LED scan frquency

DCLK>----+-----|>DATA CLK | +-------|>DATA CLK | RCLK>--+-|-----|LE |-[Rext]-+ | +-----|LE |-[Rext]-+ | | +-----------+ | | | +-----------+ | | +--------------------//----|--+ | GND +-|--------------------//----|----+ | | | | | GND | | Vcc Vcc Vcc | | | | | | | [LED1] [LED8] [LED25] GREEN | | | | | | | +--- - - ---+ +-----------+ GDAT>--|-|-----|SIN SOUT|--//---------------|SIN | DCLK>--|-+-----|>DATA CLK | +-------|>DATA CLK | RCLK>--+-|-----|LE |-[Rext]-+ | +-----|LE |-[Rext]-+ | | +-----------+ | | | +-----------+ | | +--------------------//----|--+ | GND +-|--------------------//----|----+ | | | | | GND | | Vcc Vcc Vcc | | | | | | | [LED1] [LED8] [LED25] BLUE | | | | | | | +--- - - ---+ +-----------+ BDAT>--|-|-----|SIN SOUT|--//---------------|SIN | DCLK>--|-+-----|>DATA CLK | +-------|>DATA CLK | RCLK>--+-|-----|LE |-[Rext]-+ | +-----|LE |-[Rext]-+ | | +-----------+ | | | +-----------+ | | +--------------------//----|--+ | GND +----------------------//----|----+ | GND

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John Fields

I worked on some modules with 3mm dia. tri-color LED pixels (you can see the tiny dies mounted exactly adjacent to each other within the encapsulated pixel) on a 4mm and 5mm pitch set into a 16x16 stackable pixel array. These included heatsinks on the back, black interpixel finish on the front, and each used six graphics controllers with ram and constant current drivers.

Each controller handled an 8x16 array of single-color dies within the pixels. Three driver ICs were responsible for the "upper half" and three for the "lower half," with one for each color. Each included a 7-bit settable "course" and a

7-bit settable "fine" (with its max at 1/2 of the "course" source and simply added in parallel) pair of constant current sources that set the "100%" level and could accept a 5-bit PWM number to control the brightness of each color die it controlled, from there. A programmable stagger of the column drivers allowed for reduced EMI. There was also some color "bleed" controls, as well.

Separate voltage sources were recommended for each color, because of dissipation. If you set up all of the voltages to the minimum required for the blue LEDs, then the red LED constant current drivers would dissipate more heat.

I'd imagine that units like this must greatly simplify the production of the kinds of LED video screens you are talking about. Did you use these kinds of things? (I've still got a bunch of them in a box, here.)

Jon

The sensors in peripheral vision are "faster". It's easy to perceive irritating flicker that disappears when you look straight at it. If you have a CRT monitor, try looking sideways relative to the screen. Presumably an evolutionary adaptation that helps you perceive things coming at you fast from the edges of vision...

Best regards, Spehro Pefhany

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I read in sci.electronics.design that Don Klipstein wrote (in ) about 'LED scan frquency', on Mon, 28 Feb 2005:

Flicker perception depends on the relative brightness of the source compared to its background. Above 60 Hz, the curve is VERY steep. You need a large increase in brightness even at 65 Hz to see the flicker.

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

front. ================================== This is proof of Darwins theory of natural selection. The cavemen that didnt hear the saber toothed tigers sneaking up behind them didnt get their gene for acute hearing sensitivity selected....

So true- I am always being irritated by peripheral vision flicker- but it may be more of a necessity for maintaining balance than an evolutionary adaptation.

I read in sci.electronics.design that Spehro Pefhany wrote (in ) about 'LED scan frquency', on Mon, 28 Feb 2005:

Yes. In the same vein, sounds from behind provoke a stronger reaction than those from the front. Discovered in early stereo-via-headphones and binaural recording experiments.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

for each LED precisely because of the problem Fred highlighted - which I independantly discovered when as a boy I made a mux'd LED display that was initially too dim to read, then too hot to live long (my excuse is I was 12...)

scan at any frequency you want. the eyer refreshes at about 25Hz, so as long as you pick > 50Hz you're fine.

a better way? given Freds point, more parts and less multiplexing. Or drive the shit out of the LEDs.

Cheers Terry

I read in sci.electronics.design that BobG wrote (in ) about 'LED scan frquency', on Mon, 28 Feb 2005:

Yes, of course; it's clearly a survival trait.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

allowed

Hi Jon,

the later ones use LED driver chips, less solder joints that way. Our first screens were entirely discrete. Separate supplies for blue/green and red. all running from +48Vdc, via a nice little planar smps. Efficiency is a big issue - especially at 25kW or so.

Cheers Terry