[sdiy] SMD LED intensity and lenses

[rsdio at audiobanshee.com]

PWM is only limited to global brightness if you design the circuit that way. I've designed LED controller circuits with upwards of 180 separate PWM channels with 512 steps of brightness per LED component, in which case the brightness was not global at all.

As for PWM control versus current control, they're both the same to the LED. You can get the same effect either way, visibly. The question is which way is more difficult for your circuit design. There are chips available which automate the current control, and for those solutions you need to provide a current sensing resistor of reasonable accuracy in the current path of the LED. Beyond that, there are multichannel LED driver chips with 16 independent LED outputs and a serial interface that allows register-based direct control of the current for each channel. They don't even require a current sensing resistor in your circuit because it's provided internally to the chip. These chips almost seem like cheating, but they can be just as cheap as CPU PWM solutions.

In contrast, PWM can be as simple as a GPIO port, current limiting resistor, and LED. However, if the number of LEDs is greater than the number of GPIO pins on your CPU, then you end up needing to spend more anyway, making these fancy current-controlled LED driver chips feasible.

Brian Willoughby
Sound Consulting


On May 24, 2017, at 4:56 PM, Mike HEQX <mike at heqx.com> wrote:
> I would agree with pwm John, but if you have matching issues that are related to manufacturing variations or human perception then pwm only controls global brightness even if it is per color. Outside of that you need a physical tweak.
> 
> On 5/24/2017 12:52 PM, John Speth wrote:
>> I think the right way to equalize LED intensity is using PWM.  Controlling it by varying the current will be difficult. - JJS
>> 
>> On 5/24/2017 2:52 AM, Rutger Vlek wrote:
>>> Hi guys/girls,
>>> 
>>> I'm finishing up a new product that will contain a multi-color LED ring, which I plan to do with SMD leds in 4 different colors. In the prototype I had a hard time getting the intensities of the various colors of LEDs to match. I've now learned that not all manufacturers spec according to a comparable norm, so I've decided to stick with Kingbright components that at least have a reference to a uniform norm in their datasheets to make their reported intensities more or less comparable between colors and between leds. I can design in an option to regulate supply current to various colors individually, so technically I can compensate for any differences in intensity between leds.
>>> 
>>> However, now I'm considering which leds to start with and have options for 0603 leds with a dome lens or flat lens. There will be lightpipes in front of the leds to bring light to the front panel, so is there any advantage or disadvantage to the type of lens? And more importantly, will I run into issues if I combine leds with a dome lens from one color (and spec'd brightness) with other types of lenses? I suspect the lightpipes will straighten out a big portion of the leds angle of dispersion, but I'm still hesitant to mix and match, as I feel the resulting brightness at the end of the lightpipe may vary.
>>> 
>>> Any suggestions?