(You can use this fact to distinguish between multiple connected Arduinos and have them do different things. For instance, I have one Arduino that is “/dev/tty.usbserial-A4001lZf” and another Arduino that is “/dev/tty.usbserial-A7006xIM”)

When controlling LEDs there are usually two things you have to worry about:
- power draw — how much current needed for your LED array
- addressing — how to control a specific LED in your array

By saying external power source, you get around the power draw issues. An Arduino on a USB port can supply about 500mA of current. A standard LED draws max about 20mA of current. So you can have 500/20 = 25 LEDs off a single USB-powered Arduino. Of course, the Arduino chip itself can’t supply enough power directly to do all 25, you’d need some external driver transistors to help out. Or use LED driver chips.

As for addressing, an Arduino has 13 digital I/O pins. If you set those up as a matrix of 6 pins by 7 pins, you could address 42 LEDs with just the Arduino. But since you include LED driver chips and shift registers in the mix, those chips can do all the hard work and you just talk to them via multi-node serial protocols like I2C or SPI. There are LED driver chips which can address 24 LEDs at a time and are meant to be bussed together in a long string. So you could have maybe 100 of these chips and be able to have 240 individually-addressable LEDs.

But now you have to start worrying about logical addressing. If you think about what an Arduino program to control a few hundred LEDs would look like, you’d probably have a big array in memory, with each array byte is the brightness value of one LED. (many call it a “framebuffer” after what video cards do) A standard Arduino has 1024 bytes of RAM, of which about 256 bytes of this is used by various libraries like Serial. Your program’s non-framebuffer variables take up some of that too. So let’s just say that you have 512 bytes of RAM free for your framebuffer. That means 512 LEDs is your maximum.

Of course, if you don’t need to remember the value for each and every LED (like if you’re doing algorithmic stuff where a “next” LED value can be computed by a small number of “previous” LED values), then your memory requirements can be much smaller than one-byte-per-LED.

If you’re asking your question for more then theoretical curiosity, like say for a particular application, it might be better to say what you’re trying to do. Like most things in technology, there are many options, so the problem needs to be defined better so a solution can be properly fitted to it.

I’ve been playing around with MAX7219’s and an Arduino Duemillanove, I’m wondering if you can tell me the theoretical limit of how many non-rgb Leds the arduino can control individually (with an external power source if need be).

Thanks
Mik

This time you were right – the XBee adapter from Adafruit was easy to make and get working. There was really no need to stack an XBee shield on a motor shield. We used 4 wires from the XBee adapter right to the motor shield. We also used Limor’s tip to use the analog ports for digital ports 14-19 which worked out nice. We posted some details on http://sports4nerds.blogspot.com/.

Ps – now we have new problems – the h-bridge on the motor shield is not providing full power to our DC motors and overheating. We are going to try stacking another h-bridge or find another that can handle more amps.

Thanks again for your help.
Jules

What you should be able to do, but it’s not as compact, is to use Adafruit’s Xbee Adapter Kit plugged into a solderless breadboard. See the bottom of the XBee adapter Arduino page for how to hook it up to an Arduino. (be sure to solder in straight headers instead of the right-angle ones)

If you’re concerned about how to connect multiple shields to an Arduino, the current standard practice is to stack Arduino shields on top of one another. Many shields come with “stacking headers” to enable this. Or you can get a stacking header kit from Adafruit or a stacking header kit from NKC Electronics. NKC also carries Xbee shield kits, in case you want to save some money on Xbee and like to solder.