Use two Texas Instruments TLC5940s! The chips allow you to control the intensity of 16 LEDs per chip. Theres a library for controlling them but if you get stuck, just ask.

You won't need the resistors on your LEDs as the 5940 takes care of limiting the current for you. All you need to limit the current to 20mA is one 2k resistor between the IRef and GND pins on the 5940. Also, it doesn't matter what voltage you drive your LEDs at.

Hi

This is my first post to this forum, and the first time I've done anything with the Arduino. I have a little experience with electronics, having built a few projects using the Parallax Propellor.

I have built a DMX receiver using a Propellor chip and I was going to use this to drive several TLC5940 LED drivers. The idea was to have the Propellor in a 'hub' connected to several 'tiles', each containing a TLC chip to drive 16 LEDs. Unfortunately while the system worked perfectly when joined with a very short cable, as soon as I used a longer cable all kinds of interference was introduced which made the system unusable. It was suggested on the Parallax forum that I use line drivers to boost the signal level so it would work over long cable distances, but the chips worked out to be quite expensive.

While Googling for ideas I came across the TLC5940 project on this website. After a little research I was shocked to discover that one Atmel CPU and associated parts would cost less than the 5-way serial line-driver setup I was considering using! Now I want to build my tiles with an Atmel cpu and a TLC chip, and use a serial interface to control each LEDs brightness. This will allow me to use anything that can generate a serial string to control the tiles.

I want to use RS485-differential signalling to talk to my tiles as this allows very long cables (its used for DMX as well). I only need to send data to them, not receive anything back again. I am currently in the process of breadboarding my first Arduino using the schematics and instructions from this site. The schematics use a 7404N hex inverter chip in the serial interface. My question is this : will I need the 7404N chip when I implement the RS485 interface, or can I just connect an RS485 chip such as the MAX487 directly to the processor?

I don't think that custom software would necessarily give you more control. Have a look at the MagicQ software. I think you will be surprised at just how powerful it is.

I only have 10 rgb leds at the moment. Work gets in the way of me playing! If I were to make a low-res video screen, it would be very low-res. Each led 'pixel' would need 3 dmx channels so one 512 channel dmx universe would only give me 170 pixels. For your application, you'd need 12 universes! MagicQ can handle this many universes, but you'd need some expensive Artnet to DMX interfaces.

The data rate shouldn't be a problem though. For a full 512 channel dmx datastream, you'd get at most 44 updates per second. Having more than one universe won't be a problem as they get sent in parallel.

I would stick with DMX if I were you. I've built a system using an Arduino, two TLC5940 LED driver chips, and 10 Piranha RGB LEDs. They use DMX and I am using MagicQ lighting software on my PC to control them. The software allows me to have complete control over the color of each LED so that if I had enough of them, I could build a low-resolution LED video screen. The software allows me to create a grid of 'pixels' and to have scrolling text or moving images as well as color effects. It is well worth a look.

I get all my bits from uk.farnell.com. Free postage on orders over £20, and its on your doorstep the next day (the next morning usually)

If you use hyperterminal then it is easy to save any incoming serial data. Just connect to your Arduino, and then go to the 'Transfer->Capture Text' menu option. Select a file to save your data and away you go! All incoming serial data will then be automatically saved for you.

You could always buy the bits from Farnell and breadboard your own Arduino while you experiment! Thats what I'm doing at the moment.