Set the LED color tone from red to green

　　The circuit in Figure 1 can create 32° hues of light using red and green LEDs. A constant current is divided into two parts. One part flows through a red LED and the other part flows through a green LED. The current through the red LED can be changed from 0~%, thereby also changing the complementary green LED current (the sum of the two is 100%). When this happens, the eye receives any hue of light mixed with red and green. Roughly speaking, from red to green, it passes through orange, amber and yellow. It can be set to any of the 32 hues between red and green, such as orange, amber and yellow.

　　IC3 is an Analog Devices AD5228 resistive DAC with a 1-in-32 resolution, which determines the resolution of the circuit. In this application, the resistive DAC functions as a digital potentiometer. The position of its actuator can be manually set by briefly grounding its pull-up and pull-down control terminals. The resistive DAC has no memory, so this setting must be done every time the power is turned on.

　　Keeping the AND pin at logic low, the actuator position will increase or decrease at a rate of one step every 0.25 seconds, so the color of the output light will gradually change (Figure 2). In addition, the LED color displayed at power-on can be preset. In the high preset, the color is 100% red when powered on. In the low preset, it is preset at the midpoint of the resistive DAC, so the color at power-on is 50% red and 50% green, and the color seen is yellow.

　　The circuit uses two LEDs from IC1, a high-performance tri-color ASMT-MT00 LED from Avago Technologies. The blue LED is not used. However, any of the other five red/green, red/blue, blue/red, green/blue, or blue/green combinations can be connected instead of the green/red combination used in this circuit.

　　Although the sum of the currents flowing through the red and green LEDs is close to one-quarter of the nominal current of each LED, it is still bright enough that the light emitted by IC1 should not be viewed directly with the naked eye at a distance of less than 1 foot.

　　IC2, IC3, and IC4 form a low-side source of dual complementary analog voltages (Reference 1). The resistive DAC replaces the traditional potentiometer in the previous design example. These complementary analog voltages are the input voltages to the two power stages formed by transistor Q1 and mid-power transistor Q2.

　　The power stage (a voltage-to-current converter consisting of two bipolar transistors in cascade and an op amp) drives two LEDs. The circuit senses the output current across resistor RE. Resistor RB eliminates leakage current from the two bipolar transistors in the cascade. These power stages work even with only one bipolar transistor instead of two. The cascaded bipolar transistors provide precision for the voltage-to-current converter. With a single power transistor, the associated error is about 1/β, while when cascaded, the error is about 1/(β1β2), where β1 and β2 represent the current gains of the bipolar transistors, about 300 and 100, respectively. The error comes from the current through resistor RE, which is the sum of the output current and the base current of transistor Q1.

　　Keeping it low, feeding a 50% duty cycle, 0.05 Hz frequency logic waveform into the foot can produce a slow, periodic, quasi-continuous color "wave" from red to green to black.