Much has already been written about the creation of high-brightness LEDs and the revolutionary possibilities that are now available for intelligent lighting. The range of colours possible using LED mixing is too great to ignore and too powerful to avoid. With this transition, many lighting designers working with traditional methods are discovering that critical knowledge barriers lie in their path: they can no longer rely on a simplistic current- or voltage-control methodology. Instead, intelligence in the form of microcontrollers or FPGAs is needed for such basic functions as scrolling through colours or adjusting accuracy instantly.
Bridging the gap
Within this transition to the world of microcontrollers lies the knowledge gap. C programming, general microcontroller terminology and such can prove a costly barrier to entry, as many lighting-design houses have turned to expensive consultants to stay in the game. The amount of time necessary to come up to speed on MCU fundamentals can mean the loss of a bid on a national landmark. A microcontroller especially needs to overcome two particular inherent hurdles with LEDs. Temperature degrades the luminous flux of LEDs and modifies the dominant wavelength. To compensate for these changes, engineers essentially have to use a thermistor or temperature sensor and then create some form of a two- or three-dimensional look-up table to accurately adjust the dimming waveform to compensate for a reduction in luminous intensity. The second hurdle is the variance of particular LEDs, even if they are of the same part number. LEDs are binned dependent on their luminous flux, dominant wavelength and forward voltage. The difference of a single dominant wavelength bin can be noticeable to the human eye. Again, multidimensional look-up tables have to be created, since the designer will not know which bin he will receive until the reel arrives at the production floor. To make matters even worse, bin definitions vary from one LED manufacturer to another, so additional software look-up tables have to be created for multiple brands of LEDs. All in all, it becomes obvious that microcontrollers are not only necessary, but a vital component of accurate color mixing. It is also obvious that using a microcontroller can be far more complicated than at first envisioned, ratcheting up the knowledge requirement necessary to complete a fully functional design.
Colour programming
One recently released method for overcoming these LED shortcomings is Cypress' EZ-Color programmable solution using PSoC Express. This embedded-system-design tool eliminates the need for writing code. The tool itself works at the system level instead of at the design level. In other words, it uses specific functions instead of "for" loops. Those functions themselves are real-world devices, such as sensors, thermistors, or LED drivers. EZ-Color and PSoC Express enable a visual embedded system-design approach, eliminating the need to learn colour-mixing science while also removing the need to create multidimensional look-up tables for a thermistor or for binning. Taking a simple example, EZ-Color will turn on three LEDs (red, green, and blue) to create a mixed colour. Using the 1931 CIE chromaticity diagram (Figure 1) we shall create the colour (.1, .12), which should be vaguely purplish.
Figure 1: Choosing a colour and selecting your temperature sensors and thermistors.
First, a designer can look to the built-in catalogue to discover the Lumileds K2 3LED colour-mixing driver. This catalogue is split up into four sections: inputs and outputs are functional drivers (the real-world devices discussed above), while the other two tabs will be discussed below. An input would be a sensor; an output would be a Triple Luxeon colour-mixing driver. The driver name is so specific (not only the LED manufacturer, but also the part number) because it has been regulated to K2's specific functionality. This already eliminates a significant amount of software overhead from a typical colour system. After dragging the driver onto the screen and naming it, a properties window appears that lets designers select the specific bin or temperature sensor to drive their colour-mixing equation. To set the output colour, EZ-Color drivers use what are called "transfer functions". These are variables stored in memory and needed to drive the internal algorithm under the surface. The ColorMix driver requires four inputs, "enable, relative flux, CIE x, and CIE y". To learn more about the driver and these inputs, it is always a good idea to read the driver datasheet. To establish these four inputs, another PSoC Express device is needed: a valuator, which is found in the same catalogue as the inputs/outputs. To understand the principle, think variables, only better. PSoC Express offers either set variables that would relate to "# defines" in normal C, or logic-based variables such as a state machine or a status encoder. In our simplistic example, our four inputs only require standard valuators (Figure 2).
Figure 2: Selecting the variables for the chosen colour.
While building the design, PSoC Express will first offer the EZ-Color part catalogue. Only the parts that can handle the specific code-size and resource requirements of the design you have created will be shown. Once the part is selected, a dynamic pinout is available; the project is now complete. This whole process took less than 10 min. A schematic, individual datasheet and custom firmware are all now ready for review. The schematic itself currently uses the National LM3402 buck converter, but EZ-Color is by no means locked into that choice: any power converter that is able to have a dimming waveform as an input can be used. Additional external devices, such as colour sensors, temperature sensors, accelerometers, tachometers, buttons, or voltage monitors, can be added using the inputs tab in the "catalog" menu. After completing the design, PSoC Express offers a monitor function to make sure it is free of errors. This function includes a graphical tool called the "Tuner", which creates the CIE chart dependent on the LED bins selected in the "properties" menu. PSoC Express can easily handle multiple pixels using the same EZ-Color device.
Beyond simple colour schemes
Additional logic valuators available in the tool can create unique colour-mixing functions, and any variable that is part of the transfer function can be modified by a priority encoder or set-point region. As such, state-machine logic can easily be created to control the "x" and "y" portions of the transfer function, scrolling through the available colour gamut. Additional examples are available on the PSoC Express start page as express designs. These are complete designs with no C necessary.
