Wednesday, October 27, 2021

Bi-Colour LED bars.

I am evaluating bi-colour LEDs that are RED/GRN, and can be mixed to ORANGE/YELLOW. The two candidates are SunLED GMDKVGX10D and KingBright DC10EGWA.

I've found that the SunLED has a bright GREEN with an even brighter RED. You need to drive the green channel with at least double the current of the red, to match them. I find that 1kΩ and 2kΩ work for my application.

The KingBright, on the other hand, doesn't show much difference between the RED and GRN brightness. But both are far less bright than the SunLED. Even though the KingBright has the term "High Efficiency" in the data sheet? When I limit both RED and GRN with 470Ω I find it is still quite a bit dimmer than the SunLED at 1kΩ/2kΩ.

What both models do the same: they have the GRN/RED LEDs share their cathode, not their anode. A common-anode design would have been so much more convenient. You would have been able to drive it with a constant current LED driver that is a current sink. All constant current LED drivers that I could find are current sinks, so none of them can be used with these LEDs. So not Texas Instruments TLC591x, nor TLC592x and neither a Maxim6966 would work. I consider this a strange design decision.

Another feature I miss, is an 8-segment version of these bar-graphs. It's just more convenient to work with 8-bit quantities. Or in the case of bi-colour bar-graphs: 16-bit would be preferable over 10-segment 20-bit units. Monochromatic, and fixed-colour bar-graphs come in more varied sizes, like 8-segment, 10-segment and even 12-segments sometimes.

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