A friend of mine recently asked about making a small LED sign for his business that can scroll text. It seems that buying such signs is pretty expensive. To give him a little demonstration of how it would work I made a small 3x3 matrix of LEDs using a 74HC595 shift register. Using multiplexing it is possible to run a 4x4 matrix with only one shift register but I didn't have that many matching loose LEDs on hand. I was able to draw some simple patterns using an MSP430 and even do some PWM to dim the LEDs.
One thing that surprised me was that the cheap red LEDs I used burned out almost instantly when hooked up to the supply rail without a resistor. Running an LED without any kind of current limiting is of course a bad idea but it was a surprise that these failed so quickly. Others I have experimented with worked without any noticeable damage for a few seconds when hooked up to the
Also, although these LEDs appear to be identical and were bought at the same time from the same store, one of them seems to glow at least twice as bright as the others. Some LEDs I have used seem to glow a little dimmer than others from the same batch but I've never seen one like this that glows so much brighter. I'm not sure what causes this.
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