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

["Followup-To:" header set to aus.electronics.]
On 2005-12-29, Mark Haase <mehaase@gmail.com> wrote:
> In article <dovdbp$o4k$1@news-01.bur.connect.com.au>,
>  MC <MC@nonexistent.place> wrote:
>
>> 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.

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)

> 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?

another way would be to matrix them with capacitors and scan at a
reasonable speed.

    |             //   
    +-->|--+-[R]->|--.
    |      |         |
    |     ===        |
    |      |         |
  --|------+---------+-- ROW
    |
   COL 
    
    
    

-- 

Bye.
   Jasen

On Thu, 29 Dec 2005 12:15:32 +1100, MC <MC@nonexistent.place> wrote:

>>>
>>>Not a trivial or cheap project beyond around 150 x 40 pixles.
>> 
>> Definitely.
>> 
>> But wouldn't a 720 x 576 panel be one heck of a project!
>
>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.

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.

>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.

Yes, price and self-"madeability" of this: horrible.

Mochuelo wrote:
> On Thu, 29 Dec 2005 12:15:32 +1100, MC <MC@nonexistent.place> wrote:
> 
> 
>>>>Not a trivial or cheap project beyond around 150 x 40 pixles.
>>>
>>>Definitely.
>>>
>>>But wouldn't a 720 x 576 panel be one heck of a project!
>>
>>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.
> 
> 
> 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.

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 !!

>>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.
> 
> 
> Yes, price and self-"madeability" of this: horrible.

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.

test