Abstract:
A bias offset voltage circuit for controlling one or more devices is disclosed. The bias offset voltage circuit includes a three voltage sources connected in series and a switching element. Each voltage source includes a positive terminal and a negative terminal. The switching element includes a positive input terminal, a negative input terminal, and an output terminal. The negative terminal of the first voltage source is connected to the negative input terminal. The positive terminal of the third voltage source is connected to the positive input terminal. The terminals of the second voltage source are used to drive a first device. The output terminal of the switching element drives a second device. The bias offset voltage circuit may be used to provide proper voltages to each of the devices where the higher-supplied voltage could damage the device supplied with the lower supplied voltage, or vice versa.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims benefit under 35 U.S.C. § 119(e) of U.S. provisional application Ser. No. 60/501,803, filed on Sep. 10, 2003. 

   FIELD OF INVENTION 
   The present invention relates generally to electronic circuits for providing a voltage source to a system, and, more particularly, to electronic circuits for providing two or more voltage sources in electronically programmable and/or controllable signs. 
   BACKGROUND OF THE INVENTION 
   Traditional signs have been based upon printed materials, paper, plastic, metal, etc., and are therefore not programmable. Accordingly, they are not easily changed. In an attempt to overcome this problem, electronically programmable and/or controllable signs have been in existence for many years. For example, liquid crystal diode (LCD) displays, cathode ray tube (CRT) displays, and other electrically-addressable displays will display an image in response to applied electric signals or fields. However, such signs typically require a large amount of electricity, since they must provide illumination in order to be visible to a viewer. 
   Electrical twisting-cylinder and rotary ball displays, such as those described in U.S. Pat. Nos. 4,126,854 and 4,143,103, incorporated herein by reference in their entirety, have been developed to overcome the problems with previous programmable signs. Twisting-cylinder displays, rotary-ball displays and related displays have numerous advantages over conventional displays, such as LCDs and CRTs, since they are suitable for viewing in ambient light, they retain an image indefinitely in the absence of an applied electric field, and they can be made to be very lightweight and/or flexible. For further advantages of such displays, see U.S. Pat. No. 5,389,945, incorporated herein by reference in its entirety. Such displays are referred to herein as “electric paper” displays. An example of such a display is a SmartPaper™ display, from Gyricon Media, Inc. 
   One method of applying an electric field to electric paper display elements is by using a thin film transistor (TFT) active matrix array. A TFT active matrix array is composed of an array of TFTs. A TFT is a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) implemented using thin film technology. A TFT uses thin films, made of either amorphous silicon (a-Si) or polycrystalline silicon (p-Si), and a glass substrate. Current flows between the source and drain of a TFT when a voltage is applied to its gate. Thus, by connecting the gate of a TFT to the power rail of the TFT active matrix array, a TFT is turned on (at a positive voltage). If the gate of a TFT is connected to the ground rail, the TFT is turned off (at zero voltage). 
   It is advantageous to use TFT active matrix arrays with electric paper displays because a localized electric field used to rotate one or more rotatable elements within an electric paper display may be generated from each TFT. TFT active matrix arrays are used as active switches for each picture element or pixel. A TFT active matrix array is similar to a DRAM array. In other words, each display element is at the intersection of horizontal and vertical lines and may be addressed by enabling both lines. A TFT-based display may typically employ one transistor for each sub pixel of the display. Three sub-pixels—pertaining to red, green and blue electric paper display elements—may be used to make one pixel in a color display. Alternatively, a single sub-pixel may be used to make one pixel in a monochromatic display. A sub-pixel may include multiple rotatable elements. 
   Numerous circuits for powering electronic devices exist.  FIG. 1  illustrates an exemplary prior art circuit for electrically powering an electronic device. The circuit of  FIG. 1  includes a voltage source  110  and a switching element  115 . The voltage source  110  possesses a positive terminal  111  connected to a positive input terminal  116  of the switching element  115  and a negative terminal  112  connected to a negative input terminal  117  of the switching element  115 . The positive terminal  111  of the voltage source  110  is also connected to a power rail of a first device  120 . The negative terminal  112  of the voltage source  110  is also connected to a ground rail of the first device  120 . The first device  120  of  FIG. 1  is a thin-film transistor (TFT) active matrix array. The TFT active matrix array  120  includes a plurality of TFTs. 
   The switching element  115  selects between its positive input terminal  116  and its negative input terminal  117  to determine the voltage on its output terminal  118 . The output terminal  118  of the switching element  115  is connected to a second device  125 : The second device  125  shown in  FIG. 1  is an electric paper display. An electric paper display  125  includes an array of rotatable elements, such as bichromal balls, and a conductive layer  126 . One or more rotatable elements correspond to each TFT in the TFT active matrix array  120 . Likewise, one or more TFTs can correspond to each rotatable element. 
   The output terminal  118  of the switching element  115  is connected to a conductive layer  126  of an electric paper display  125 . If the positive input terminal  116  is selected, then the conductive layer  126  is driven to a positive voltage. If the negative input terminal  117  is selected, then the conductive layer  126  is driven to zero volts. For each rotatable element, if a voltage differential between the conductive layer  126  and a TFT associated with the rotatable element is greater than or equal to a threshold voltage, the rotatable element rotates into an orientation displaying a first hemisphere to a viewer of the electric paper display  125 . If the voltage differential is reversed, the rotatable element rotates into an orientation displaying a second hemisphere to a viewer. If the voltage differential between the TFT and the conductive layer  126  is less than a threshold voltage or zero, then the rotatable element maintains its present orientation. Thus, by altering the connection of the gates of each TFT in the TFT active matrix array  120  to the power or ground rails and by selecting the voltage level of the conductive layer  126  of the electric paper display  125 , via the switching element  115 , the rotatable elements are aligned to display known patterns. 
   One problem with the implementation of the prior art circuit occurs when the operating voltage for components of the first device  120  is less than the operating voltage for components in the second device  125 . In this case, if the operating voltage of the second device  125  were used to power the first device  120  as well, the first device  120  could be damaged and experience a vastly decreased lifespan. 
   What is needed is an electronic circuit that overcomes this disadvantage of prior art circuits by providing proper voltage levels to all devices within a system. 
   SUMMARY OF THE INVENTION 
   The present invention relates to providing proper voltage levels to one or more devices within a system. In a preferred embodiment, the present invention relates to providing more than one voltage source to electric paper including rotatable elements, such as bichromal beads, selected by a thin film transistor active matrix array. 
   In a preferred embodiment of the present invention, a bias offset voltage circuit includes a first voltage source, a second voltage source, a third voltage source, and a switching element. The first voltage source includes a first positive terminal and a first negative terminal and has a first voltage differential equal to the voltage difference between the first positive terminal and the first negative terminal. The second voltage source includes a second positive terminal and a second negative terminal and has a second voltage differential equal to the voltage difference between the second positive terminal and the second negative terminal. The third voltage source includes a third positive terminal and a third negative terminal and has a third voltage differential equal to the voltage difference between the third positive terminal and the third negative terminal. The switching element includes a positive input terminal, a negative input terminal and an output terminal. The first negative terminal is electrically connected to the negative input terminal. The first positive terminal is electrically connected to the second negative terminal. The second positive terminal is electrically connected to the third negative terminal. The third positive terminal is electrically connected to the positive input terminal. In a further embodiment, the circuit includes a first device having a power plane electrically connected to the positive terminal of the second voltage source and a ground plane electrically connected to the negative terminal of the second voltage source and a second device having a power plane electrically connected to the output of the switching element. 
   In an embodiment, the first device is a thin film transistor active matrix array including a plurality of thin film transistors and the second device is a rotatable element display including a plurality of rotatable elements. Each of the plurality of thin film transistors may correspond to one of the plurality of rotatable elements. In an embodiment, the second voltage differential is less than an optimal voltage for rotating each of the plurality of rotatable elements in the rotatable-element display. In a further embodiment, the sum of the first voltage differential and the second voltage differential is greater than or equal to the optimal voltage for rotating each of the plurality of rotatable elements in the rotatable element display. In a further embodiment, the sum of the second voltage differential and the third voltage differential is greater than or equal to the optimal voltage for rotating each of the plurality of rotatable elements in the rotatable element display. 
   In an embodiment, the first voltage differential is approximately equal to the third voltage differential. In an embodiment, the first voltage differential is approximately equal to 20 volts, the second voltage differential is approximately equal to 60 volts and the third voltage differential is approximately equal to 20 volts. 
   In a preferred embodiment, a bias offset voltage circuit includes a first voltage source, one or more pairs of voltage sources, and one or more switching elements. The first voltage source has a first positive terminal and a first negative terminal. The one or more pairs of voltage sources each have a negative voltage source and a positive voltage source. Each negative voltage source includes a positive terminal and a negative terminal. Each positive voltage source includes a positive terminal and a negative terminal. The one or more switching elements each include a positive input terminal, a negative input terminal, and an output terminal. Each positive input terminal is electrically connected to a positive terminal of a positive voltage source of a pair of voltage sources. Each corresponding negative input terminal is electrically connected to the negative terminal of the corresponding negative voltage source of the pair of voltage sources. The first positive terminal is electrically connected to the negative terminal of the positive voltage source of a first pair of voltage sources. The first negative terminal is electrically connected to the positive terminal of the negative voltage source of a first pair of voltage sources. The negative terminal of the positive voltage source of each subsequent pair of voltage sources is electrically connected to the positive terminal of the positive voltage source of the preceding pair of voltage sources. The positive terminal of the negative voltage source of each subsequent pair of voltage sources is electrically connected to the negative terminal of the negative voltage source of the preceding pair of voltage sources. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Aspects, features, benefits and advantages of the embodiments of the present invention will be apparent with regard to the following description, appended claims and accompanying drawings where: 
       FIG. 1  illustrates a prior art circuit for electrically powering an electronic circuit; and 
       FIG. 2  illustrates an exemplary circuit for electrically powering an electronic circuit according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Before the present compositions and methods are described, it is to be understood that this invention is not limited to particular compositions, methodologies or protocols described, as these may vary. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope of the present invention which will be limited only by the appended claims. 
   It must also be noted that as used herein and in the appended claims, the singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, reference to a “voltage source” is a reference to one or more voltage sources and equivalents thereof known to those skilled in the art, and so forth. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Although any methods, devices and material similar or equivalent to those described herein can be used in the practice of testing of embodiments of the present invention, the preferred methods, devices, and materials are now described. All publications mentioned herein are incorporated by reference. Nothing herein is to be construed as an admission that the invention is not entitled to antedate such disclosure by virtue of prior invention. 
     FIG. 2  illustrates an exemplary circuit for electrically powering an electronic circuit according to an embodiment of the present invention. The bias offset voltage circuit  200  of the present invention may include a first voltage source  210 , a second voltage source  215 , a third voltage source  220 , and a switching element  225 . The first voltage source  210  may include a first positive terminal  211  and a first negative terminal  212 . The second voltage source  215  may include a second positive terminal  216  and a second negative terminal  217 . The third voltage source  220  may include a third positive terminal  221  and a third negative terminal  222 . The switching element  225  may include a positive input terminal  226 , a negative input terminal  227  and an output terminal  228 . 
   The first positive terminal  211  may be electrically connected to the second negative terminal  217 . The second positive terminal  216  may be electrically connected to the third negative terminal  222 . The first negative terminal  212  may be electrically connected to the negative input terminal  227 . The third positive terminal  221  may be electrically connected to the positive input terminal  226 . The output terminal  228  may be selectably connected to either the positive input terminal  226  or the negative input terminal  227  at any given time. 
   The bias voltage offset circuit  200  of the present invention may be used to drive a first device  230  and a second device  235 . The second positive terminal  216  may be electrically connected to the power rail of the first device  230 , and the second negative terminal  217  may be electrically connected to the ground rail of the first device  230 . In a preferred embodiment, the first device  230  may be a TFT active matrix array including an array of TFTs. At any given time, each TFT may be connected to either the power rail or the ground rail of the TFT active matrix array  230 . The connection of each gate may be altered to connect to the other of the power rail and the ground rail at a future time. 
   In the preferred embodiment, the second device  235  may be an electric paper display. The electric paper display  235  may include an array of rotatable elements, such as bichromal balls, and a conductive layer  236 . Each TFT in the TFT active matrix array  230  may correspond to one or more rotatable elements. Likewise, each rotatable element may correspond to one or more TFTs. The conductive layer  236  may be electrically connected to the output terminal  228  of the switching element  225 . For each rotatable element, if a voltage differential between the conductive layer  236  and a TFT associated with the rotatable element is greater than a threshold voltage, the rotatable element may rotate into an orientation displaying a first hemisphere to a viewer of the electric paper display  235 . If the voltage differential is reversed, the rotatable element may rotate into an orientation displaying a second hemisphere to a viewer. If the voltage differential between the TFT and the conductive layer  236  is less than a threshold voltage, then the rotatable element may maintain its present orientation. Thus, by altering the connection of the gates of each TFT in the TFT active matrix array  230  to the power or ground rails and by selecting the voltage level of the conductive layer  236  of the electric paper display  235 , via the switching element  225 , the rotatable elements may be aligned to display known patterns. 
   In a preferred embodiment, the voltages supplied by each of the voltage sources  210 ,  215  and  220  may be selected based on the operating voltage of the first device  230  and the second device  235 . For example, where the first device  230  is a TFT active matrix array, the voltage supplied by the second voltage source  215  may be selected to be approximately a maximum value at which the TFT active matrix array  230  may operate. In the example, where the second device  235  is an electric paper display, the voltages supplied by each of the first voltage source  210  and the third voltage source  220  may be selected to be greater than or equal to the differential between an optimal voltage for rotation of a rotatable element in the electric paper display  235  and the voltage supplied by the second voltage source  215 . Preferably, the voltages supplied by each of the first voltage source  210  and the third voltage source  220  may be less than the threshold voltage for rotation of a rotatable element in the electric paper display  235 . In a preferred embodiment, the second voltage source  215  may supply a voltage of approximately 60 volts, and the first voltage source  210  and the third voltage source  220  may each supply a voltage of approximately 20 volts. 
   In the preferred embodiment, four voltage differential states may be achieved between the conductive layer  236  and a TFT. Two states may occur if the switching element  225  connects the positive input terminal  226  to its output terminal  228 . In this case, the third positive terminal  221  may be connected to the conductive layer  236  of the electric paper display  235 . 
   If a TFT in the TFT active matrix array  230  is connected to the power rail, then the TFT may be at a voltage equal to the voltage at the second positive terminal  216 . In this case, the voltage differential between the TFT and the conductive layer  236  of the electric paper display  235  may equal the voltage differential between the third positive terminal  221  and the third negative terminal  222 . In a preferred embodiment, one or more rotatable elements corresponding to the TFT may maintain their current orientation under these circumstances because the voltage differential across the one or more rotatable elements (i.e., the voltage supplied by the third voltage source  220 ) may preferably be less than the threshold voltage for rotation of the one or more rotatable elements. 
   If a TFT in the TFT active matrix array  230  is connected to the ground rail, then the TFT may be at a voltage equal to the voltage at the second negative terminal  217 . In this case, the voltage differential between the TFT and the conductive layer  236  of the electric paper display  235  may equal the voltage differential between the third positive terminal  221  and the second negative terminal  217 . In a preferred embodiment, one or more rotatable elements corresponding to the TFT may align to display a first hemisphere under these circumstances because the voltage differential across the one or more rotatable elements (i.e., the sum of the voltage supplied by the third voltage source  220  and the voltage supplied by the second voltage source  215 ) may preferably be greater than the threshold voltage for rotation of the one or more rotatable elements. 
   Two more states may occur if the switching element  225  connects the negative input terminal  227  to its output terminal  228 . In this case, the first negative terminal  212  may be connected to the conductive layer  236  of the electric paper display  235 . 
   If a TFT in the TFT active matrix array  230  is connected to the power rail, then the TFT may be at a voltage equal to the voltage at the second positive terminal  216 . In this case, the voltage differential between the TFT and the conductive layer  236  of the electric paper display  235  may equal the voltage differential between the second positive terminal  216  and the first negative terminal  212 . In a preferred embodiment, one or more rotatable elements corresponding to the TFT may align to display a second hemisphere under these circumstances because the voltage differential across the one or more rotatable elements (i.e., the sum of the voltage supplied by the first voltage source  210  and the voltage supplied by the second voltage source  215 ) may preferably be greater than the threshold voltage for rotation of the one or more rotatable elements. 
   If a TFT in the TFT active matrix array  230  is connected to the ground rail, then the TFT may be at a voltage equal to the voltage at the second negative terminal  217 . In this case, the voltage differential between the TFT and the conductive layer  236  of the electric paper display  235  may equal the voltage differential between the first positive terminal  211  and the first negative terminal  212 . In a preferred embodiment, one or more rotatable elements corresponding to the TFT may maintain its current orientation under these circumstances because the voltage differential across the one or more rotatable elements (i.e., the voltage supplied by the first voltage source  210 ) may preferably be greater than the threshold voltage for rotation of the one or more rotatable elements. 
   In an alternate embodiment, a plurality of switching elements may be used in place of the single switching element  225  shown in  FIG. 2 . Each of the plurality of switching elements may be connected to a conductive layer covering a portion of the second device  235 . The use of multiple conductive layers may permit a plurality of regions to be created within the second device  235 . Alternatively, a plurality of second devices may each have one or more conductive layers each attached to one of the plurality of switching elements. Each conductive layer may switch independently of the other conductive layers. 
   In an alternate embodiment, a plurality of devices may replace the first device  230  shown in  FIG. 2 . Each of the plurality of devices may have its power rail connected to the positive terminal of the second voltage source  215  and its ground rail connected to the negative terminal of the second voltage source  215 . The use of a plurality of devices in place of the first device  230  may allow multiple devices to be operated from the same bias offset voltage circuit. 
   In an alternate embodiment, the second voltage source  215  may control a switching element, such as switching element  225  in  FIG. 2  instead of a first device. Similarly, the outputs of the voltage sources that are connected to the input terminals of the switching element  225  may be used to supply the power and ground rails of a device, such as the first device  230  in  FIG. 2 . 
   In an alternate embodiment, a bias offset voltage circuit may include one or more additional pairs of voltage sources. Each pair of voltage sources may include a positive voltage source and a negative voltage source. The positive voltage source may include a positive terminal and a negative terminal. The negative voltage source may include a positive terminal and a negative terminal. The first additional pair of voltage sources may have the positive terminal of the negative voltage source connected to the negative terminal of the first voltage source and the negative terminal of the positive voltage source connected to the positive terminal of the third voltage source. Each subsequent additional pair of voltage sources may be similarly connected to the preceding pair of voltage sources. 
   In an embodiment, each additional pair of voltage sources may have the negative terminal of the negative voltage source attached to ground rails of one or more devices and the positive terminal of the positive voltage source attached to power rails of one or more devices. In an alternate embodiment, the negative terminal of the negative voltage source may be attached to a negative input terminal of one or more switching elements and the positive terminal of the positive voltage source may be attached to a positive input terminal of the one or more switching elements. In a further embodiment, one or more devices and one or more switching elements may each be attached as stated above.