Low power uniform high luminous intensity digital display

• US 3,992,873 A
• Filed: 05/19/1975
• Issued: 11/23/1976
• Est. Priority Date: 05/19/1975
• Status: Expired due to Term

• Current Assignee: International Product Development Limited
• Original Assignee: International Product Development Limited
• Inventors: Awalt, Rockland Karl
• Primary Examiner(s): Jackmon, E. S.
• Application Number:
  US05/578,697
• Time in Patent Office: 554 Days
• Field of Search: 58/50 R, 58/85.5, 58/4 A, 58/58, 340/334, 340/336, 340/337
• US Class Current: 368/68
• CPC Class Codes: G04G 21/08 Touch switches specially adapted for time-pieces
• CPC Class Codes: G04G 9/0017 in which the light emitting display elements may be activated at will or are controlled in accordance with the ambient light
• CPC Class Codes: G04G 9/102 using multiplexing techniques
• CPC Class Codes: G04G 9/107 provided with means for displaying at will a time indication or a date or a part thereof

Abstract

A light emitting digital display for use in subminiature assemblies such as miniaturized wrist watches. A crystal controlled frequency reference operates in conjunction with integrated circuitry for providing scanning of a number of display digits and for scanning of individual segments in each digit. The digit scan frequency is above the frequency resolution capability of an observers eye and the segment scan feature allows only one segment in the entire display to be energized at any instant. The segment duty cycle is therefore on the order of the ratio of one to the total number of segments in the display, which allows high peak current to be passed through each segment while maintaining a low average current. The high peak current provides for a high luminous intensity display. A finger switch having two exposed electrodes is actuated by placing a biological segment across the electrodes thereby causing the display to appear. An associated photosensitive device provides a signal indicative of ambient light level for continuously varying the brightness of the display to afford a desired contrast to the eye of an observer. The photosensitive device is also sensitive to the brightness level of individual segments and provides a signal for instantaneous adjustment of current through each segment to provide for uniform brightness therebetween. A set switch is provided including a Hall effect device for sequencing the display to a predetermined point by placing a magnetic field proximate to the assembly while actuating the finger switch.

Claims

1. A light emitting digital display circuit energized by a source of electric power, comprising

a plurality of light emitting devices for providing discrete light emitting segments responsive to electric current passing therethrough

a digit array formed by relative physical positioning of said discrete light emitting segments so that a plurality of light emitting alphabetical and numerical symbols may be formed by predetermined combinations of selected ones of said light emitting segments

means operating at a first frequency for enabling said digit array for a first period determined by said first frequency,

means for scanning said light emitting segments at a second frequency for coupling each of said selected ones of said segments in sequence to the source of electric power for a second period determined by said second frequency within said first period,

means for controlling the luminous intensity of light emitted by each of said selected ones of said segments to substantially the same level relative to ambient light,

a switch for selectively providing an actuating signal for said digit array,

whereby said digit array produces an illuminated symbol having uniform light intensity from all segments in the symbol when said actuating signal is provided while said digit array is enabled.

2. A light emitting digital display as in claim 1 wherein said switch comprises first and second externally accessible electrodes, said first electrode being coupled to the source of electric power,

and an electric switch having an input connected to said second externally accessible electrode and having an output,

so that when the resistance of a biological segment is imposed between said first and second electrodes said actuating signal is provided.

3. A light emitting digital display as in claim 2 wherein said electronic switch comprises first and second complementary field effect transistors with the source of the first and the drain of the second connected to said output,

the drain of the first and the source of the second coupled to different electrical potentials provided by the source of electrical power, and the gates of both the first and second field effect transistors connected to said input.

4. A light emitting digital display as in claim 1 wherein said plurality of light emitting devices are light emitting diodes.

5. A light emitting digital display as in claim 4 wherein said light emitting diodes emit green light.

6. A light emitting display as in claim 1 together with a plurality of additional digit arrays

said digit array and additional digit arrays being enabled in sequence for said first period by said means operating at a first frequency,

means for continuously controlling the brightness of said digit array and additional digit arrays to provide substantially the same optical stimulation as ambient light level changes.

7. A light emitting display as in claim 1 wherein said first frequency is above the frequency resolution of an observer'"'"'s eye and said second frequency is a higher multiple of said first frequency depending upon the number of segments in said digit array.

8. A light emitting display as in claim 7 wherein said digit array is a first digit array together with second, third and fourth digit arrays,

said digit arrays being enabled in sequence for said first period by said means operating at a first frequency,

said first digit array having nine segments,

said second digit array having eight segments,

said third and fourth digit arrays having seven segments each,

and a colon having one segment located between said second and third digit arrays

whereby said second period is one eighth of said first period so that eight segments are scanned while each digit array is enabled and each segment has a one 30second duty cycle.

9. A light emitting digital display circuit as in claim 1 wherein the source of electric power has terminals for coupling to the circuit, together with a capacitor connected across the terminals for storing a charge provided by the source,

so that when said selected ones of said segments pass electric current the voltage at the terminals is sustained by the charge on said capacitor, and the circuit remains energized during brief periods of disconnection from the source of electric power.

10. A light emitting digital display circuit as in claim 1 wherein the means operating at a first frequency includes a current mirror connected to said digit array for controlling the magnitude of the electric current passing through said selected ones of said segments to a preadjusted level,

together with means connected to said current mirror for continuously controlling the brightness of said digit array to provide substantially the same optical stimulation for variations in ambient light levels.

11. A light emitting digital display as in claim 10 wherein said means for controlling the luminous intensity of light emitted by selected ones of said segments includes a photo sensitive device providing a control output signal responsive to light input, and a high pass filter for receiving said control output signal and providing a high frequency control signal coupled to said current mirror and responsive to light levels at said second frequency for changing the electric current level therethrough,

and wherein said means for continuously controlling the brightness of said digit array includes a low pass filter for receiving said control output signal and providing a low frequency control signal coupled to said current mirror and responsive to ambient light levels for changing the electric current level therethrough from said preadjusted level.

12. A light emitting digital display as in claim 1 whrein said means for scanning includes a clock providing a predetermined reference frequency, a first divider circuit for receiving said reference frequency and providing a plurality of segment drive signals in sequence having said second period dwell time, a plurality of segment driver transistors having inputs connected one each to ones of said plurality of segment drive signals and outputs connected to one of said plurality of light emitting devices,

and wherein said means operation at a first frequency includes a second divider circuit coupled to said reference frequency and providing a digit drive signal having said first period dwell time, and a current mirror having an input coupled to said digit drive signal and an output connected to said plurality of light emitting devices.

13. A light emitting digital display circuit as in claim 1 wherein said plurality of light emitting alphabetical and numerical symbols are formed in a predetermined sequence by said digit array, together with

means for stepping said digit array through said predetermined sequence at a rapid rate, said last named means producing a set signal when said switch is actuated so that a predetermined symbol may be formed by said digit array.

14. A light emitting digital display circuit as in claim 13 wherein said means for stepping includes a Hall effect device for producing said set signal when placed in a magnetic field.

15. A light emitting digital display comprising

a clock for providing a stable reference frequency output

a segment scan circuit coupled to said reference frequency output having a segment scan frequency and producing a plurality of sequential segment control signals in each segment scan cycle, each segment control signal having a first dwell time,

a digit scan circuit coupled to said references frequency output having a digit scan frequency above the frequency response of the eye and producing a plurality of sequential digit control signals in each digit scan cycle, each digit control signal having a second dwell time at least as long as the period of said segment scan frequency,

a plurality of digit arrays,

a plurality of segments in each of said digit arrays arranged in relative position to represent predetermined symbols when electric current is passed through selected segments causing said segments to emit light,

a plurality of means for enabling said digit arrays having inputs coupled to ones of said plurality of sequential digit control signals and outputs connected to said plurality of segments, said last named means being adjusted to pass a predetermined level of electric current,

a plurality of segment drivers having inputs coupled to ones of said plurality of sequential segment control signals and outputs connected to ones of said plurality of segments in each of said digit arrays,

a pair of exposed electrodes,

a switch responsive to placement of the resistance of a biological segment across said pair of electrodes for actuating said segment scan circuit, whereby said selected segments have a short duty cycle equivalent to said first dwell time during each digit scan cycle so that higher electric current levels may be passed through said segments for producing higher brightness levels there from while maintaining average current at or below levels for longer duty cycles.

16. A light emitting digital display as in claim 15 together with

a photo sensitive device providing a light intensity control signal responsive to light input,

a high pass filter for receiving said light intensity control signal and providing a high frequency control signal coupled to said means for enabling said digit arrays and responsive to light levels at said segment scan frequency for controlling the electric current level therethrough to provide uniform brightness of light emitted from selected segments, and a low pass filter for receiving said light intensity control signal and providing a low frequency control signal coupled to said means for enabling said digit arrays and responsive to ambient light levels for controlling the electric current level therethrough relative to said predetermined level so that uniform brightness is provided by light emitted from digit arrays for differing ambient light levels.

17. A light emitting digital display as in claim 15 wherein said predetermined symbols are presented in timed sequence related to said reference frequency, together with means for setting any desired symbol to initially appear at said digit array, said means for setting operating only when said switch is actuated.

18. A light emitting display as in claim 15 together with power terminals coupled to said segment scan circuit adapted to receive power from an electrical power source and a capacitor connected across said power terminals whereby power source terminal voltage is sustained during passing of electric current through said selected segment by the charge on said capacitor and electric current and segment brightness is therefore more uniform.

19. A light emitting digital display circuit comprising

a clock for providing a reference frequency,

a segment scan circuit for providing a repeating segment scan cycle output when enabled,

said scan cycle output being related to said reference frequency,

said segment scan cycle having a plurality of sequential segment signals therein,

a plurality of light emitting segments for receiving said segment signals within a segment scan cycle,

a plurality of digit arrays formed by said segments for providing predetermined symbols according to selected segments being energized therein,

a digit scan circuit for providing a repeating digit scan cycle output related to said reference frequency,

said digit scan cycle having a plurality of sequential digit signals therein,

said segment scan cycle having a period equivalent to the dwell time of one of said sequential digit signals,

a switch for enabling said scan circuit,

a plurality of segment drivers for receiving ones of said plurality of sequential segment signals and for passing current to a connected segment,

a plurality of digit drivers for receiving ones of said plurality of sequential digit signals, thereby sequentially enabling ones of said plurality of digit arrays,

whereby said segments are scanned in each digit array during the dwell time of each of said sequential digit signals, and segment duty cycle is reduced to the ratio of said sequential segment signal dwell time to said digit scan cycle period so that high peak current may be passed through selected segments to obtain a high brightness level while average current therethrough is minimized.

20. A light emitting digital display circuit as in claim 19 together with a photo sensitive device for providing a control signal output responsive to light input,

a high pass filter for receiving said control signal output and producing a segment control signal resulting from light input to said photo sensitive device at the frequency of said sequential segment signals

said digit drivers having a preset current level

said segment control signal being coupled to said digit drivers for continously varying the preset current level therethrough and through said selected segments to obtain uniform brightness between selected segments.

21. A light emitting digital display as in claim 20 together with

a low pass filter for receiving said control signal output and producing a digit control signal resulting from ambient light input to said photo sensitive device

said digit control signal being coupled to said digit drivers for continuously varying the preset current level therethrough and through said selected segments to obtain a uniform display contrast for different ambient light levels.

22. A method of presenting a light emitting digital display comprising the steps of

scanning a number of digits,

scanning a number of segments in each digit for each digit scan,

presetting a current level for passing through each segment, whereby only one segment passes current at a time,

whereby a high current level may be passed through each segment for a short time period to obtain high luminous intensity therefrom while using low average current,

sensing the ambient light level, and controlling the preset current level to vary relative to the ambient light level,

thereby providing an adjusted current level to obtain desired display contrast continuously as ambient light level changes.

23. A method as in claim 22 together with the step of sensing the light level emitted by each segment,

controlling the adjusted current level to vary for obtaining uniform brightness from all segments.