[sdiy] SMD LED intensity and lenses

[rsdio at audiobanshee.com]

On May 25, 2017, at 12:49 AM, Rutger Vlek <rutgervlek at gmail.com> wrote:
> @John: yes, PWM would have been nice, but this is an entirely analog design, without a microcontroller.

PWM can be accomplished with an entirely analog circuit. You merely need an oscillator that can generate a square wave that has a variable pulse width. This can be accomplished with a sawtooth oscillator core and a comparator. The simplest design would not necessarily allow Voltage Control of the frequency, but that's okay because PWM generally has a fixed frequency anyway. The Pulse Width could easily be Voltage Controlled by simply introducing the CV to the comparator input. An implementation like this should be highly efficient, with very little Wattage lost in the analog circuits compared to straight linear control of brightness via current (unless the current control was switch mode). Of course, you might want to use discrete transistors to drive the LEDs rather than op-amp outputs.


> @Mike: thanks. Initially, I matched several colors of LEDs together for the LED ring that had similarly spec'ed intensities (in lumen). However, upon testing I found these values weren't comparable at all. Then I discovered Kingbright spec two values, one according to some weird magical standard, and one according to an ISO norm. The latter gave me hope intensities would become comparable, and now I'm deciding on which LEDs to order. I have a subset of options that include both dome and flat lenses, and I'm wondering if it is a problem to get a mixture of these into the led ring. I'll install a current limiting resistor per color in the led ring (consisting of roughly 5 green, 4 orange, 3 red, 2 blue LEDs), so I can later even out the brightnesses between colors by adjusting the resistor.

You should keep in mind that intensity (in lumens) is not the only manufacturing variable. The wavelength in nanometers is also a variable, and that affects the color as well. LED manufacturers use "bins" to categorize how well each individual LED falls into specified ranges. Within a bin, the intensity and wavelength can only vary a little bit. This is true of discrete, single-color LEDs as well as RGB LEDs.

However, it costs a lot of money to purchase a set of LEDs that all fall within the same bin, because this requires serious testing. When you buy a bag of LEDs through a reseller like Mouser or Digi-Key, they usually are not binned. That means you can get widely ranging values for intensity and wavelength. As far as I know, you need to buy LEDs in bulk, directly from the manufacturer, in order to get them to be closely matched. This also costs quite a bit more than the discounted prices you can find for bulk LED parts that have not been binned.

This information is second hand, because I was working on the circuitry and firmware for a system with several hundred RGB and monochrome LEDs. When the first prototypes came in, we had widely varying color matching, and product management was not satisfied. Once our parts procurement people starting shopping for matched LEDs, we found that the price was significantly higher for matched LEDs. It took a great deal of effort to find affordable LEDs that were binned, and therefore likely to match. We even had to come up with a way of marking LED boards with lot numbers, so that the repair facility would not replace a board from one binned lot with a board from another lot - otherwise the color hue would show an obvious change from one side of the product to the other (there were six PCBs with LEDs on each of them).

The alternative to buying pre-tested LEDs in bins is to do the testing yourself and then adjust a component like a resistor in order to match individual LEDs. You'd need a lot of different resistor values on hand to get good matching. Given the process, it would be very "costly" in terms of process to tweak resistor values for every individual LED. It's far easier to pay someone else to match the LEDs and then use the same, precision resistor value for all LEDs.

Brian Willoughby
Sound Consulting