Full colour message display from Red, Blue, Green LEDS

[emufambirwi]

I want to embark on a project to design a display system based on a PIC16F877 controlling an array of red, blue green LEDs to produce a picture or text in full colour?

The initial idea I have is to arrange the LEDs as follows
RGBRGBRGBRGBRGBRGB
GBRGBRGBRGBRGBRGBR
BRGBRGBRGBRGBRGBRG
RGBRGBRGBRGBRGBRGB ........etc where R G and B represent Red, Green and Blue LEDs respectively.

The activation of each column is controllrd by a BC557 transistor while the pattern to be displayed comes from the PIC's ports directly.

I would be happy if you could assist in developing the idea so it become a reality.

 

[suicidalmonkey]

I don't have much experience with micro controllers, so I won't comment on their aspect of you idea.

What I will say however, is to consider this idea logically. Each pixel in you display is going to contain 3 LED's. Each LED is 3 - 5 mm wide, therfore each pixel is going to be somewhere 3 x 9 and 5 x 15 mm in size.

This is not going to make the display very clear, unless it is viewed from a reasonable distance (greater than 10 - 15 meters), in which case you would need to make the display huge.

Why not consider making a display 50 x 10 LED's in size, of the one colour. And use it to display text messages and simple paterns.

 

[justDIY]

I want to embark on a project to design a display system based on a PIC16F877 controlling an array of red, blue green LEDs to produce a picture or text in full colour?

sounds like a big project.

to save space, I recommend using RGB leds, so you have all three color chips inside one 3 or 5 mm package (or look at 1210 SMT package rgb's)

wire the LEDs as an anode row cathode column matrix, using common anode LEDs. You can control each color by sinking the cathode side (using a 74HC595 + ULN2803), and using some beefy high side transistors (p-type fet or darlington) on the rows.

You can store your pictures (patterns) and text as bitmaps in an eeprom, and scan them out to the array one row at a time (to keep power dissipation low).

That only gets you a handful of primary colors (8 including black) ... to get more colors, you'll have to do some fancy programming; time domain division, which will pulse the different colors on and off very rapidly to simulate shading. Now you have a full color LED display screen.

Toshiba (as I recall) had a nice whitepaper on methods of building and driving RGB led matricies - I will see if I can find the link for you.

 

[Dan East]

I linked to some of Mouser's RGB LEDs in this thread.

Dan East

 

[panic mode]

how many RGB pixel in total? what is refresh rate?
what is image source and how often image updates(changs)?
is display supposed to operate smoothly during image updates
(no slow down or pousing in display refresh)?

 

[evandude]

I've seen such displays, often using separate LEDs for each color as you intended... however, they are generally very large signs (5 feet tall and 8 feet wide or more) and they only look half decent at a distance... and even then you can clearly see the separation between colors in each pixel.

Whether you use single package RGB LEDs, or use individual LEDs, the cost is bound to be astronomical... RGB LEDs are often a few dollars apiece, and I doubt you could easily find them for less than $1 each. individual red and green LEDs are a dime a dozen, but blue ones can be expensive; and most likely for a project like this you would want the LEDs that are clear in the off state, which are usually more expensive as well. By the time you get an array big enough to display anything interesting, you'd probably be looking at a few thousand dollars worth of LEDs, and needing a whole lot of lines to control them. I think you could do plenty of interesting things with a single-color dot matrix LED display, in which case you could probably be fine with run-of-the-mill red LEDs, which you can get for as little as 1.5 cents apiece quite easily.

 

[justDIY]

or, through some crafty programming, you can achieve a large number of colors using a bipolar LED, perhaps not the most accurate color rendering. red+green leds are very inexpensive as well.

 

[emufambirwi]

I admit this could be a massive project, considering there is massive shortages of these components in this part of world. I am now considering the option of using just the Red LEDs for displaying stuff. But currently my programmer is down and I have to make a new one

 

[justDIY]

here's the link to the PDF i mentioned - great whitepaper on driving LED matrixes for full color applications

https://engineering.dartmouth.edu/~engs031/hansen/demos/Agilent-LEDmatrices.pdf