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How to build your own LED TV-like color displays?

Discussion in 'Embedded' started by Guest, Dec 28, 2005.

  1. Guest

    Guest Guest

    There are some companies manufacturing large LED based TV-like, color
    display units.


    Electronics is an amateur hobby for me and I would like to investigate (and
    eventually build) the design issues of LED based TV-Like color display
    As a starting point I would see the following grouping of issues;
    1.) Mechanical design and manufacture of the modular LED panels
    2.) Automatic LED adressing protocol (this will initialize individual
    LED panels as well as individual LED when panels are connected with each
    other to form large 2D matrix)
    3.) Selecting/sourcing bright and low-cost LED with almost identical
    4.) Digitising video into NxM color pixels (N, and M are number of
    pixels in Row and Column)
    5.) Sending the digitized image to indivually addressed LED panels
    6.) Each LED panel should be intelligent to receive part of the
    digitized video data corresponding to its own spatial position
    7.) Scanning LEDs according to received pixel data.

    Do you have any idea, information, design, URL etc. about above mentioned
    and/or any other related issue to share?


    Guest, Dec 28, 2005
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  2. Guest

    MC Guest

    Structural, comms, bandwidth and power hassles will excalate
    at an x by y rate. (at least).

    Consider reliabilty, repair, 'fail gracefully', self-test
    and fault-diagnosis issues.

    Add in variations in LED colour and luminance with age
    and manufacturer and manufactured date.

    Not a trivial or cheap project beyond around 150 x 40 pixles.
    MC, Dec 28, 2005
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  3. Definitely.

    But wouldn't a 720 x 576 panel be one heck of a project!

    Dave :)
    David L. Jones, Dec 28, 2005
  4. These are large displays that draw a lot of power (some modules are specced
    at 0.5KW and higher per module)! That one is going to need special cameras and other gizmos to drive it
    as well as possibly its own power station!
    Well that all depends on what you are used to. The bits described above are
    LARGE systems with teams of people working on different parts.
    Not unsurmountable, if you use a scalable design methodology. But will
    probably require lots of LED drivers to keep LEDs 'ON' until a new value
    appears, CRT and LCD have a persistence that keeps the 'dot' bright for
    longer than the sweep of 'addressing' each 'dot'.

    Please also consider what resolution display you want to make and then
    total up the power involved in doing this and how you are going to drive
    the POWER.

    If all LEDs permanently left ON for QVGA resolution and various LED

    Total LEDs 320 x 240 = 76800

    @ 10mA per LED 0.01 * 76800 = 768A (yes AMPS)

    @ 5mA per LED 0.05 * 76800 = 384A

    One line of LEDs ON at a time = 320 * 0.01 = 3.2A

    Sorting out what POWER distribution and LED control is a CRUCIAL part of
    this sort of design. These figures are true even if they are ALL turned
    on at the same time for a short period of time, just the same current
    is seen for a shorter period of time repeatedly.

    How to turn on LEDs, for how long and how many at the same time has effects
    on power and how you see the image (too dim, pulsing, beat frequencies).
    Line at a time can cause vertical beat frequency 'cycling' through the
    picture, groups at a time on can cause bands or other distortions, where
    as single LED at a time on
    Being sure you can get colour balance across the 'screen' will be the main
    issue there, where by you see the same level and mix of colours for the
    same colour at all points on the screen (within a _small_ tolerance) as
    the eye will pick out the differences.
    Your real problems are not so much the digitising, but synchronising and
    scaling/cropping when you have different number of LEDs (pixels) compared
    to input resolution and how to handle it.
    Not forgetting if you are taking PAL or NTSC video, you have the
    de-interlacing to deal with.
    The real crucial issue is anything beyond 1bit depth per colour (R, G and B)
    is a complicated distribution

    analog - switching and distribution without signal loss across
    the dispaly modules

    digital - transferring all the digital levels, between the modules
    (lots of wires or lots of higher speed serial transmission)
    Which requires some form of intelligent controller that parcels out the
    data (or provides clocking signals to say module start/end).
    Shift register controls (chips or PLD) are probably the easiest method
    for scan control, and cascadeable between modules. See also above about
    power and duty cycle of LEDs.
    Look at some of the Disco Dance floor designs for how people have done
    projects with modular LED panels if nothing else for driving tehniques.
    You will need a higher density of LEDs.

    Such as

    Paul Carpenter, Dec 28, 2005
  5. You are probably going to be overrun with the new OLED/PLED
    Technology. Samsung has demonstrated a 40" OLED Display panel.
    The resolution is 1280x800, which is just over 3 million LEDs.
    Stacking smaller panels, in stead of individual LEDs probably makes
    more sense for future products.


    Anton Erasmus
    Anton Erasmus, Dec 28, 2005
  6. Guest

    MC Guest

    hmmm, that'll need around 4m x 3m of surface area if
    you use 3mm leds.
    Awkward for personal viewing, but if you don't close
    the curtains then I'm sure that the neighbours will
    appreciate it <g>

    I'll let you pay for the 240 Amps (peak) to run it.

    And 3x 414720 high luminance Leds at (say) $0.10 each
    comes to around $124k.
    oh, you don't want a visible-in-daylight display ?,
    fine, we'll use ordinary cheap Leds at $0.01 each
    to reduce the price of the visible-bits to around $12k.
    MC, Dec 29, 2005
  7. Guest

    Mark Haase Guest

    Where on earth would you source 240 amps from? I don't have such a
    circuit in my house.

    Also, the time to fabricate, by hand, would be incalculable.

    My question is can the LEDs be operated in various intensities, or is it
    only full-off/full-on? If its it the latter, how do you produce gray? If
    its the former, how to you duty-cycle modulate millions of LEDs?
    Mark Haase, Dec 29, 2005
  8. 240 amps at ~3V for the LEDs is only about 8 Amps at 110V. The power
    supply shouldn't be that tough to build.
    Not really---at least if you have a decent computer and Excel.
    I suspect that it would be similar to the fashion in which you modulate
    millions of pixels in an LCD display. Kodak is making OLED displays
    with 521x218 resolution now---this should just be a step up in size!


    Mark Borgerson
    Mark Borgerson, Dec 29, 2005
  9. I think that was the point.
    Long, yes. Incalculable, probably not.
    Usually the latter. You can operate them at variable intensity
    by varying the amount of current, but that's difficult. To
    acheive "gray" you usually use pulse-width-modulation to vary
    the duty cycle.
    Pulse width modulation.
    Good question.
    Grant Edwards, Dec 29, 2005
  10. Guest

    MC Guest

    This current would be at around the 3 (or so) volt level, so
    you'll only need about 750W (peak) from the mains.
    Not if you get it hand-assembled in China !
    I once spend half a day populating and soldering a 256 Led panel. yetch.
    To create colour, you need different intensities of the primary colours,
    and the usual technique is to use pulse-width-modulation (PWM) because
    it reduces power consumption and heat.
    You'll also need colour correction of some sort because the Leds
    you buy now will have different characteristics from the
    ones you buy in 6 months time. (even if it's the same part number).
    You can reduce this to some extent by buying a particular 'bin'
    of tested and sorted devices, but the price then goes up a lot.

    As to how to control millions of LEDs.., that I'll leave as an
    exercise for you <g>,
    but bear in mind that control and communication techniques which
    work ok for a small number of pixels tend not to scale-up very
    well when the numbers get into the realm of thousands of pixels.

    Most sign manufacturers organise pixels in groups that can be
    easily manufactured, installed and repaired as one entity.
    eg. vertical strips of 2 x 16 pixels or 8 x 8 pixel panels,
    etc etc etc.
    MC, Dec 29, 2005
  11. Guest

    Jasen Betts Guest

    ["Followup-To:" header set to aus.electronics.]
    you could use a step-down transformer (etc), leds only need about 3V each,

    or possibly run the LEDs in a number of series chains with transistors to
    bypass individual leds (to make them dark(er)) and probably some sort of
    switching regulator at the head of each chain..... and probably some sort of
    custom IC doing all the switching.

    if one IC can handle 10 leds 7680 ICs would be needed...
    the IC could do PWM to modulate the brightness.... and only a little over
    24A would be needed (at aroound 30V)
    another way would be to matrix them with capacitors and scan at a
    reasonable speed.

    | //
    | | |
    | === |
    | | |
    --|------+---------+-- ROW
    Jasen Betts, Dec 29, 2005
  12. Guest

    Mochuelo Guest

    Where did you get that number from?
    If he uses 20 mA LEDs, and turns on one row at a time, the peak
    current would be 720*0.02=14.4 A. That is 43.2 W peak at 3 V. The duty
    ratio per LED would be 1/576. I would prefer 172.8 W and 1/144. A
    modern, high-brightness 3 mm LED produces a lot of light at duty
    ratio=1. At 1/144, probably one single pixel still produces more W/m^2
    at the eyes of an observer that is located at a distance such that
    s/he sees the 2.16 x 1.73 m (min) screen covering the same angle as a,
    say, 32" CRT display with only one pixel on.
    Yes, price and self-"madeability" of this: horrible.
    Mochuelo, Dec 29, 2005
  13. Guest

    MC Guest

    eh ? what's wrong with a 2 orders of magnitude error ? <g> <g>

    I assumed a non-multiplexed display to keep the brightness up
    for outdoor use.
    So 720 x 576 x 3 colours = 1244160 leds, and in the worst case
    scenario they are *all* driven on at 20mA = 24.8 kA !!
    MC, Dec 29, 2005
  14. the worse display ive seen was one that used 4 leds per pixel 2red 1blue
    1green. the best display ive seen used composite led's which were 5MM
    led's with 4 di's per 'package' 2blue 1red 1green. 4 elements in a 5mm led.

    if you wanted to you could bring the display down to VCD resolution
    (350x288 or what ever the vcd res is), it wouldnt be HD or SD but still
    perfectly usable.
    matt2-amstereo, Dec 30, 2005
  15. Guest

    rosaldo Guest

    rosaldo, Jan 3, 2006
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