United States Patent l19J
`Fisler
`
`(11)
`
`(45)
`
`4,090,189
`May 16, 1978
`
`[75)
`
`[54) BRIGHTNESS CONTROL CIRCUIT FOR LED
`DISPLAYS
`Inventor: Charles F. Fisler, New Hartford,
`N.Y.
`(73) Assignee: General Electric Company, Syracuse,
`N.Y.
`(21] Appl. No.: 688,474
`[22] Filed:
`May 20, 1976
`Int. CJ.2 ...................... G09B 13/00;.H05B 39/00;
`[51]
`H05B 41/00
`(52] U,S, Cl, .................................... 340/335; 340/340;
`340/324 R ; 315/169 TV
`(58] Field of Search ............ 315/169 R, 169 TV, 208,
`315/245, 291, 297; 340/334, 335, 340, 154,324
`R
`
`[56]
`
`3,493,956
`
`References Cited
`U.S. PATENT DOCUMENTS
`2/1970 Andrews et al . .................... 340/334
`
`Primary Examiner-Maynard R. Wilbur
`Assistant Examiner-T. M. Blum
`Attorney, Agent, or Firm-Marvin A. Goldenberg
`
`ABSTRACT
`[57]
`A brightness control circuit for use with light emitting
`(LED) displays, or comparable electronic displays that
`are energized from a source of DC potential that
`supplies periodic pulses of constant peak current to the
`display elements, the display brightness being con•
`trolled as a function of the pulse duty cycle so as to
`achieve a uniform and continuous control of the display
`over a relatively wide range of brightness levels, ex(cid:173)
`tending particularly into the lower brightness region.
`Energizing current is coupled to the display by a tran(cid:173)
`sistor switching means actuated at a given frequency
`and with a duty cycle that is a function of the brightness
`control setting.
`
`7 Claims, 8 Drawing Figures
`
`5
`
`SWITCH CONTROL
`
`(4
`
`PULSE
`GENERA.OR
`
`I~
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1031, Page 1
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`VIZIO Ex. 1031 Page 0001
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`

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`U.S.Patent
`
`May 16, 1978
`
`Sheet 1 of 2
`
`4,090,189
`
`Fies. 1.
`
`5
`
`14
`
`PULSE
`GENERAiOR
`
`I~
`
`34
`-"-y}y (
`
`MINIMUM
`B'e.1GlfH1ESS
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`28
`24
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`''lAAA M~IUM
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`'FiG, 28. { 1~~~~ BRIGATNESS
`t - -
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`•
`HIGH
`+ 1G . 2C. ~~ t -~~-~ -B~IGHTuESS
`t - -
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1031, Page 2
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`VIZIO Ex. 1031 Page 0002
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`

`

`U.S.Patent
`
`May 16, 1978
`
`Sheet 2 of 2
`
`4,090,189
`
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`VIZIO Ex. 1031 Page 0003
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`1
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`4,090,189
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`2
`input of the transistor switching means responds to the
`voltage across the capacitor and derives at its output a
`drive signal having a duty cycle that is dependent upon
`the relative time said capacitor voltage is above and
`5 below Vth. Thus, the threshold voltage sensing transis(cid:173)
`tor provides.the transistor switching means with a pre(cid:173)
`cise on/ off operation.
`In accordance with a further aspect of the invention,
`circuit means are provided for periodically actuating
`the threshold voltage sensing transistor irrespective of
`the voltage across the capacitor, to ensure that this
`transistor will not remain continuously in a single oper(cid:173)
`ating state should the capacitor voltage fail to be re(cid:173)
`duced below Vth during discharge.
`
`BRIGHTNESS CONTROL CIRCUIT FOR LED
`DISPLAYS
`
`BACKGROUND OF THE INVENTION
`The invention pertains to electronic displays and to
`control circuitry for controlling the light output of
`these displays. In the more common type of brightness
`control circuit for electronic displays, the current sup(cid:173)
`plied to the display'elements is controlled as a function 10
`of the amount of illumination desired from the display.
`This is normally done by adjusting a resistance through
`which the energizing current flows, or by adjusting the
`supply voltage as applied through an emitter follower
`circuit. In addition to being inefficient and wasteful of 15
`energy, these forms of control have a limited range over
`which the illumination can be linearly controlled and
`tends to turn off completely at low brightness. As a
`related matter, the control circuit may be subject to
`temperature instabilities and excessive variations in 20
`component tolerances, giving rise to a nonuniform il(cid:173)
`lumination from the display elements.
`
`30
`
`SUMMARY OF THE INVENTION
`It is accordingly one object of the invention to pro- 25
`vide an improved brightness control circuit for LED
`and comparable electronic displays that provides a con(cid:173)
`tinuous control of the display elements over a wide
`range of brightness levels, extending particularly into
`the low brightness region.
`Another object of the invention is to provide a bright(cid:173)
`ness control circuit that provides uniform illumination
`from the display elements over a wide range of bright(cid:173)
`ness levels.
`A further object of the invention is to provide a 35
`brightness control circuit which is of relatively simple
`circuit configuration and may be inexpensively con(cid:173)
`structed.
`Another object of the invention is to provide a bright(cid:173)
`ness control circuit that is highly reproducible on a mass 40
`production basis.
`These and other objects of the invention are accom(cid:173)
`plished in accordance with one aspect of the invention
`by a brightness control circuit for controlling the cur(cid:173)
`rent flow from a source of energizing potential to an 45
`electronic display, the output of said potential source
`being coupled through a transistor switching means for
`supplying pulses of approximately constant peak cur(cid:173)
`rent to the display elements. The transistor switching
`means is controlled so as to provide a periodic on/off 50
`operation having a duty cycle that is varied to control
`the brightness of the display. The operation of the tran(cid:173)
`sistor switching means is controlled as a function of a
`drive signal of approximately constant peak voltage
`derived from a capacitive charge-discharge circuit. 55
`This circuit includes a capacitor that is charged through
`a serially connected charge circuit means which in(cid:173)
`cludes a brightness control resistor whose resistance is
`adjusted for a selected condition of brightness to deter(cid:173)
`mine the initial rate of charge of said capacitor voltage, 60
`the capacitor being periodically and briefly discharged
`through a discharge transistor. During the charge time
`the capacitor voltage is made to exceed a given thresh(cid:173)
`old voltage Vth, and during discharge the capacitor
`voltage is reduced toward a reference level that is 65
`below Vth. A threshold voltage sensing transistor hav(cid:173)
`ing its input coupled to the capacitor through a resistor
`voltage divider circuit and its output coupled to the
`
`BRIEF DESCRIPTION OF THE DRAWING
`While the specification concludes with the claims
`which particularly point out and distinctly define that
`subject matter which is regarded as the invention, it is
`believed that the invention will be more clearly under(cid:173)
`stood when considering the following detailed descrip(cid:173)
`tion taken in connection with the accompanying figures
`of the drawing in which:
`FIG. 1 is a schematic circuit diagram of a brightness
`control circuit: for controlling the illumination of an
`LED display;
`FIGS. 2A, 2B, 2C and 2D are graphs of various
`waveforms pertaining to the operation of the circuit of
`FIG. 1; and
`FIGS. 3A, 3B and 3C are also graphs of various
`waveforms pertaining to the circuit operation.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`With reference to FIG. 1 of the drawing, there is
`illustrated a schematic circuit diagram of a brightness
`control circuit for use with an LED or comparable
`electronic disp'lay, which includes a source of DC po(cid:173)
`tential Bl that is applied through a transistor switching
`means 2 to an LED display 4. Transistor switching
`means 2 is in the form of a Darlington pair of NPN
`transistors 3 and 5 connected with their collectors
`joined together to Bl, and providing a constant peak
`voltage output at the emitter of transistor 5 in an emitter
`follower configuration with the components of the
`LED display 4. A second sow:ce of DC potential Bl is
`applied through a brightness control resistance 6, com(cid:173)
`posed of a tapped resistor 8 connected in series with a
`fixed resistor 10, to one side of a charge-discharge ca(cid:173)
`pacitor 12. For an efficient operation of the transistor
`switching means 2, the voltage of Bl is made greater
`than that of Bl. The other side of capacitor 12 is con(cid:173)
`nected to ground. The tap for resistor 8 has one fixed
`terminal connected to the junction of resistors 8 and 10
`and a movable contact selectively positioned at a point
`on resistor 8 between the extreme maximum and mini(cid:173)
`mum brightness positions. The resistor 8 thus provides a
`variable resistance for developing a voltage across ca(cid:173)
`pacitor 12 in accordance with the level of brightness
`desired from the display.
`An NPN transistor 14 is coupled in shunt with capac(cid:173)
`itor 12 for periodically discharging capacitor 12. The
`collector of transistor 14 is connected to the one side of
`capacitor 12 and its emitter is connected to ground. A
`pulse train from pulse generator 16, which may be of
`conventional form, is coupled through a first differenti(cid:173)
`ating capacitor 18 to the base of transistor 14. In re(cid:173)
`sponse to the positive spikes of the differentiated pulse
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1031, Page 4
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`VIZIO Ex. 1031 Page 0004
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`4,090,189
`
`Y f oy
`
`Type 2Nl4J6
`
`Light Emilling Diodes
`
`3
`4
`In the operation of the circuit of FIG. 1, capacitor 12
`train, transistor 14 is periodically and briefly turned
`fully on for discharging the capacitor U. The one side
`is charged from source B2 through the brightness con-
`trol resistors 8 and 10. The movable contact 9 is set
`of capacitor 12 is also coupled through a resistor 20 to
`along the resistor 8 between maximum and minimum
`the base of a further NPN transistor 22, the base also
`being connected through a resistor 23 to ground. This 5 brightness positions to adjust the brightness level of the
`transistor has its emitter connected to ground and its
`display. As will be seen, actual control of the display's
`collector connected through a resistor 24 to source B2
`light output brightness is accomplished through con-
`and to the input of the Darlington pair at the base elec-
`trolling the duty cycle of the drive signal applied to
`trode of transistor 3. The transistor 22 responds to the
`transistor switching means 2.
`voltage across capacitor 12 to become fully conductive 10 Changing the position of contact 9 adjusts the RC
`when this voltage exceeds a threshold level that is estab-
`time constant for cfuarging the capacitor 12, which is
`lished by the Veb of transistor 22, and to be nonconduc-
`determined predominantly by the amount of resistance
`tive when the capacitor voltage is below the threshold
`of resistor 8 connected into the circuit, the resistance of
`resistor 10 and the capacitance of capacitor U . In ad-
`level. Transistor 22 derives a drive signal for the transis-
`tor switching means 2 at its collector that is of constant 15 justing this time constant, the rate at which voltage is
`developed across the capacitor 12 is correspondingly
`peak voltage and has a duty cycle that is a function of
`the relative time the capacitor voltage is above and
`adjusted. The rate of voltage build-up is employed to
`control the duty cycJe of the drive signal for switching
`below the threshold level.
`The LED display 4 is schematically represented as a
`means ~. as will be more clearly seen.
`is provided
`matrix of current paths coupled in parallel, each path 20 A d1schar~e path for capacitor U
`t~ough trans1~tor 14. _As previously noted, the positive
`including the serial connections of a light emitting diode
`26, a fixed resistor 28 and a switch 30. The switch 30 is
`spikes of the differentiated pulses from pulse generator
`illustrated as a mechanical component for simplicity but
`16 ac~ to briefly tu~ fully on f:an~istor 14 for inserting
`in a practical embodiment both the resistor 28 and
`the. ~ischai:ge pa~ mto the _ctrcu1t. A graph of these
`switch 30 would normally be incorporated in a transis- 25 posltl~e spikes _is 1llus':ated !n FIG. 2D. The time for
`tor switching device. The diodes 26 have their anodes
`char~g capac1_tor ~2 1s the llme between positive volt-
`joined together. The switches 30, with one terminal at
`a~e spikes, which IS ~he pulse period less the spike
`ground, are selectively operated by a conventional
`width. I~ th_e embod~ment under consideration, the
`switch control circuit 32 for connecting different com-
`P~lse penod 1~ convemently _1~60 second and the spike
`binations of LED current paths into the circuit in accor- JO 'Y1dth approXJmat~Jy ~/10 millisecond. Th_e _c~arge RC
`dance with a particular information to be displayed. It is
`time constant, which 1s a measure of the 1mt1~ rate of
`noted that the fixed resistors 28 are of equal value so
`chang~ of voltage developed across the capacitor, may
`that pulses of constant peak current are supplied to each
`be a?Justed from less ~han a millisecond, which is a
`of the LED elements connected into the circuit, irre-
`f~actlon of the charge u~e'. to several tim':5 the charge
`spective of the number of such connected elements.
`35 ~e such as over I~ mill1secon?s· !he d1sch~ge RC
`A second differentiating capacitor 34, having one side
`time const~t fo: opt11num ope_ra_lton 1s substantially less
`than thespi.!<e width of 1/10 ~111iseconds so as to permit
`coupled to the output of pulse generator 16 and its other
`side coupled to the base of transistor 22, applies a differ-
`complete d1scharg~ of capac1~0~ 12.
`.
`.
`. When c~n~ct 9 1s set for ~um bn_ghtness, which
`entiated pulse to said transistor. In response to the nega-
`tive and positive spikes of the differentiated pulses, 40 ~se~ a mmunu_m am_ount of resistance mto the charge
`transistor 22 is caused to periodically turn off and tum
`circuit, volta~e is r_a~i?IY developed across the capaci-
`on, respectively, irrespective of the voltage across the
`tor 12_at maximum irutt_al rate of change, as shown in the
`capacitor U. Of particular importance are the negative
`capacitor volt~ge vs. time cui:ve of 1'.11G. 2A. ~t is see~
`spikes which in periodically turning off transistor 22
`that the capacitor voltage rapidly builds_ up to us maxi-
`ensure that the display cannot tum fully off durin 45 mum v~ue and remams a_t this value duru_ig most of the
`settings of low brightness.
`g
`ch~rge ~me. Correspond1~gly, as the settmg of contact
`9 is adJust~d for s~cc~ss1vely greater brightness, the
`In considering one exemplary embodiment of ap-
`charge res1stan~e is mcreased . to develol.' . ~oltages
`Iicant's invention the circuit of FIG. 1 ma em I
`across the capacitor 12 at successively lower irutial rates
`~e folio
`· g co '
`t t
`d
`w~ mponen ypes an component va ues, 50 of change. FIGS. 2B and 2C show capacitor voltage
`.
`wh1~h_3:e given b~ way ?f example and not mtended to
`curves for selected medium and h'gh b · ht
`d"-
`. 1
`be lim1tmg of the mvention:
`.
`.
`.
`.
`ng ness con 1
`llons, respectively, bearmg m mmd there may be numer-
`ous other brightness settings, each exhibiting its own
`capacitor voltage curve. In FIG. 2B the voltage devel-
`55 ops to about half the maximum value during the charge
`time, and in FIG. 2C it develops to a relatively low
`value. From FIGS. 2A, 2B and 2C, it is seen that the
`capacitor 12 is charged at an initial rate corresponding
`to the selected brightness setting to a resulting voltage,
`60 and is then rapidly discharged, the charge-discharge
`operation being done in a cyclical manner at an estab(cid:173)
`lished frequency. While the voltage developed across
`the capacitor at the end of the charge time is a function
`~f the initi~ r~te o_f change of voltage and the charge
`65 time, of pnnc1pal importance to the operation of the
`circuit is the initial rate of change of voltage and the
`ratio of the time the voltage is above and below an
`established threshold level V th.
`
`Transistors
`3, 5, 14, 22
`~
`26
`Resistors
`7
`10
`20
`23
`24
`28
`Capacitors
`12
`18, 34
`Source Potential
`Bl
`B2
`
`2 Megohm
`4.7 Kohm
`390 Kohm
`IOOKohm
`27 K ohm
`300ohm
`
`.05 mf
`470 pf
`
`6 volts
`27 volts
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1031, Page 5
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`VIZIO Ex. 1031 Page 0005
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`5
`During the time when the capacitor 12 voltage ex(cid:173)
`ceeds the threshold level Vth, transistor 22 will con(cid:173)
`duct. The threshold level Vth is determined by the Veb
`of transistor 22 as voltage multiplied by resistors 20 and
`23, and may be expressed by the equation:
`Vth = Veb (R20 + R23/R23)
`
`Upon conduction of transistor 22 its collector voltage is
`reduced, which applies a signal to the base of transistor 10
`3. This causes the transistors of switching means 2 to be
`nonconducting and thereby prevents energization of the
`LED display 4. Conversely, during the time when the
`capacitor 12 voltage is below the threshold level Vth,
`transistor 2.2 is turned off and its collector voltage in- 15
`creased to cause the transistors of switching means 2 to
`conduct and thereby apply energizing current to the
`display 4.
`Thus, the ratio of the time the capacitor 12 voltage is
`below Vth to the time it is above Vth determines the 20
`duty cycle of the drive signal applied from the collector
`of transistor 22 to the base of the transistor 3 of switch(cid:173)
`ing means 2. The brightness of the display is directly
`related to the magnitude of the duty cycle.
`In the operation under consideration, Veb was ap- 25
`proximately 0.6 volts, Vth approximately 3 volts and
`the voltage across the display 4 in its energized condi(cid:173)
`tion was approximately 6 volts.
`As illustrated in FIG. 2A, showing the capacitor
`voltage curve for a minimum brightness condition, the 30
`capacitor voltage during initiation of the charge period
`rapidly exceeds Vth, which is indicated as equal to
`about 3 volts, and remains above this level until the
`discharge period when the capacitor voltage falls pre(cid:173)
`cipitously. In an optimum operation, the voltage falls to 35
`zero but, as will be seen, it may not always do so. At the
`initiation of the subsequent charge period, it again rap(cid:173)
`idly increases to exceed Vth. The duty cycle of the
`drive signal from the collector output of transistor 22
`for this operation of the circuit is very low as shown by 40
`pulses A in FIG. 3A. Pulses E are due to negative volt(cid:173)
`age spikes applied to the base of transistor 22 through
`differentiating capacitor 34, an additional feature of the
`circuit as will be discussed.
`In FIG. 2B, illustrating a medium level of brightness, 45
`the capacitor voltage rises less rapidly and takes more
`time to exceed Vth. Accordingly, the duty cycle of the
`drive signal is increased from that previously consid(cid:173)
`ered, as shown by the pulses B in FIG. 3B. Referring to
`FIG. 2C, illustrating a high brightness condition, the SO
`capacitor voltage rises relatively slowly so as to exceed
`Vtb in the latter portion of the charge period. Thus, the
`duty cycle of the drive signal in this type operation is
`relatively high, as shown by the pulses C in FIG. JC.
`As previously mentioned, because of an unavoidable 55
`imprecision in the circuit operation due to component
`tolerances and the like, for conditions of minimum and
`low brightness, as shown by the capacitor voltage curve
`in FIG. 2A, the capacitor voltage may not be fully
`discharged during the brief discharge period and there- 60
`fore may continuusly remain above the threshold Vth.
`Should this occur, the duty cycle would become zero
`and the display turned fully off. To avoid such occur(cid:173)
`rence, the negative spikes of the differentiated pulses
`formed by capacitor 34 in being applied to the base of 65
`transistor 22, ensure that this transistor will briefly tum
`off at least once each cycle. This is illustrated by the
`pulses E in FIGS. 3A and 3B. Because of the briefness
`
`4,090,189
`
`6
`ofthis action, there is no significant effect on the overall
`circuit operation in -respect to the brightness control.
`In addition, it may be ,appreciated •that the positive
`spikes of the differentiated pulses·from capacitor 34 in
`5 being.applied to-.the .base,o(transisto.r 22 at the same
`time the transisto~ 14 is' made conducting will tend to
`maintain conduction of transistor -22 during the dis(cid:173)
`charge period. This will reduce the duty cycle slightly
`fo.r each brightness setting. However, since the dis(cid:173)
`charge period is very short, the overall operation of the
`control circuit is not significantly affected by this ac-
`tion.
`It may be a_ppreciated that numerous changes and
`modifications can be made to the present circuitry with(cid:173)
`out exceeding the teachings herein provided, and the
`appended claims are intended to include within their
`range all such changes and modifications.
`What is claimed as new and desired to be secured by
`Letters Patent of the United States is:
`1. A brightness control circuit for controlling the
`current flow from a source of energizing potential to an
`electronic display for thereby controlling the light out(cid:173)
`put brightness of said display, comprising:
`a. a charge-discharge capacitor coupled to a point of
`fixed reference voltage,
`b. charge circuit means for developing a voltage
`across said capacitor that will exceed a given
`threshold voltage Vth with respect to said refer(cid:173)
`ence voltage,
`c. discharge circuit means for periodically reducing
`the capacitor voltage toward said reference volt-
`age,
`d. said charge circuit means including an adjustable
`element that adjusts the initial rate of change of
`said capacitor voltage in accordance with a se(cid:173)
`lected display brightness so that the time required
`for the voltage to exceed Vth is likewise in accor(cid:173)
`dance with said display brightness,
`e. threshold voltage sensing means for deriving a
`drive signal having a duty cycle that is a function of
`the relative time said capacitor voltage is above
`and below Vth,
`f. switching means for coupling said source of ener(cid:173)
`gizing potential to said display, and
`g. means for applying said drive signal to said switch(cid:173)
`ing means so as to control its operation as a func(cid:173)
`tion of said duty cycle.
`2. A brightness control circuit as in claim 1 wherein
`said charge circuit means is connected in series with
`said capacitor and the adjustable element thereof com(cid:173)
`prises an adjustable resistor.
`3. A brightness control circuit as in claim 2 wherein
`said discharge circuit means includes a discharge tran(cid:173)
`sistor whose emitter-collector circuit is connected in
`parallel with said capacitor, said discharge transistor
`being periodically actuated for providing a brief and
`rapid discharge of said capacitor.
`4. A brightness control circuit as in claim 3 wherein
`said threshold voltage sensing means includes a further
`transistor whose base-emitter circuit is in a path in par(cid:173)
`allel with said capacitor so that its operating state is
`determined by said capacitor voltage, said drive signal
`appearing at the collector of said further transistor.
`5. A brightness control circuit as in claim 4 wherein
`said threshold sensing means also includes a resistor
`divider circuit having one resistor arm in said path and
`another arm io parallel with said base-emitter circuit,
`whereby the voltage across the base-emitter of said
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1031, Page 6
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`VIZIO Ex. 1031 Page 0006
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`4,090,189
`
`7
`further transistor is a fraction of said capacitor voltage
`as determined by said resist.or divider circuit.
`6. A brightness control circuit as in claim 5 wherein
`said drive signal is of approximately constant peak volt•
`age and said switching means includes a transistor s
`means connected in an emitter follower configuration
`for applying to said display pulses of approximately
`constant peak current.
`7. A brightness control circuit as in claim 5 that fur-
`
`8
`ther includes actuating means for providing periodic
`and brief actuation of said further transistor irrespective
`of said capacitor voltage for ensuring that this transistor
`will not remain continuously in a single operating state
`should the capacitor voltage fail to be reduced below
`Vth during discharge.
`• • • • •
`
`10
`
`IS
`
`20
`
`2S
`
`30
`
`3S
`
`40
`
`4S
`
`50
`
`ss
`
`60
`
`6S
`
`(cid:47)(cid:50)(cid:58)(cid:40)(cid:54) 1031, Page 7
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`VIZIO Ex. 1031 Page 0007
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