Playing with LED Matrices

After completing all the ARDX experiments, I already had in mind what I would use for my first real project... a dot matrix clock using 8x8 LED matrices!

A LED matrix is a fancy piece of hardware having 64 LEDs (in my case 128, 64 red and 64 green leds), which have not as many pins. The idea is that with multiplexing, you can light one row at a time, and keep looping through all rows, so it looks all LEDs are lit, but actually are lit 1/8th of the time. (For some reason it seems more logical to me to loop through the 8 rows, in stead of the columns. Especially when you want to build a LED matrix showing the time or a nice message using 8 rows and many many columns.)

With a bit of research, I found a webshop called Sure Electronics offering 10 pieces 8x8 bicolor LED matrices. (With a bit of shipping costs, still very cheap. Actually I found their ebay webshop and ordered there.)

Controlling one 8x8 LED matrix required a lot of wires, 8 row and 8 column wires. But the ARDX kit comes with a 74HC595 shift register. Using this shift register, the number of required wires reduce a bit. We'd have: ground, vcc, data, clock, latch and 8 column wires. That is still 13 wires...

But when using multiple shift registers to power the columns, the number of required wires stays the same... The the shift registers pass on the data (on/off values of LEDs) and the row wires power the complete row of all LED matrices.

The ShiftOut tutorial provides all the details needed to wire everything up and even some code samples to get you started. Probably in some future post, I'll unveil my code and explain the fine details of controlling multiple shift registers for multiple LED matrices.

My project is a simple clock, that would require 32 colums, four 8x8 LED matrices, to display "HH:MM" in a 5x8 font. That requires quite a lot wires to put on a breadbord, or even two. So I bought a prototype circuit board, and I'll solder everything together. For now I've put 2 LED matrices on my breadboards, and I'm displaying only the hours.

At first I used a small resistor for every rows, adding to the number of connections, but if you carefully calculate every aspect of your setup, it might just work without the resistors... Tinkerlog.com thoroughly explained LEDs and why the would require a resistor. My LED matrix specs have the following relevant numbers:

• Max current: 20mA
• Max puls current: 100mA (pulse <= 10ms and duty < 1/10)

So trying to follow the calculations, I must admit, I lost it a bit... In my case, not all LEDs are lit and they are lit 1/8th of the time. Having 1 LED lit in a row, is something different from having approximately 10 lit on average. But after going through the calculations, a small 36 Ohm resistor would be required. Not having such a resistor and having more than one LED lit at a time, made me omit the resistors. The LEDs now shine nice and bright... ;-) but not so bright that they might burn. (Few days later I've bought 30 Ohm resistors, and adding them does almost not change the brightness. Leaving them out should not be a problem, as long as more than a few LEDs per row are lit.)

Tomorrow I'll start soldering everything together on a prototype circuit board. I guess it will take some time to solder all those pieces...

After that I'll add an alarm and programming buttons, DCF-77, some nice case with touch snooze switch?