LED circuit design

[dingo]

Hi

I am a film maker but don't have knowledge of LED circuit's.

The project : RGB LED light box ( 6' x 10' ) DMX / Flicker free /
Individual circuit boards mounted behind defuser ( what size boards ? )

In order to produce an even glow I thought that wide angle Strawhead 8-mm / 140-degree would be
the way to go. Please advise for better solutions. The spacing would be 2" x 2" . On a 2' x 3' circuit
board we would need 216 RGB 8mm Strawhead 140degree LED's.

In order to produce xtra white light as a key - light I added 85 xtra white 8mm Strawhead /
140degree LED's. The spacing would be 4" x 4" .
On a 2' x 3' circuit board we would need 85 LED's.


Could you please guide me in the right direction.
I do need the design of the LED circuit and the design of the LED circuit boards.

thanks in advance

 

[Kandela]

I don't know how much help I could be, seems like a big project.

- Firstly, I don't know what led's you're talking about, and a quick google search didn't bring anything up. Can you post some specs or a datasheet? Are the led's cheapo little 20-50mA one's or bigger? Without knowing the spec's I really can't suggest too much.. Initially your 2" spacing and 4" spacing sounds fine.

-With that many led's I would ditch any conventional driver ideas. I don't know anything about DMX, but I would at a bare minimum suggest audrino, or other PIC based control, if not totally computer based. I also would suggest PWM as the way to go as far as controlling the output, colors, hues, brightness, mixing, etc.. PWM is by FAR the most efficient and stable way of operating RGB leds, as far as I know.

-You're also not saying how much control you need over the system... Do you need control over each led? or blocks of leds acting as pixels or just one large display acting and fading as a whole? With such a big project ALOT more details is a good start!

 

[Curt R]

For photo work you must use an analog driver circuit otherwise you will get strobe interference with a pwm driver at
low light power outputs when adjusting the RGB LEDs for color matching. Also you should have Amber LEDs in the mix
for better CRI matching along with cool white LEDs.

LEDs made by CREE or the Philips Rebel are the proper choice as they will maintain a very good color balance over on time
and a very long LED life. With a project like this involving time and money, do not throw away good money on cheap LEDs.
Some of these Chink LEDs are only good for 10's of hours of run time no matter what they advertise.

Curt

 

[Kandela]

Curt; what do you mean by strobe interference? Like as in, taking pictures and having the shutter be so quick it actually is as faster or faster than the PWM rate, causing false colors and fuzzyness??

If i'm on the right track I guess it makes sense, but I don't see why you couldn't just increase the PWM speed to a cycle time of that shorter than a shutter time...?? In any case, if you can dim by turning the led off at certain intervals, is better than C.C. dimming..

Also, he did say film work, i assumed he meant videos... I believe most videos are shot at 30-40 fps?? That is plenty slow enough to use PWM.. I could be on the wrong track...

If you're talking about interference with drivers, there's def a way to isolate/buffer each properly.

I can see how an analog control setup would be much more so a "TRUE" color palate, but I don't understand why you're advising against PWM.

-not grumbling, just curious! :shrug:

 

[Curt R]

With still picture work and using 3, 4 or 5 LEDs and PWM, one or more of the LEDs may have to be turned down
to such a low frequency that one or more of the colors may be too dim or even disappear. When you go to a
film movie or an electronic capture system even at a 200 cps PWM level, the recording can be compromised.
Some professional systems can detect pulses at 2000 Hz. With an analog system you can adjust the output level
down to zero. Typical PWM systems have a dimming ratio of from 200 to 1000 of their base frequency, but not
down to zero. Each LED in an analog system can be controlled with a 10 turn pot for precise adjustment.

Curt

 

[qwertyydude]

In movie making your light source must be completely continuous. It's no problem with conventional light sources as incans and hmi's have slow fade times so light output stays constant even for the brief time AC current switches. Led's turn off as soon as the applied voltage dips below vF. Even if you had 400 hz pwm frequency it would still be visible as strobing if you had to use fast shutter speeds.

Although for the original posters requirements I would have simplified the design with a few high powered led's like 8xP7 leds. And to warm it up if necessary an array of 4 each red and green high powered cree's. This way you don't need the blue led's because I doubt you'll need the cooler temperatures past neutral white. I find P7's to be better for color rendition on camcorders than either MC-E's or most cheapy 5mm leds. If you put each on an analog power controller it's only 3 knobs. One for brightness, one for green and one for red. This is a very easy control system without the need for advanced programming and anyone who's played with combining gels or adjusting hue and tint settings manually could get the hang of getting the correct lighting color.

I used to have a 100 led showerhead light and can tell you just one P7 flashlight easily beat it so if 85 leds is enough light for you, you can get away with probably just one or two P7's and one red and one green cree led.

 

[Kandela]

Hmm, makes sense. Makes me think of the iphone video of a plane's prop. Strange that it's that way, but I'd def listen to these peeps. Analog is gonna be the way to go if you're including anything but the human eye. YA! :naughty:

"the more you know"

 

[Watashi]

I think he's referring to these LEDs. I'm thinking of using these for a reef tank project.

http://stores.ebay.com/LED-DNA/0-5w...15644011&_sid=868265971&_trksid=p4634.c0.m322