The circuit in Figure 1 can use red and green LEDs to create a 32° tone of light. A constant current is divided into two parts. One part flows through one red LED and the other part flows through a green LED. The current through the red LED can be varied from 0 to %, which also changes the complementary green LED current (the sum of the two is 100%). When this happens, the eye receives any shade of light that is mixed with red and green. Roughly speaking, orange, amber and yellow are passed from red to green. You can set any of the 32 shades between red and green, such as orange, amber, and yellow.
IC3 is Analog Devices' AD5228 resistive DAC with a resolution of 32 out of 1 to determine the resolution of the circuit. In this application, the function of the resistive DAC is a digital potentiometer. The position of the rotor can be manually set by grounding its pull-up and pull-down control terminals for a short time. Resistive DACs do not have a memory, so this setting must be made after each power-up.
Keeping the foot and logic low, the position of the moving piece will be incremented or decremented by one step every 0.25 seconds, so the color of the output light will gradually change (Figure 2). In addition, it is also possible to preset the color tone of the LED displayed at power-on. At high presets, the color is 100% red when powered up. At low presets, it is preset at the midpoint of the resistive DAC, so the color at power-up is 50% red and 50% green, and yellow is seen.
The circuit uses two LEDs in IC1, which is Avago Technologies' high-performance tri-color ASMT-MT00 LED. Blue LEDs are not used. However, it is also possible to connect any of the other five red/green, red/blue, blue/red, green/blue, or blue/green combinations 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 a quarter of the nominal current of each LED, there is still a high brightness, so the illumination of 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 a dual complementary analog voltage (Reference 1). The resistive DAC replaces the traditional potentiometer in the previous design example. These complementary analog voltages are the input voltages of the two power stages, which are composed of transistor Q1 and medium power transistor Q2.
The power stage (a voltage-to-current converter consisting of two bipolar transistor cascades and one op amp) drives two LEDs. The circuit senses the output current on the resistor RE. The resistor RB eliminates the leakage current of the two bipolar transistors in the cascade. These power stages work even with only one bipolar transistor (rather than two). Cascaded bipolar transistors provide precision for voltage-to-current converters. With a single power transistor, the correlation error is about 1/β, and when using cascading mode, the error is about 1/(β1β2), where β1 and β2 represent the current gain of the bipolar transistor, which are 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.
It will remain low, feeding a 50% duty cycle, 0.05 Hz frequency logic waveform to generate a slow, periodic quasi-continuous color "wave" from red to green to black.
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