20/8/2015 Experimental Hardware: Playing with LED Matrices

http://xp­ware.blogspot.com/2010/07/playing­with­led­matrices.html 1/2

Experimenting and prototyping with Arduino

Experimental Hardware

Friday, July 2, 2010

Playing with LED MatricesAfter completing all the ARDX experiments, Ialready had in mind what I would use for myfirst real project... a dot matrix clock using 8x8LED matrices!

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

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

Controlling one 8x8 LED matrix required a lot of wires, 8 row and 8 column wires. But the ARDX kitcomes with a 74HC595 shift register. Using this shift register, the number of required wires reducea 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 staysthe same... The the shift registers pass on the data (on/off values of LEDs) and the row wirespower the complete row of all LED matrices.

The ShiftOut tutorial provides all the details needed to wire everything up and even some codesamples to get you started. Probably in some future post, I'll unveil my code and explain the finedetails 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 Ibought a prototype circuit board, and I'll solder everything together. For now I've put 2 LEDmatrices 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 youcarefully 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 matrixspecs 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

arduino (6)

ardx (2)

clock (2)

coding (1)

data logger (1)

LCD (1)

LED matrix (2)

performance (1)

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20/8/2015 Experimental Hardware: Playing with LED Matrices

http://xp­ware.blogspot.com/2010/07/playing­with­led­matrices.html 2/2

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Posted by Jasper at 2:31 AM

Tagged with: arduino, clock, LED matrix

and they are lit 1/8th of the time. Having 1 LED lit in a row, issomething different from having approximately 10 lit onaverage. But after going through the calculations, a small 36Ohm resistor would be required. Not having such a resistorand having more than one LED lit at a time, made me omit theresistors. The LEDs now shine nice and bright... ;­) but not sobright that they might burn. (Few days later I've bought 30Ohm resistors, and adding them does almost not change thebrightness. Leaving them out should not be a problem, as longas more than a few LEDs per row are lit.)

Tomorrow I'll start soldering everything together on aprototype circuit board. I guess it will take some time to solderall those pieces...

After that I'll add an alarm and programming buttons, DCF­77,some nice case with touch snooze switch?

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