated. The red LED 4a, green LED 5a, and blue LED 6a in the background region 32a, disposed in a background light blending cavity 9a, may be activated in a similar fashion. 
     The display segments are optically separated from adjacent background regions by opaque walls. In the display segment 31f, the walls 7a and 7b have generally smooth inclined surfaces defining an obtuse angle with the support 10 and defining a display light blending cavity 8f therebetween. Alternatively, the wall surfaces may be rough to further promote diffusion of the light signals. In the background region 32g, the inclined walls 7b and 7c similarly define a background light blending cavity 9g therebetween. In a similar fashion, the display light blending cavity 8b is defined by the walls 7c and 7d, background light blending cavity 9a is defined by the wall 7d and housing 30, and background light blending cavity 9f is defined by housing 30 and wall 7a. The width of the top surfaces of the opaque walls is uniform and distinctly less than the width of the display segments or the background regions so as to minimize the boundaries therebetween. The top surfaces of the opaque walls, top surfaces of the display segments, and top surfaces of the background regions are in the same plane to allow wide angle observation of the display device. Although the walls and light blending cavities are shown to be of certain shapes and dimensions, it is envisioned that they may be modified and rearranged. 
     In FIG. 3 is shown a block diagram of a variable color display system of the invention which includes a variable color display device 11, display decoder 23 for converting input codes to displayable codes for displaying a desired character by activating appropriate groups of display segments, segment color control 21 for illuminating the display segments in a desired color, and background color control 22 for illuminating the background area in a color definitely different from the color of the display segments. The segment color control and background color control may be independent, as illustrated, or background color may be derived from the segment color, as will be pointed out subsequently. 
     Proceeding now to the detailed description, in FIG. 4 is shown a simplified schematic diagram of a one-character 7-segment variable color display element with variable color background of the invention. The circuit employs a common anode 7-segment decoder driver which may be substantially conventional with the qualification that it must be capable of driving a triad of LEDs in each display segment rather than a single LED. 
     Each display segment of the display device 11 includes a triad of closely adjacent LEDs: a red LED 1, green LED 2, and blue LED 3 which are adapted for producing a composite light signal of a variable color. To facilitate the illustration, the LEDs are designated by segment letters, e.g., red LED in the segment b is shown at 1b, green LED in the segment d is shown at 2d, and blue LED in the segment f is shown at 3f. The background area is comprised of background regions adjacent the display segments but alternatively may be integral. Each background region includes a triad of closely adjacent LEDs: a red LED 4, green LED 5, and blue LED 6 which are adapted for producing a composite light signal of a variable color. As much as possible, the background regions are designated by letters of adjacent display segments. 
     The cathodes of all red, green, and blue LED triads are interconnected in each display segment and electrically coupled to respective outputs of the decoder 24. The anodes of all display red LEDs 1a, 1b, 1c, 1d, 1e, 1f, and 1g are interconnected to form a common electric path referred to as a display red bus 12. The anodes of all display green LEDs 2a, 2b, 2c, 2d, 2e, 2f, and 2g are interconnected to form a like common electric path referred to as a display green bus 13. The anodes of all display blue LEDs 3a, 3b, 3c, 3d, 3e, 3f, and 3g are interconnected to form a like common electric path referred to as a display blue bus 14. 
     In a similar fashion, the anodes of all background red LEDs 4a, 4b, 4c, 4d, 4e, 4f, 4g, and 4h are interconnected to form a common electric path referred to as a background red bus 16. The anodes of all background green LEDs 5a, 5b, 5c, 5d, 5e, 5f, 5g, and 5h are interconnected to form a like common electric path referred to as a background green bus 17. The anodes of all background blue LEDs 6a, 6b, 6c, 6d, 6e, 6f, 6g, and 6h are interconnected to form a like common electric path referred to as a background blue bus 18. The cathodes of all red, green, and blue LED triads in each background region are grounded. 
     The display red bus 12 is connected to the output of a non-inverting tri-state buffer 25a capable of sourcing sufficient current to illuminate all display red LEDs. The display green bus 13 is connected to the output of a like buffer 25b. The display blue bus 14 is connected to the output of a like buffer 25c. 
     The background red bus 16 is connected to the output of an inverting tri-state buffer 26a capable of sourcing sufficient current to illuminate all background red LEDs. The background green bus 17 is connected to the output of a like buffer 26b. The background blue bus 18 is connected to the output of a like buffer 26c. 
     It would be obvious to those skilled in the art that current limiting resistors should be connected in series with all LEDs in the circuit to constrain current flow. Such resistors are omitted in the interest of clarity. 
     The six tri-state buffers 25a, 25b, 25c, 26a, 26b, and 26c can be simultaneously enabled, by applying a low logic level signal to the ENABLE input of an inverter 27, and disabled by applying a high logic level signal therein. When the buffers 25a, 25b, 25c, 26a, 26b, and 26c are disabled, all six buses are effectively disconnected, and all display segments and background regions are completely extinguished. 
     When the buffers are enabled, the color of the display segments may be controlled by applying proper combinations of logic level signals to the bus control inputs RB (red bus), GB (green bus), and BB (blue bus). Since the display segments are driven by non-inverting buffers and background regions by inverting buffers, the display segments illuminate in the selected color, and the background regions illuminate in substantially complementary color. 
     The operation of the display element shown in FIG. 4 will be now explained on example of illuminating digit `1` in green color. To exhibit decimal number `1`, a BCD code 0001 is applied to the inputs A0, A1, A2, A3 of the common anode 7-segment decoder driver 24. The decoder 24 develops low logic levels at its outputs b, c, to illuminate equally designated display segments, and high logic levels at all remaining outputs, to extinguish all remaining display segments. 
     To illuminate the display element in green color, the bus control input GB is raised to a high logic level, while both remaining bus control inputs RB and BB are maintained at a low logic level. As a result, the output of the buffer 25b rises to a high logic level. The current flows from the output of the buffer 25b, via display green bus 13 and green LED 2b, to the output b of the decoder 24, and, via green LED 2c, to the output c of the decoder 24. As a result, the display segments b, c illuminate in green color. Since the bus control inputs RB and BB are at a low logic level, the outputs of inverting buffers 26a  and 26c rise to a high logic level. The current flows from the output of the buffer 26a via all red LEDs 4a to 4h in parallel to ground. The current also flows from the output of the buffer 26c via all blue LEDs 6a to 6h in parallel to ground. As a result of blending the red and blue light signals in each background region, the entire background area illuminates in substantially purple color. The overall effect is numeral `1` illuminated in green color on substantially purple background, as shown in FIG. 1a. 
     To exhibit decimal number `3` in red color, a BCD code 0011 is applied to the inputs A0, A1, A2, A3 of the decoder 24. The decoder 24 develops low logic levels at its outputs a, b, c, d, g, to illuminate equally designated display segments, and high logic levels at all remaining outputs, to extinguish all remaining display segments. 
     To illuminate the display element in red color, the bus control input RB is reaised to a high logic level, while both remaining bus control inputs GB and BB are maintained at a low logic level. As a result, the output of the buffer 25a rises to a high logic level. The current flows from the output of the buffer 25a, via display red bus 12 and red LED 1a to the output a of the decoder 24, via red LED 1b to the output b of the decoder 24, via red LED 1c to the output c of the decoder 24, via red LED 1d to the output d of the decoder 24, and via red LED 1g to the output g of the decoder 24. As a result, the segments a, b, c, d, g illuminate in red color. Since the bus control inputs GB and BB are at a low logic level, the outputs of inverting buffers 26b and 26c rise to a high logic level. The current flows from the output of the buffer 26b via all green LEDs 5a to 5h in parallel to ground. The current also flows from the output of the buffer 26c via all blue LEDs 6a to 6h in parallel to ground. As a result of blending the green and blue light signals in each background region, the entire background area illuminates in substantially blue-green color. The overall effect is numeral `3` illuminated in red color on substantially blue-green background, as shown in FIG. 1b. 
     To exhibit decimal number `7` in blue color, a BCD code 0111 is applied to the inputs A0, A1, A2, A3 of the decoder 24. The decoder 24 develops low logic levels at its outputs a, b, c, to illuminate equally designated segments, and high logic levels at all remaining outputs, to extinguish all remaining display segments. 
     To illuminate the display element in blue color, the bus control input BB is raised to a high logic level, while both remaining bus control inputs RB and GB are maintained at a low logic level. As a result, the output of the buffer 25c rises to a high logic level. The current flows from the output of the buffer 25c, via display blue bus 14 and blue LED 3a to the output a of the decoder 24, via blue LED 3b to the output b of the decoder 24, and via blue LED 3c to the output c of the decoder 24. As a result, the display segments a, b, c illuminate in blue color. Since the bus control inputs RB and GB are at a low logic level, the outputs of inverting buffers 26a and 26b rise to a high logic level. The current flows from the output of the buffer 26a via all red LEDs 4a to 4h in parallel to ground. The current also flows from the output of the buffer 26b via all green LEDs 5a to 5h in parallel to ground. As a result of blending the red and green light signals in each background region, the entire background area illuminates in substantially yellow color. The overall effect is numeral `7` illuminated in blue color on substantially yellow background, as shown in FIG. 1c. 
     The invention may be now briefly summarized. The method was disclosed of selectively exhibiting display units in a variable color by causing a display area to be illuminated in a selected color and by causing a background area substantially surrounding the display area to be illuminated in a substantially complementary color to more effectively exhibit the display units. 
     A variable color display device was disclosed that comprises a plurality of variable color display areas arranged in a pattern and adapted to be illuminated in groups in a selected color to selectively exhibit a plurality of display units and a variable color background area substantially surrounding the display areas and adapted to be illuminated in a color definitely different from the color of the display areas. 
     All matter herein described and illustrated in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. It would be obvious that numerous modifications can be made in the construction of the preferred embodiment shown herein, without departing from the spirit of the invention as defined in the appended claims. 
     CORRELATION TABLE 
     This is a correlation table of reference characters used in the drawings herein, their descriptions, and examples of commercially available parts. 
     
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#    DESCRIPTION            EXAMPLE                                       
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1    display red LED                                                      
2    display green LED                                                    
3    display blue LED                                                     
4    background red LED                                                   
5    background green LED                                                 
6    background blue LED                                                  
7    opaque wall                                                          
8    display light blending cavity                                        
9    background light blending cavity                                     
10   support                                                              
11   variable color display device                                        
12   display red bus                                                      
13   display green bus                                                    
14   display blue bus                                                     
16   background red bus                                                   
17   background green bus                                                 
18   background blue bus                                                  
21   segment color control                                                
22   background color control                                             
23   display decoder                                                      
24   common anode 7-segment decoder driver                                
                            7446                                          
25   non-inverting buffer   74LS244                                       
26   inverting buffer       74LS240                                       
27   inverter               part of                                       
                            74LS240,4                                     
30   housing                                                              
31   display area                                                         
32   background area                                                      
34   light scattering material                                            
35   top surface of display area                                          
36   top surface of background area                                       
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