Multicolored LEDs Create Secondary Colors

Any student of physics knows mixing two primary color light sources produces a secondary color. For instance, mixing the primary colors red and green creates the secondary color yellow. There are three primary colors—Red, Blue, and Green. This process is easily seen in tricolor and RGB LEDs.

There is a disadvantage in this method. As two primary colors are necessary for generating a secondary color, two LEDs must remain turned on at the same time. Therefore, generating a secondary color means consuming twice the current a primary color requires. In battery powered circuits, the operating current of the LED indicator may be a significant fraction of the total current, and using the same current for generating both primary and secondary colors would be an advantage.

Using a sequencing method can generate balanced secondary colors from RGB, tricolor and bicolor LEDs, while using the operating current of a single LED. The sequencing method offers uniform intensities between the primary and secondary colors, and lower power dissipation. An added advantage of using the sequencing method with bicolor LEDs is keeping a simple pc-board layout with two pins while it produces three colors. Using the sequence method with RGB LEDs produces white light while consuming the operating current of a single LED.

The sequencing method works because it takes advantage of a property of the human eye. This is called persistence of vision, wherein images in the human eye persist for about sixty milliseconds after light from the object ceases to enter the eye. For instance, when a glowing coal is moved about in the dark, the eye sees a continuous red line.

When the human eye sees different primary colors flashed sequentially and quickly from one point, they appear to overlap in time, while the brain interprets the colors to be secondary colors, or, depending on the color components, even white.

Experiments with multiple primary-colored LEDs show that the above flash sequence should repeat every 25 milliseconds or lower, for the eye to treat the effect as a solid secondary color. In fact, the flash rate can go down to one microsecond, before the human eye can detect the degradation of the secondary color. Therefore, any clock source, say a convenient 40 Hz, should be adequate for creating secondary colors.

For the eye to properly see the mixed colors, the primary-color LEDs must be physically very close together, such as on a semiconductor chip. As an added advantage, diffused lenses are better, as this offers a wider viewing angle.

When using bicolor LEDs, the driver has to be bidirectional, as the LEDs are placed back-to-back in the chip. Moreover, currents for the three LEDs may have to be adjusted to achieve color balance between the primary and secondary colors. In addition, color balancing may be required also as LEDs have different intensities and efficiencies as the human eye sees them.

This correction can be done in one of two ways. As each LED has a current limiting resistor in series, the value of these resistors may be tweaked to achieve the necessary differentiation in individual currents. The other option is to keep the same current but tweak the duty cycle.