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Changing LED hue from red to green

19 Mar 2015  | Marian Stofka

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Although the sum of currents flowing through the red and green LEDs is approximately one-fourth of the nominal per-LED current, the radiance is high, and you should not look directly at the lid of IC1 when it is on from a distance of less than approximately 1 foot.

IC2, IC3, and IC4 comprise a low-side source of two complementary analogue voltages (Reference 1). The resistive DAC replaces the classic potentiometer in the earlier Design Idea. These complementary analogue voltages are the input voltages for the two power stages comprising transistor Q1 and midrangepower transistor Q2.

The power stage—voltage-to-current converters you make by cascading two bipolar transistors and an op amp—drives each of the two LEDs. The circuit senses output current at resistor RE. The RB resistors eliminate the leakage currents of both bipolar transistors in the cascaded series. These power stages would be functional even with one bipolar transistor instead of two. The cascaded bipolar transistors provide precision in the voltage-to-current converter. With a single power transistor, the relative error would be approximately 1/?, whereas using the cascaded series, the error is approximately 1/(?1?2), where ?1 and ?2, the current gains of the bipolar transistors, are approximately 300 and 100, respectively. The error results from the current flowing through resistor RE, which is the sum of the output current and the base current of transistor Q1.

You can use this circuit in industries ranging from entertainment to toys; it may eventually find use in experimental psychology and in modern fine arts, which involves the use of optoelectronics.

Holding low and feeding a 50% duty cycle, 0.05Hz-frequency logic waveform to the pin produces a slow, periodic, quasicontinuous "waving" of the colour from red to green and back.


Reference
Stofka, Marian, "Amplifiers deliver accurate complementary voltages," EDN, Sept 23, 2010, pg 44.


About the author
Marián Štofka is with Slovak University of Technology in Bratislava, Slovakia.


This article is a Design Idea selected for re-publication by the editors. It was first published on October 20, 2010 in EDN.com.


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