Evaluating LEDs for ACRIS

I got sidetracked (what else is new) from my audio visualization project and things have been progressing somewhat slowly.  First of all, my oscilloscope has randomly decided to stop displaying channel 2, so I need to fix that at some point so that I can use my spectrum analyzer again.

Anyways, I recently received four RGB LEDs from DealExtreme.  DX sells two types, so I bought two of each to determine which one to buy for my lighting project.  The first one I saw is on a star board and is cheaper than the second, which is enclosed in its own casing.

First, a few pictures:

These pictures were taken while the LEDs were being driven with very little current.  The star emitter is supposedly rated at 350mA for each channel and when each color is driven at this current, the LED is extremely bright.  I initially anticipated needing around 20-25 LEDs for each wall lamp, but I can see that 5-10 will definitely suffice.

I think I know why the star emitter is so cheap.  It’s labeled incorrectly.  The emitter, like most others, is common anode, meaning that the + terminal is common to all three colors.  The markings on the board are indeed wrong.  The enclosed emitter is also common anode.

Here’s a graph of some preliminary tests of driving the diodes at different voltages.  I pushed the enclosed one a lot harder, so I will need to set up a more rigorous test for complete characterization of the diode.  Also, I wasn’t able to heatsink the encased emitter, but stuck the star emitter on a big block of aluminum with some thermal compound.

As is fairly easy to see, the voltage curves look very similar between the two emitter models, except for the green channel.  However, it is also important to note that at 350mA, the first LED did not get anywhere near its rated operating voltage.  I haven’t pushed it far enough to see just how much current is necessary for it to be at is operating voltage, but the diode is certainly bright enough at 350mA.  I don’t want to overly degrade the diode by pushing it too far, or my wall lights will keep breaking.

I’ll probably go with the star emitter because it’s easier to attach a bunch of them to a heatsink block.  I’ll definitely need some kind of diffuser for the lamp, though.

The enclosed diodes are great for custom LED projects where you only need one or two of them; I wouldn’t recommend it for creating a compact array, though.  I might use them in my computer case or something; I could make them change color depending on processor load.

2 Responses to “Evaluating LEDs for ACRIS”


  • Hey Joe, that looks pretty sweet! I also read about your audio visualization project using an FPGA, and it sounds sweet as well. I’ve been working on a very similar project, haha! I’ve never used FPGA’s though, so instead my system architecture involves doing all of the singal processing inside of my computer. I’ll have a separate Arduino board that sends PWM signals to strings of LED’s connected in series, and the Arduino will be commanded based on the signal processing results from the main computer. Right now I have an Arduino on order with a few LED’s (one of which is a diffused RGB LED to mess around with!), and I have some real time FFT code in Java that I’m almost done writing (which explains my current interest in FFT’s). So, no FPGA but a computer instead 🙂 Your architecture sounds really interesting, and I can’t wait to see how it turns out! We’ll have to throw some parties to test out the systems 😛

  • Realtime FFT in Java? Man, I’m impressed; I was worried about doing anything in software because I assumed the latency would be way too high, especially since the data has to be queued out of the system into the Arduino for PWM control. I’m really far behind on the project though; I still haven’t ordered the FPGA yet.

    I just bought a bunch of LED controllers today that take serial input, so I don’t even have to do any PWM now. Controlling common anode LEDs is tricky because you need to use constant current sink drivers. The LED driver I bought was the TLC5926. You might be able to bypass the Arduino step and instead use a serial port on your computer along with these chips to reduce latency if you find that communicating with Arduino is too slow.

    We’ll have to throw some parties to test out the systems 😛

    That sounds like a good idea. 🙂

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