Y
`
`O
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`United States Patent [191
`Fisler
`
`[111
`[45]
`
`4,090,189
`May 16, 1978
`
`[54] BRIGHTNESS CONTROL CIRCUIT FOR LED Primary Examiner—Maynard R. Wilbur
`DISPLAYS
`Assistant Examiner-T. M. Blum
`[75] Inventor: Charles F. Fisler, New Hartford,
`Attorney’ Agent’ or ?rm-Mm A’ Goldenberg
`
`_
`
`' N.Y.
`_
`
`-
`
`[57]
`
`ABSTRACT
`
`,
`[51] Int. CLZ .................... .. G09B 13/00; HOSB 39/00;
`H9513 41/
`zsgo/agig 3:290:32],
`[52] U.S. c1. .........................
`R’
`/
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`[58] Field of Search .......... .. 315/169 R, 169 TV, 208,
`315/245’ 291’ 297; 340/334’ 335’ 34o’ 154’ 32;
`
`[73] Asslgnee: gegeml Electnc Company’ Syracuse’ A brightness control circuit for use with light emitting
`'
`'
`(LED) displays, or comparable electronic displays that
`[21] APPL Net-1 688,474
`are energized from a source of DC potential that
`[22] Filed:
`May 20’ 1976
`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_
`tending particularly into the lower brightness region.
`Energizing current is coupled to the display by a tram
`sistor switching means actuated at a given frequency
`and with a duty cycle that is a function of the brightness
`control setting.
`
`[56]
`
`3,493,956
`
`References Cited
`U.S. PATENT DOCUMENTS
`2/1970 Andrews et al. .................. .. 340/334
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`7 Claims, 8 Drawing Figures
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`TCL 1031, Page 1
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`U. S.Patent
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`May 16, 1978
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`Sheet 1 of 2
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`4,090,189
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`B2
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`‘FIG. 1.
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`MIN\M U M
`BHGHTNESS
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`TCL 1031, Page 2
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`U. S.Patent
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`May 16, 1978
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`Sheet 2 of 2
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`4,090,189
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`TCL 1031, Page 3
`TCL 1031, Page 3
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`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
`below Vth. Thus, the threshold voltage sensing transis
`tor providesthe transistor switching means with a pre
`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
`ating state should the capacitor voltage fail to be re
`duced below Vth during discharge.
`
`1
`
`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
`plied to the display: elements is controlled as a function
`of the amount of illumination desired from the display.
`This is normally done by adjusting a resistance through
`which the energizing current ?ows, or by adjusting the
`supply voltage as applied through an emitter follower
`circuit. In addition to being inef?cient and wasteful of
`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
`component tolerances, giving rise to a nonuniform il
`lumination from the display elements.
`
`15
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`BRIEF DESCRIPTION OF THE DRAWING
`While the speci?cation concludes with the claims
`which particularly point out and distinctly de?ne that
`subject matter which is regarded as the invention, it is
`believed that the invention will be more clearly under
`stood when considering the following detailed descrip
`tion taken in connection with the accompanying ?gures
`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 display, which includes a source of DC po
`tential B1 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 B1, and providing a constant peak
`voltage output at the emitter of transistor 5 in an emitter
`follower con?guration with the components of the
`LED display 4. A second source of DC potential B2 is
`applied through a brightness control resistance 6, com
`posed of a tapped resistor 8 connected in series with a
`?xed resistor 10, to one side of a charge-discharge ca
`pacitor 12. For an ef?cient operation of the transistor
`switching means 2, the voltage of B2 is made greater
`than that of B1. The other side of capacitor 12 is con
`nected to ground. The tap for resistor 8 has one ?xed
`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
`mum brightness positions. The resistor 8 thus provides a
`variable resistance for developing a voltage across ca
`pacitor 12 in accordance with the level of brightness
`desired from the display.
`An NPN transistor 14 is coupled in shunt with capac
`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 ?rst differenti
`ating capacitor 18 to the base of transistor 14. In re
`sponse to the positive spikes of the differentiated pulse
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`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‘
`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
`ness control circuit that provides uniform illumination
`from the display elements over a wide range of bright
`ness levels.
`A further object of the invention is to provide a
`brightness control circuit which is of relatively simple
`circuit con?guration and may be inexpensively con
`structed.
`Another object of the invention is to provide a bright
`ness control circuit that is highly reproducible on a mass
`40
`production basis.
`These and other objects of the invention are accom
`plished in accordance with one aspect of the invention
`by a brightness control circuit for controlling the cur
`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
`rent to the display elements. The transistor switching
`means is controlled so as to provide a periodic on/off
`operation having a duty cycle that is varied to control
`the brightness of the display. The operation of the tran
`sistor switching means is controlled as a function of a
`drive signal of approximately constant peak voltage
`derived from a capacitive charge-discharge circuit.
`This circuit includes a capacitor that is charged through
`a serially connected charge circuit means which in
`cludes a brightness control resistor whose resistance is
`adjusted for a selected condition of brightness to deter
`mine the initial rate of charge of said capacitor voltage,
`the capacitor being periodically and brie?y discharged
`through a discharge transistor. During the charge time
`the capacitor voltage is made to exceed a given thresh
`old voltage Vth, and during discharge the capacitor
`voltage is reduced toward a reference level that is
`below Vth. A threshold voltage sensing transistor hav
`ing its input coupled to the capacitor through a resistor
`voltage divider circuit and its output coupled to the
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`train, transistor 14 is periodically and brie?y turned
`fully on for discharging the capacitor 12. The one side
`of capacitor 12 is also coupled through a resistor 20 to
`the base of a further NPN transistor 22, the base also
`being connected through a resistor 23 to ground. This
`transistor has its emitter connected to ground and its
`collector connected ‘through a resistor 24 to source B2
`and to the input of the Darlington pair at the base elec
`trode of transistor 3. The transistor 22 responds to the
`voltage across capacitor 12 to become fully conductive
`when this voltage exceeds a threshold level that is estab
`lished by the Veb of transistor 22, and to be nonconduc
`tive when the capacitor voltage is below the threshold
`level. Transistor 22 derives a drive signal for the transis
`tor switching means 2 at its collector that is of constant
`peak voltage and has a duty cycle that is a function of
`the relative time the capacitor voltage is above and
`below the threshold level.
`The LED display 4 is schematically represented as a
`matrix of current paths coupled in parallel, each path
`including the serial connections of a light emitting diode
`26, a ?xed resistor 28 and a switch 30. The switch 30 is
`illustrated as a mechanical component for simplicity but
`in a practical embodiment both the resistor 28 and
`switch 30 would normally be incorporated in a transis
`tor switching device. The diodes 26 have their anodes
`joined together. The switches 30, with one terminal at
`ground, are selectively operated by a conventional
`switch control circuit 32 for connecting different com
`binations of LED current paths into the circuit in accor
`dance with a particular information to be displayed. It is
`noted that the ?xed resistors 28 are of equal value so
`that pulses of constant peak current are supplied to each
`of the LED elements connected into the circuit, irre
`spective of the number of such connected elements.
`A second differentiating capacitor 34, having one side
`coupled to the output of pulse generator 16 and its other
`side coupled to the base of transistor 22, applies a differ
`entiated pulse to said transistor. In response to the nega
`tive and positive spikes of the differentiated pulses,
`transistor 22 is caused to periodically turn off and turn
`on, respectively, irrespective of the voltage across the
`capacitor 12. Of particular importance are the negative
`spikes which in periodically turning off transistor 22
`ensure that the display cannot turn fully off during
`settings of low brightness.
`In considering one exemplary embodiment of ap
`plicant’s invention, the circuit of FIG. 1 may employ
`the following component types and component values,
`which are given by way of example and not intended to
`be limiting of the invention:
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`In the operation of the circuit of FIG. 1, capacitor 12
`is charged from source B2 through the brightness con
`trol resistors 8 and 10. The movable contact 9 is set
`along the resistor 8 between maximum and minimum
`brightness positions to adjust the brightness level of the
`display. As will be seen, actual control of the display’s
`light output brightness is accomplished through con
`trolling the duty cycle of the drive signal applied to
`transistor switching means 2.
`Changing the position of contact 9 adjusts the RC
`time constant for charging the capacitor 12, which is
`determined predominantly by the amount of resistance
`of resistor 8 connected into the circuit, the resistance of
`resistor 10 and the capacitance of capacitor 12. In ad
`justing this time constant, the rate at which voltage is
`developed across the capacitor 12 is correspondingly
`adjusted. The rate of voltage build-up is employed to
`control the duty cycle of the drive signal for switching
`means 2, as will be more clearly seen.
`A discharge path for capacitor 12 is provided
`through transistor 14. As previously noted, the positive
`spikes of the differentiated pulses from pulse generator
`16 act to briefly turn fully on transistor 14 for inserting
`the discharge path into the circuit. A graph of these
`positive spikes is illustrated in FIG. 2D. The time for
`charging capacitor 12 is the time between positive volt
`age spikes, which is the pulse period less the spike
`width. In the embodiment under consideration, the
`pulse period is conveniently l/60 second and the spike
`width approximately 1/ l0 millisecond. The charge RC
`time constant, which is a measure of the initial rate of
`change of voltage developed across the capacitor, may
`be adjusted from less than a millisecond, which is a
`fraction of the charge time, to several times the charge
`time such as over 100 milliseconds. The discharge RC
`time constant for optimum operation is substantially less
`than the spike width of l/ 10 milliseconds so as to permit
`complete discharge of capacitor 12.
`When contact 9 is set for minimum brightness, which
`inserts a minimum amount of resistance into the charge
`circuit, voltage is rapidly developed across the capaci
`tor 12 at maximum initial rate of change, as shown in the
`capacitor voltage vs. time curve of FIG. 2A. It is seen
`that the capacitor voltage rapidly builds up to its maxi
`mum value and remains at this value during most of the
`charge time. correspondingly, as the setting of contact
`9 is adjusted for successively greater brightness, the
`charge resistance is increased to develop voltages
`across the capacitor 12 at successively lower initial rates
`of change. FIGS. 2B and 2C show capacitor voltage
`curves for selected medium and high brightness condi
`tions, respectively, bearing in mind there may be numer
`ous other brightness settings, each exhibiting its own
`capacitor voltage curve. In FIG. 2B the voltage devel
`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,
`and is then rapidly discharged, the charge-discharge
`operation being done in a cyclical manner at an estab
`lished frequency. While the voltage developed across
`the capacitor at the end of the charge time is a function
`of the initial rate of change of voltage and the charge
`time, of principal 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 Vth.
`
`Transistors
`3, 5, 14, 22
`Diodes
`26
`Resistors
`7
`10
`20
`23
`24
`28
`Capacitors
`12
`18, 34
`Source Potential
`B1
`B2
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`Type 2N3416
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`Light Emitting Diodes
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`2 Megohm
`4.7 K ohm
`390 K ohm
`100 K ohm
`27 K ohm
`300 ohm
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`.05 mf
`470 pf
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`6 volts
`27 volts
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`Vth = Veb (R20 + 1223/1123)
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`During the time when the capacitor 12 voltage ex
`of this action, there is no signi?cant effect’on the overall
`circuit operation in respect tothe brightness control.
`ceeds the threshold level Vth, transistor 22 will con
`In addition, it maybe ‘appreciated-‘that the positive
`duct. The threshold level Vth is determined by the Veb
`spikes of the differentiated‘ pulses-from capacitor 34 in
`of transistor 22 as voltage multiplied by resistors 20 and
`beingapplied to.-.the .basezof transistor 22 at the same
`23, and may be expressed by the equation:
`time the transistor, 14 'is‘made conducting will tend to
`maintain conduction of transistor -22 during the dis
`charge period. This will reduce the duty cycle slightly
`for; each brightness ‘setting. However, since the dis
`charge period is very short, the overall operation of the
`control circuit is not signi?cantly affected by this ac~
`tion.
`It may be appreciated that numerous changes and
`modi?cations can be made to the present circuitry with
`out exceeding the teachings herein provided, and the
`appended claims are intended to include within their
`range all such changes and modi?cations.
`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 ?ow from a source of energizing potential to an
`electronic display for thereby controlling the light out
`put brightness of said display, comprising:
`a. a charge-discharge capacitor coupled to a point of
`?xed 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
`ence voltage,
`0. 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
`lected display brightness so that the time required
`for the voltage to exceed Vth is likewise in accor
`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
`gizing potential to said display, and
`g. means for applying said drive signal to said switch
`ing means so as to control its operation as a func
`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
`prises an adjustable resistor.
`3. A brightness control circuit as in claim 2 wherein
`said discharge circuit means includes a discharge tran
`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
`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 in parallel with said base-emitter circuit,
`whereby the voltage across the base-emitter of said
`
`Upon conduction of transistor 22 its collector voltage is
`reduced, which applies a signal to the base of transistor
`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 22 is turned off and its collector voltage in
`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
`duty cycle of the drive signal applied from the collector
`of transistor 22 to the base of the transistor 3 of switch
`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
`proximately 0.6 volts, Vth approximately 3 volts and
`the voltage across the display 4 in its energized condi
`tion was approximately 6 volts.
`As illustrated in FIG. 2A, showing the capacitor
`voltage curve for a minimum brightness condition, the
`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
`cipitously. In an optimum operation, the voltage falls to
`zero but, as will be seen, it may not always do so. At the
`initiation of the subsequent charge period, it again rap
`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
`pulses A in FIG. 3A. Pulses E are due to negative volt- '
`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
`ered, as shown by the pulses B in FIG. 3B. Referring to
`FIG. 2C, illustrating a high brightness condition, the
`capacitor voltage rises relatively slowly so as to exceed
`Vth 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. 3C.
`As previously mentioned, because of an unavoidable
`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
`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
`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 brie?y turn
`off at least once each cycle. This is illustrated by the
`pulses E in FIGS. 3A and 3B. Because of the briefness
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`4,090,189
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`further transistor is a fraction of said capacitor voltage
`as determined by said resistor 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
`means connected in an emitter follower con?guration
`for applying to said display pulses of approximately
`constant peak current.
`7. A brightness control circuit as in claim 5 that fur
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`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.
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`TCL 1031, Page 7
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