`
`IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
`
`In re Patent of:
`
`Lebens et al.
`
`U.S. Patent No.:
`
`6,095,661
`
`
`
`Issue Date:
`
`August 1, 2000
`
`Appl. Serial No.:
`
`09/044,559
`
`Filing Date:
`
`March 19, 1998
`
`Title:
`
`METHOD AND APPARATUS FOR AN L.E.D.
`
`FLASHLIGHT
`
`
`PETITION FOR INTER PARTES REVIEW OF UNITED STATES PATENT
`NO. 6,095,661 PURSUANT TO 35 U.S.C. §§ 311–319, 37 C.F.R. § 42
`
`Exhibit LG-1004
`
`U.S. Patent No. 4,514,727 (“Van Antwerp”)
`
`

`

`United. States Patent
`
`[19]
`
`[11] Patent Number:
`
`4,514,727
`
`Van Antwerp
`
`[45] Date of Patent:
`
`Apr. 30, 1985
`
`[54] AUTOMATIC BRIGHTNESS CONTROL
`APPARATUS
`
`[75]
`
`Inventor:
`
`Joel C. Van Antwerp, Frisco, Tex.
`
`[73] Assignee: TRW Inc., Redondo Beach, Calif.
`
`[21] Appl. No.: 392,675
`
`[22] Filed:
`
`Jun. 28, 1982
`
`Int. Cl.3 ............................................. .. G09G 3/00
`[51]
`[52] US. Cl. .................................. .. 340/793; 358/161;
`250/214 AL; 340/812
`[58] Field of Search ............. .. 340/793, 703, 812, 791,
`340/753, 754; 358/161; 250/214 B, 214 AL,
`214 c
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,493,956
`3,889,251
`4,009,414
`4,346,331
`4,368,406
`4,386,345
`
`................. .. 340/792
`2/1970 Andrews et al.
`. 250/214 AL
`6/1975 Litman .......... ..
`
`....... .. 340/753
`271977 Beckett-Pugh .
`.. 250/214 AL
`8/1982 Hoge ................. ..
`
`., 250/214 AL
`1/1983 Kruzich et a1.
`................. .. 340/703
`5/1983 Narveson et a1.
`
`FOREIGN PATENT DOCUMENTS
`
`2434049
`
`3/1980 France .............................. .. 340/793
`
`Attorney, Agent, or Firm—Jack—A. Kanz; Robert J. Stern
`
`[57]
`
`ABSTRACT
`
`Automatic brightness control apparatus for providing
`real time control of the brightness of various types of
`displays while also providing ambient light compensa-
`tion is disclosed which includes a photodiode for pro-
`viding a current proportional to the ambient light level.
`Amplifiers, together with a load, change the photodi-
`ode current to a control voltage indicating the level of
`ambient
`light. The control voltage is provided as a
`reference input
`to a voltage comparator. The other
`input to the voltage comparator is the output of a free
`running ramp oscillator. The pulse width modulated
`logic signal comprising the output of the voltage com-
`parator is provided as the input for an output driver
`whose pulse stream output is used to control the bright-
`ness of the designated display. The duty cycle of the
`pulse stream output varies with the ambient light level.
`Several logic gates are added to the free running ramp
`oscillator to provide synchronization capability and
`assure that the ramp output waveform of the ramp oscil-
`lator will always begin with an incoming synch pulse.
`The automatic brightness control apparatus is packaged
`as a monolithic integrated circuit on a single silicon chip
`which is housed in a clear plastic 8 pin DIP.
`
`Primary Examiner—Marshall M. Curtis
`
`5 Claims, 6 Drawing Figures
`
`
`
`Exhibit LG—1004 Page 1
`
`
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`Exhibit LG-1004 Page 1
`
`

`

`US. Patent Apr. 30,1985
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`Sheetlof3
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`4,514,727
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`I6
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`I02 ’
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`Fig. 2
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`Exhibit LG—1004 Page 2
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`Exhibit LG-1004 Page 2
`
`

`

`US. Patent Apr. 30,1985
`
`Sheet20f3
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`4,514,727
`
`
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`
`
`Exhibit LG—1004 Page 3
`
`Exhibit LG-1004 Page 3
`
`

`

`US. Patent Apr. 30,1985
`
`Sheet3of3
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`4,514,727‘
`
`
`
`
`
`Exhibit LG—1004 Page 4
`
`Exhibit LG-1004 Page 4
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`

`

`1
`
`AUTOMATIC BRIGHTNESS CONTROL
`APPARATUS
`
`4,514,727
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`2
`which would maintain the reliability and cost objectives
`needed as a basis for system brightness control.
`The present invention as claimed is intended to pro-
`vide a solution to various prior art deficiencies includ-
`ing the inability of the prior art methods and devices to
`automatically provide real
`time light control of the
`various displays in a cost effective manner. Also, the
`prior art devices did not provide effective light control
`of the various displays over the large dynamic range of
`light levels to which the displays in the instrument panel
`were subjected. In addition, the prior art devices did not
`provide effective light control of the broad types of
`displays including fluorescent displays, CRT displays,
`LCD displays, etc. The prior art devices did not pro-
`vide for external sensitivity adjustment for the bright-
`ness control.
`
`DISCLOSURE OF THE INVENTION
`
`The present invention provides automatic brightness
`control apparatus for automatically controlling the
`brightness of the various displays in the instrument
`panel of an automobile by automatically adjusting for
`changes in ambient light to make the display appear to
`remain at
`the same brightness level. The automatic
`brightness control apparatus includes a photodiode
`which together with appropriate amplifiers and load
`resistor provide a control voltage which provides an
`indication of the relative light level surrounding the
`displays. The control voltage is applied as the reference
`input to a voltage comparator. A free running ramp
`oscillator provides a ramp voltage waveform output
`whose peak values are predetermined but whose peak
`to peak voltage will decrease if the primary power
`supply voltage decreases. The ramp voltage waveform
`of the free running ramp oscillator is applied as the
`other input to the voltage comparator. The output of
`the voltage comparator is a logic signal which is pulse
`width modulated and is used to control the brightness of
`the display. If the power supply voltage decreases, the
`peak to peak ramp voltage will drop but the duty cycle
`of the pulse width modulated output will increase. Vari-
`ous logic gates in conjunction with the ramp generating
`circuit provide the ability to synchronize the automatic
`brightness control apparatus with a display driver so the
`ramp waveform will always begin with an incoming
`sync pulse. The automatic brightness control apparatus
`10 is implemented as a monolithic integrated circuit on
`a single silicon chip housed in a clear plastic eight (8)
`pin DIP.
`Among the advantages offered by the present inven-
`tion is the ability to automatically provide real
`time
`light control of the various displays while also provid-
`ing ambient light compensation. The present invention
`allows this control to be done in a cost effective manner.
`The present invention compensates for changes in the
`power supply voltage for the display. Also, the present
`invention allows synchronization with a display driver.
`If several drivers are used and multiplexed, they can all
`be controlled by a single automatic brightness control
`apparatus. Furthermore, the present invention provides
`external sensitivity adjustment, analog and digital out-
`put and synchronous or asynchronous modes of opera-
`tion.
`
`DESCRIPTION
`
`1. Technical Field
`
`The present invention relates in general to display
`systems, and more particularly to apparatus for pr0vid-
`ing dynamic brightness control of display systems.
`Although the present invention is applicable for pro-
`viding automatic brightness control for various dis-
`plays, it has been found to be particularly useful in pro-
`viding automatic brightness control for various multiple
`function displays located across the instrument panel of 15
`an automobile. Therefore, without limiting the applica-
`bility of the invention to “automobile displays,” the
`invention will be described in this environment.
`
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`Sophisticated electronic displays represent a growing
`trend in panel
`instrumentation for US. automobiles.
`One location, single function displays, such as a digital
`clock, a radio, or an instrument cluster, are giving way
`to multiple function displays located across the instru-
`ment panel. In the 1980 model year, one automobile
`corporation introduced an all electronic instrument
`panel. In the 1981 model year, another automobile cor—
`poration introduced an electronic instrument cluster
`which included four digital displays for chronometer,
`odometer, speedometer and fuel supply functions. Cur-
`rently, vacuum fluorescent displays seem to offer the
`greatest
`flexibility for automobile display strategies.
`However, other technologies, such as CRTs and LCDs,
`are emerging which could have a very dramatic impact
`on automobile display design.
`2. Background Art
`The large dynamic range of light levels occurring in
`vehicles and its effect upon the visibility, by the opera-
`tor, of various displays located across the instrument
`panel established a need for some type of real time light
`controlling system for the various displays. Specifically,
`controlling display intensity, once not considered a
`critical function, becomes critical as automobile manu-
`facturers strive to make the various displays more pleas-
`ing to the consumer.
`The effect of ambient light on display intensity has
`always been a design consideration, which was nor-
`mally addressed by one or more of the following tech-
`niques: (1) arbitrarily selecting a brightness level for the
`display with the selected brightness level being con-
`trolled by discrete components, (2) providing a step
`function for the display intensity which is mechanically
`tied to the interaction of a second function, such as the
`engagement of the headlight switch, or (3) a dynamic
`brightness control system which is regulated by discrete
`components and/or computer software. Selecting a
`specific technique was normally tied to the most cost
`effective approach for each display function. As long as
`single function displays, such as a digital clock, radio or
`instrument cluster, were controlled separately,
`then
`there were no problems. As the individual functions
`were combined into more complex systems,
`it was
`found that the methods used for individual displays
`would not necessarily work in an integrated display
`system without costly system interfaces or varying de-
`grees of display intensity. Thus, there was a need for
`some type of real-time control subsystem which would
`include techniques for ambient light compensation but
`
`65
`
`Examples of the more important features of this in-
`vention have thus been summarized rather broadly in
`order that the detailed description thereof that follows
`may be better understood, and in order that the contri-
`bution to the art may be better appreciated. There are,
`
`Exhibit LG—1004 Page 5
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`Exhibit LG-1004 Page 5
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`

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`3
`of course, additional features of the invention that will
`be described hereinafter and which will also form the
`subject of the claims appended hereto. Other features of
`the present invention will become apparent with refer-
`ence to the following detailed description of a presently 5
`preferred embodiment thereof in connection with the
`accompanying drawing, wherein like reference numer-
`als have been applied to like elements, in which:
`BRIEF DESCRIPTION OF THE DRAWING
`
`10
`
`FIG. 1 is a simplified block circuit diagram illustrat-
`ing the overall system of the present invention;
`FIG. 2 is a simplified schematic diagram for generat-
`ing a voltage to set the sensitivity level of the system.
`FIG. 3 is a simplified schematic diagram of the ramp
`oscillator portion of the present invention;
`FIG. 4 is a simplified schematic diagram of the pulse
`width modulated logic output signal of the present in-
`vention;
`FIG. 5 is a simplified exemplary schematic illustrat-
`ing one application of the present invention;
`FIG. 6 is a simplified exemplary schematic illustrat-
`ing another application of the present invention.
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`
`Referring now to the drawing, and in particular to
`FIG. 1, automatic brightness control apparatus accord-
`ing to the present invention is generally referred to by
`reference numeral 10. The automatic brightness control
`apparatus 10 is implemented as a monolithic integrated
`circuit on a single silicon chip housed in a clear plastic
`eight (8) pin DIP 11 and comprises photodiode 12
`whose anode is connected to the input of current ampli-
`fier 14 by lead 16. The cathode of photodiode 12 is
`connected to voltage source VB at terminal 18. In the
`preferred embodiment, photodiode 12 is a PN junction
`diode with active area of 0.016 cm2, current amplifier 14
`is a high gain temperature-compensated current ampli-
`fier and VB is in the range of 1.2 to 1.5 volts. The 117
`output of current amplifier 14 is the amplified photocur-
`rent of photodiode 12 and is sourced out of PIN 1 from
`an open-collector PNP current source of current ampli-
`fier 14 via lead 20. II, is typically 1 to 2 ma under normal
`light conditions.
`II, is also provided as an input to the
`positive input terminal of operational amplifier 22 via
`lead 24. The output of operational amplifier 22 goes to
`the positive input terminal of first comparator 26 via
`lead 28, to the negative input terminal of operational
`amplifier 22 via lead 30 as a feedback signal and to PIN
`3 of DIP 11 via lead 32. Reference voltage VR at termi-
`nal 33 is set by the approximately 150 microamp current
`source connected to Vcc at terminal 36 driving diode
`34 whose cathode is connected to ground. In the pre-
`ferred embodiment, Vcc is approximately 12-14 volts
`DC (the battery voltage) and VR is approximately 680
`millivolts. VCC operating range is 4.5 to 24 volts. Refer-
`ence voltage V}; is provided to PIN 2 of DIP 11 via lead
`38 and to the negative input terminal of the second
`comparator 40 via lead 42. PIN 4 of DIP 11 is con-
`nected to the negative input terminal of the fourth com-
`parator 44 via lead 46. The positive input terminal of the
`fourth comparator 44 is connected to terminal 48 via
`lead 50 with a voltage of approximately é Vcc—VR
`being applied to terminal 48.
`PIN 5 of DIP 11 is connected via lead 52 to the nega-
`tive input terminal of third comparator 54, to the posi-
`tive input terminal of second comparator 40 and to the
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`negative input terminal of first comparator 26. PIN 5 of
`DIP 11 is also connected to the collector terminal of
`
`NPN transistor 56 via lead 58. The positive input termi-
`nal of third comparator 54 is connected to terminal 60
`via lead 62. A voltage equal to one-half of VCC is ap-
`plied to terminal 60.
`The output of first comparator 26 provides a first
`input to NAND gate 64 via lead 66. The output of
`second comparator 40 provides a second input
`to
`NAND gate 64 via lead 68 and also an input to the R
`input terminal of R 8 latch 70 via lead 72. The output of
`third comparator 54 provides one input to NOR gate 74
`via lead 76. The output of the fourth comparator 44 is
`provided as the other input to NOR gate 74 via lead 78
`and also as the input to one-shot multivibrator 80 via
`lead 82. The output of NOR gate 74 is provided as one
`input to NOR gate 84 via lead 86 with the other input to
`NOR gate 84 being the output of the one-shot multivi-
`brator 80 via lead 88. The_output of NOR gate_ 84 is
`provided as an input to the S input terminal of R S latch
`70 via lead 90. The 6 output of R R latch 70 provides
`the third and final input to NAND gate 64 via lead 92
`while the Q output ofR 8 latch 70 provides the input to
`the base terminal of NPN transistor 56 via lead 94. The
`emitter terminal of NPN transistor 56 is connected to
`ground via lead 96. PIN 6 of DIP 11 is connected to
`ground via lead 98. The output of NAND gate 64 is
`provided as an input to output driver 100 whose output
`is provided to PIN 7 of DIP 11. The output of output
`driver 100 is the output of the automatic brightness
`control apparatus 10 which is used in the adjustment of
`the brightness of various displays. VCC is applied to PIN
`8 of DIP 11.
`
`invention for dynamic
`the present
`To implement
`brightness control to perform the control function over
`a typical range of ambient light variances, the mono-
`lithic integrated circuit was developed to incorporate
`automatic brightness control for various display tech-
`nologies. The overall performance characteristics of the
`inventive circuit can best be achieved by reviewing the
`circuit function and analyzing each section individually.
`Photodiode 12 is a PN junction diode with an active
`area of 0.016 cm2 integrated into the monolithic chip.
`The starting material and processing used typically
`yields a responsivity of 0.35 amps per watt. If we have
`incident light extremes of 0.] mw/cm2 to 100 mw/cml,
`the following formula can be used to approximate the
`minimum/maximum current 13 of the photodiode 12:
`IB=(Ee)(R)(A), where Ee=incident light level, R=re-
`sponsivity and A=diode active area. By using the
`above formula, the dynamic range of the photodiode
`current 13 is from 0.56 microamps to 560 microamps.
`The current 13 from the photodiode 12 is amplified by
`current amplifier 14. In the preferred embodiment, cur-
`rent amplifier 14 has a gain of 500. The output of the
`current amplifier 14 is connected to an external load
`resistor 102 positioned at PIN 1 of DIP 11. Load resis-
`tor 102 is used to convert the amplified current 13 of
`photodiode 12 to a voltage and set the sensitivity level
`of the system. This portion of the system is shown sche-
`matically at FIG. 2.
`The voltage developed across load resistor 102 of
`current amplifier 14 and appearing at PIN 1 of DIP 11
`is fed to operational amplifier 22 via lead 24. In the
`preferred embodiment, operational amplifier 22 has
`unity gain. The output of operational amplifier 22, ap-
`pearing on leads 28 and 32, now serves as the control
`voltage Vcand is an indication of the relative light level
`
`Exhibit LG—1004 Page 6
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`Exhibit LG-1004 Page 6
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`

`

`4,514,727
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`5
`striking the photodiode 12. This control voltage Vc is
`one output voltage of the automatic brightness control
`apparatus 10 and can be used as an analog output which
`indicates the relative light level striking the photodiode
`12. In the preferred embodiment, the output of opera-
`tional amplifier 22 was not designed to sink or source
`much current so any load placed at PIN 3 of DIP 11
`must be limited to :1 ma maximum. The output of
`operational amplifier 22 on PIN 3 of DIP 11 may be
`used to control the brightness of a CRT or as an input to
`an A to D converter from which it can then be stored in
`memory and be used to control display brightness with
`software. This would make the unit compatible with the
`National Semiconductor COPS series of display drivers
`which will accept display brightness information from a
`microprocessor.
`The three basic types of displays used in the instru-
`ment panels of automobiles are LED, LCD and vacuum
`fluorescent. Display brightness of both LED and vac-
`uum fluorescent is affected by fluctuations in power
`supply voltage. It is desirable that the automatic bright-
`ness control apparatus 10 hold the brightness of the
`display at a constant level which will not be affected by
`power supply changes. The automatic brightness con-
`trol apparatus 10 holds the brightness of the display at a
`constant level by pulse width modulation of its output,
`on PIN 7 of DIP 11, to the display whose brightness is
`to be controlled. The development of the pulse width
`modulated output of the automatic brightness control
`apparatus 10 starts with a free running ramp oscillator.
`FIG. 3 discloses a simplified example of the free running
`ramp oscillator used in the automatic brightness control
`apparatus 10 and comprises a simple RC circuit with an
`upper and lower threshold point. The various logic
`gates of the automatic brightness control apparatus 10
`are omitted from FIG. 3 since they are for the purpose
`of providing the ability of the automatic brightness
`control apparatus 10 to synchronize with a display
`driver and will be discussed later. An external capacitor
`104 is connected between PINS 5 and 6 of DIP 11 and
`an external resistor 106 is connected between PINS 5
`and 8 of DIP 11 with Vcc being applied to PIN 8.
`Capacitor 104 is charged through resistor 106 toward
`the value of Vcc until its voltage just exceeds Q Vcc. At
`t_hat voltage value, the output of third comparator 54 to
`S input terminal of R S latch 70 goes low because the
`value of the voltage applied via lead 52 to the negative
`input terminal exceeds the value of the voltage, which is
`5 Vcc, applied to the positive input terminal of third
`comparator 54. The low value input to S input terminal
`sets the latch 70 resulting in the Q output of latch 70
`going high and driving the base terminal of NPN tran-
`sistor 56 and placing NPN transistor 56 to the “on”
`condition. With NPN transistor 56 turned on, capacitor
`104 is discharged therethrough until the voltage at PIN
`5 goes just below VR, which is approximately 680 mv.
`At that voltage value, the output of second comparator
`40 to the R input terminal of R S latch 70 goes low
`because the value of the voltage applied via lead 52 to
`the positive input terminal is less than the value of the
`voltage V}; applied to the negative input terminal of
`second comparator 40. The low value input to R resets
`the latch 70 resulting in the Q output of latch 70 going
`low and placing the NPN transistor 56 in the “off”
`condition. With NPN transistor 56 turned off, capacitor
`104 will again be charged through resistor 106 and the
`process will be repeated with a resulting ramp wave-
`form 108 on PIN 5. The switching of transistor 56 on
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`and off results in the capacitor 104 being charged and
`discharged. The frequency of this resulting ramp wave-
`form 108 of the free running ramp oscillator can be
`predicted by the formula F: 1.44/RC. In the preferred
`embodiment, the frequency is 150 Hz.
`A control voltage Vc which provides an indication of
`the relative light level striking the photodiode 12 and a
`ramp frequency from the free running ramp oscillator
`have both now been obtained. These two signals can be
`compared with a voltage comparator (first comparator
`26) and the resulting output will be a logic signal which
`is pulse width modulated. This is schematically repre-
`sented in FIG. 4. The control voltage Vcobtained from
`the current output of photodiode 12 is the input to the
`positive input terminal of first comparator 26 via lead 28
`while the ramp waveform is the input to the negative
`input
`terminal of first comparator 26. The resulting
`output waveform 110 of the first comparator 26 is pulse
`width modulated. During the time the value of the
`waveform 108 is less than the value of the reference
`voltage input, the output of the first comparator 26 is
`high. During the time the value of the waveform 108 is
`greater than the value of the reference voltage input,
`the output of the first comparator 26 is low. Note that if
`the power supply voltage, Vcc, decreases, the peak to
`peak ramp voltage of waveform 108 will drop and,
`therefore, the duty cycle of the output will increase.
`Also, the duty cycle of the output varies proportionally
`with the ambient light level.
`Again with reference to FIG. 1, the logic circuity to
`synchronize the automatic brightness control apparatus
`10 with a display driver includes fourth comparator 44.
`The trigger pulse is input on PIN 4 of DIP 11 and the
`output of fourth comparator 44 goes low during the
`time the trigger pulse is greater than the reference volt-
`age being applied to terminal 48. This low state is pro-
`vided as an input to NOR gate 74 via lead 78 and t0
`one-shot multivibrator 80 via lead 82. The output of
`one-shot multivibrator 80 provides a high input to NOR
`gate 84 via lead 88 for the predetermined duration of the
`output pulse from one-shot multivibrator 80. During
`this time, capacitor 104 is charging toward Vcc and the
`output of third comparator 54 is high with this state
`being input to the other input terminal of NOR gate 74
`via lead 76. This causes the output of NOR gate 74 to go
`low and is the other input to NOR gate 84 via lead 86.
`The output of NOR gate 84 then goes low and is input
`to the S input of R S latch 70 via lead 90 causing Q to go
`high and driving the base of transistor 56 and thereby
`discharging capacitor 104. In order to synchronize with
`a display driver, it is necessary that the ramp always
`begins with an incoming sync pulse. This capability is
`necessary for systems with multiple displays or digits
`which are multiplexed.
`In the multiplexed case,
`the
`ramp frequency should be at least four times the multi-
`plexing frequency.
`When apparatus 10 is used in a system with multiple
`displays, a particular apparatus 10 is chosen as the mas-
`ter unit. The photodiode 12 (the light level sensor) on
`each of the other apparatus 10, which are the slave
`units, is covered such that they cannot function to out-
`put a signal indicative of the light level. The output
`signal on PIN 3 of the designated master unit is fed to all
`the slave or remote units and input on PIN 1 thereof so
`all the apparatus 10 have the same duty cycle. In addi-
`tion, an output signal 110 (which is a pulse train signal)
`is then not required to be sent throughout the system,
`
`Exhibit LG—1004 Page 7
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`Exhibit LG-1004 Page 7
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`where it might cause interference, since only the DC
`control voltage Vc is sent around the system.
`The automatic brightness control apparatus 10 will
`perform in the asynchronous mode if the trigger input at
`PIN 4 of DIP 11 is held at Vcc or at any voltage level
`greater than the voltage on terminal 48. With PIN 4 at
`Vcc, the output of the fourth comparator 44 is low all
`the time. The output of the fourth comparator is input
`to NOR gate 74. The other input to NOR gate 74 is high
`and comes from third comparator 54. The output of 10
`NOR gate 74 is low and with two low inputs the output
`of NOR gate 84 is high. The Q output of latch 70 is low,
`having been reset the last time capacitor 104 was dis-
`charged,
`thereby allowing capacitor 104 to charge.
`When the voltage on capacitor 104 just exceeds é Vcc 15
`then the output of third comparator 54 goes low causing
`the output of NOR gate 74 to go high. This causes the
`output of NOR gate 84 to go low causing the Q output
`of latch 70 to go high resulting in the discharge of ca-
`pacitor 104. When the voltage of capacitor 104 reaches
`V}; (which is approximately 0.70), latch 70 is reset by
`the output of second comparator 40 and the process is
`repeated.
`If PIN 4 is continuously grounded then the ramp
`waveform will stop. With PIN 4 grounded, the output
`of fourth comparator 44 is always high as an input to
`NOR gate 74. The output of NOR gate 74 is always low
`regardless of the level of input from third comparator
`54. With the two low inputs, the output of NOR gate 84
`is high which causes Q output of latch 70 to stay low
`causing capacitor 104 to charge until a change occurs
`on the trigger input of PIN 4; therefore, capacitor 104
`will charge to Vcc.
`_ As shown in FIG. 1, NAND gate 64 has three inputs.
`Q input via lead 92 is low during the time capacitor 104 35
`is discharging and is high during the time capacitor 104
`is charging. The input via lead 68 from second compara-
`tor 40 is high except for the short period of time when
`capacitor 104 is changing from discharging to charging.
`These first two inputs on leads 68 and 92 are needed to
`hold the output of driver 100 low while capacitor 104 is
`discharging, so that the output duty cycle can approach
`zero % in a linear fasion. The input from first compara-
`tor 26 via lead 66 comprises a series of pulses whose
`rising edge is centered about the point of time when
`capacitor 104 is discharging. The width of the pulse is
`directly proportional to the level of ambient light mea-
`sured by photodiode 12. When all inputs to NAND gate
`64 are high, the output is low. The amount of time the
`output of NAND gate 64 is low is, for all practical 50
`purposes, directly proportional to the amount of time
`the output of first comparator 26 is high. The output of
`NAND gate 64 is input to output driver 100 whose
`output goes to PIN 7. The output “on time” is the time
`when the ramp voltage is less than the reference voltage 55
`derived from photodiode 12 and the ramp voltage is
`synchronized with the trigger signal. The output is a
`totem pole output which will sink 20 ma or source up to
`50 ma. As the ambient light level goes up, the width of
`‘ the output pulses increases (the duty cycle increases) 60
`which increases the brightness of the display. As the
`ambient light level decreases, the opposite occurs and
`the brightness of the display is decreased.
`With reference to FIG. 5, the invention is disclosed in
`an application for controlling the brightness of a stan- 65
`dard LED. A variable resistor 112 is connected be-
`tween PINS 1 and 2 which allows the operator to vary
`and set the light sensitivity level as desired. Resistor 112
`
`4O
`
`45
`
`8
`is the load resistance of the current amplifier 14 and can
`range from 25K ohm to 200K ohm. The buffered analog
`response of the circuit is then available at PIN 3. Resis-
`tor 106 is 100K ohm and capacitor 104 is 0.1 microfarad.
`Resistor 114 in series with LED 116 is 360 ohm. VCC is
`12 plus or minus 3 volts. If the automatic brightness
`control apparatus 10 is used in a battery controlled
`system and the battery voltage drops, the control appa-
`ratus 10 will increase its duty cycle and cause the dis-
`play it is controlling to maintain a constant light output.
`With reference to FIG. 6, the invention is disclosed in
`an application for controlling the brightness of vacuum
`fluorescent displays. Variable resistor 112 is adjusted
`for a brightness level that is pleasing to the eye. Resistor
`118 is added to keep the display from going completely
`off when apparatus 10 is in total darkness. The auto-
`matic brightness control apparatus 10 is capable of ad-
`justing the display brightness from 0 to 100%. Segments
`120 are off with 0 volts applied and are on with +12 v
`applied. The output of apparatus 10 is applied to grid
`122 of the vacuum fluorescent display 124.
`It will be appreciated that the present invention pro-
`vides apparatus for automatically providing real time
`control of the brightness level of various displays while
`also providing ambient light compensation. The present
`invention also compensates for changes in the power
`supply voltage for the display. Features of the present
`invention include external sensitivity adjustment, ana-
`log and digital output and synchronous or asynchro-
`nous modes of operation.
`Thus, it is apparent that there has been provided in
`accordance with this invention, an automatic brightness
`control apparatus that substantially incorporates the
`advantages set forth above. Although the present inven-
`tion has been described in conjunction with specific
`forms thereof, it is evident that many alternatives, modi-
`fications, and variations will be apparent to those skilled
`in the art in light of the foregoing disclosure. Accord-
`ingly, this description is to be construed as illustrative
`only and is for the purpose of teaching those skilled in
`the art the manner of carrying out the invention. It is
`understood that the forms of the invention herewith
`shown and described are to be taken as the presently
`preferred embodiments. Various changes may be made
`in the shape, size and arrangement of parts. For exam-
`ple, equivalent elements may be substituted for those
`illustrated and described herein, parts may be reversed,
`and certain features of the invention may be utilized
`independently of other features of the invention. It will
`be appreciated that the various modifications, alterna-
`tives, variations, etc. may be made without departing
`from the spirit and scope of the invention as defined in
`the appended claims.
`I claim:
`
`1. Apparatus for receiving voltage from a power
`supply and for producing a periodic electrical output
`signal whose average value is proportional to the ambi-
`ent light level and independent of fluctuations in the
`power supply voltage, comprising:
`i
`a photosensor circuit for producing an electrical sig-
`nal proportional
`to the ambient
`light
`level and
`substantially independent of the power supply volt-
`age;
`
`a ramp oscillator circuit for producing a periodic
`electrical signal having a ramp waveform whose
`amplitude is proportional to the power supply volt-
`age; and
`
`Exhibit LG—1004 Page 8
`
`Exhibit LG-1004 Page 8
`
`

`

`4,514,727
`
`9
`an output circuit for comparing the photosensor sig-
`nal to the ramp signal and producing a periodic
`output signal whose duty cycle is the portion of
`time the photosensor signal exceeds the ramp sig-
`nal.
`.
`‘
`2. The apparatus of claim 1, wherein the output cir-
`cuit comprises:
`a latch circuit having a first input and having an out-
`put which produces said periodic output signal;
`and
`
`a first comparator having a first input connected to
`receive the photosensor signal, a second input con-
`nected to receive the ramp signal, and an output
`connected to the first input of the latch.
`3. The apparatus of claim 2, wherein the output cir-
`cuit further comprises:
`a second comparator having a first input connected to
`receive a reference voltage, a second input con-
`nected to receive the ramp signal, and an output;
`
`10
`wherein the latch further includes a second input
`connected