Testing my 8mm diffused RGB LEDS…
For my 8X8X8 RGB LED cube, I needed 512 LEDs and I purchased 600, plenty for the build and some spares.
I started to wonder about the probability of brightness variations between the LEDs and decided to find a way to individually test those 600 RGB LEDS and ideally to pick 512 (and some spares…) closely matched for brightness levels in all 3 colors. I wanted to do this before I soldered them into that cube as I know that any repairs/replacements would be a serious pain !!
Following Jon Burroughs suggestion, I purchased a ‘RGB Color Sensor with IR filter - TCS34725’ from Adafruit and put together a jig made of some ABS pipe and couplings to create a mini dark room. I downloaded the Adafruit’s TCS34725 library and put together a test sketch that drives the LED under test through brightness values 0,1,2,4,8,16,32,64,128, 255 for colors RED, GREEN, BLUE and WHITE. This actually tests that each brightness bit is working properly in the LED’s PWM internal circuits. For each step in brightness, the sensor measures the light intensity of all 3 RGB colors and White color. I actually push the data to 2 devices to also verify the data out of the LED under test and that also gives me a visual of the test in progress.
That ends up being a huge set of collected data for a single complete pass through my test. 10 brightness steps X 4 colors X 4 measurements each = 160 measurements per LED. Actually for each of these 160 measurements, I ended up creating a table with a minimum and a maximum value for each measurement. I updated these values as I ran more LEDs through the tests as I did find variations between LEDs.
Now due to the LED’s PWM control of brightness and the sensor’s data integration times I had to make the test long enough that I did not get too much variation in the measurements because of when the sensor was measuring within the LED’s PWM cycle !
That ended up being an excellent GO-NOGO test for the LEDs and although I did find variations of the measured values between the LEDs, when I visually compared LEDs that were at the minium range for a given value and another that was at the maximum, I could not really see a difference at all so I gave up that idea.
Here’s a few pictures of that test jig…