There are some companies manufacturing large LED based TV-like, color display units.
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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 units. 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 specs. 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?
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 currents
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
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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.
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
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.
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?
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! ;-)
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.
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 , 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.
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.
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.
eh ? what's wrong with a 2 orders of magnitude error ?
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 !!
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.
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