Create secondary colors from multicolored LEDs

-July 24, 2003

It is well-known that simultaneously mixing two primary-color light sources, such as red and green, creates a secondary color, such as yellow. This mixing process commonly occurs in tricolor LEDs. One disadvantage of this method of generating a yellow color is that the LED must use twice the current because both the red and the green LEDs must be on. In battery-powered circuits, the LED indicator's operating current may be a significant fraction of the supply current, so using the same current to generate both primary and secondary colors is advantageous. The operating-current savings may be significant in telecom-line-card applications involving thousands of line cards or large-panel RGB LED displays. This Design Idea proposes a sequencing method to generate balanced secondary colors from bicolor, tricolor, and RGB LEDs, using only one LED's operating current. Advantages include lower power dissipation and more uniform intensities between primary and secondary colors. Using the sequencing method also allows a bicolor LED to now produce three colors and keep a simpler pc-board layout using two rather than three pins. In addition, you can also produce white light with RGB LEDs using the sequencing method.

The method uses the property of images to persist in the human eye for several tens of milliseconds. If different primary colors flash sequentially and quickly enough from one point, humans see them as overlapping in time, and the brain interprets them to appear as secondary colors or even white, depending on the color components. Experimentation with two or three primary-color LEDs shows that the flash sequence must complete within approximately 25 msec or less to produce a solid secondary color or white light. In testing for an upper limit, you can use flash rates to 1 MHz to produce this effect without degrading secondary colors. Thus, you can use any convenient clock source higher than 40 Hz to create secondary colors. Note that the primary-color LEDs must be physically close together, such as on a semiconductor chip, for the eye to properly mix the light. Diffused lenses also allow a wider viewing angle. These combinations are commercially available as bicolor, tricolor, and RGB LEDs.

Figure 1 shows the various LED-circuit configurations, and Figure 2 shows the timing to generate all three colors from bicolor and tricolor LEDs, although using only one LED's operating current. Note that the driver for the bicolor LED must be able to sink and source current. You may have to provide color balance between the primary-color LEDs to ensure that the secondary colors appear properly. The LEDs have different efficiencies and intensities as the human eye sees them, and these parameters need correcting. For tricolor LEDs in a common-anode or -cathode configuration and 50% duty cycle, the correction is easy to effect by adjusting the current-limiting resistors. Alternatively, you can use one current-limiting resistor and then vary the duty cycle to provide the necessary color balance. For two-leaded, bicolor LEDs, it is easier to adjust the duty cycle to produce the correct secondary color than to use additional circuitry. The waveforms at the bottom of Figure 2 illustrate duty-cycle control to achieve secondary-color balance for both bicolor and tricolor LEDs.

Using a sequenced bicolor LED to generate three colors has packaging advantages, particularly when you vertically stack several LEDs. Previously, stacked, tricolor LEDs needed to use a through-hole assembly, because the middle lead would be inaccessible if the devices were surface-mounted. Because the bicolor LED has only two pins, you can vertically stack several of them and bend out the leads for surface mounting. The generation of secondary colors can also extend to RGB LEDs (Table 1). You can achieve color balancing by adjusting the current-limiting resistors or the duty cycle. You can program three pins from a microcontroller's port to sequence through the various primary-color combinations.

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