If you’re just joining me, I’m trying to redo the LED lighting on this 46′-tall antenna:
My earlier experiments were all about speed, trying to increase the frame rate from last year’s abysmal 17 Hz to something better than 60 Hz. I decided I could easily double the number of LEDs and use single Teensy 3.2 with the OctoWS2811 adapter, and get 72Hz.
Honestly I could stop right now, but I was still interested in doing some testing with APA102c LEDs. These are what Adafruit calls DotStars. The big difference is that they have an extra pin for a clock signal.
Here are some of the pros and cons of the different options for LED strips that you might consider for blinky lights in the desert:
| WS2812b | WS2815 | APA102C |
|---|---|---|
| 5V strips, which need power injection occasionally to avoid discoloration at the end of the strip | Uses 12V for power. (The signal is still 5V). This means you don’t have to inject power as often into the middle of the strip to prevent color loss. | 5V strips, which need power injection occasionally to avoid discoloration at the end of the strip |
| If one LED fails, the rest of the strip will not work. | Provides a “backup path” for data that skips an LED. That means if a single LED fails, the rest of the strip continues to function. Note that the backup feature is not always helpful; a completely break in the strip will still kill your strip. | If one LED fails, the rest of the strip will not work. |
| Slow PWM refresh rate (400Hz); doesn’t look great in video and doesn’t provide persistence-of-vision | Fast refresh rate (2KHz) looks great in video cameras | Even faster refresh rate (4KHz – 20KHz) — provides amazing color dithering at lower brightnesses |
| Slow protocol with 800kHz data rate If you want 60Hz refresh rates you are limited to 550 pixels | Slow protocol with 800kHz data rate If you want 60Hz refresh rates you are limited to 550 pixels | Arbitrarily fast data rate of at least 24MHz You’re probably limited by the speed of your controller and code |
| Only need one conductor for data. (Nice option to use twisted pairs with GND + DATA, so, for example, CAT-6 can carry four lines of data | Only need one conductor for data. (Nice option to use twisted pairs with GND + DATA, so, for example, CAT-6 can carry four lines of data | Requires separate conductors for data and clock. |
| Currently $12/meter/144pixels | Currently $15/meter/144pixels | Currently $19/meter/144 pixels |
By the way, these things change all the time. The manufacturers are kind of unreliable and often update their products without notice. They especially love to flip the color order. I’ve bought the exact same product one month later (to replace a failed strip) and discovered that the new version had Red and Green flipped so I couldn’t really replace the failed strip. Be careful and test things yourself!
Next, I want to test how fast the APA102c really is. Can I really drive thousands of pixels with just with a single pin, no OctoWS2811? And do they really look better? I decided to wire up 4-meter strings of each to a Feather M4 and see how they did.
In terms of frame rate, as expected, I got faster refresh with the APA102. In a test with 576 pixels, I got an update rate of 52Hz with the WS2815 and 167Hz with the APA102.
In terms of how things looked: at full brightness, there was really no difference. But once the brightness got below 128 you started to notice that the APA102 did a much better job of rendering dim colors. In fact at very low brightnesses like 16 the WS2815 had basically dropped out completely but the APA102 was still showing pretty much the same hues as it showed at full brightness.
In conclusion, for bright outdoor applications, there is a lot to like about the WS2815, especially the fact that you can do less power injection and the resilience to single-pixel failures. For more subtle, indoor applications where you might run at lower brightness, APA102 might be preferable. I’m going to stick with the WS2815 for the antenna.