Chapter 4, Problem 73E

The LED circuit in Fig. 4.89 is used to mix colors to achieve any desired color in the RGB color palette. Use LTspice and the circuit model representing LEDs as voltage sources to see how changing the resistance R₁ from 100 to 1 kΩ affects the color, with the other resistors fixed at R₂ = R₃ = 100 Ω. You can do this using a parameter sweep statement by defining a variable such as {rvariable} (including the curly brackets) in the value for R₁. Then include a SPICE directive such as .step param rvariable 100 1k 20 to step the variable from 100 to 1000 in steps of 20. (a) Plot the current of all three LEDs as a function of R₁ and explain the result. (b) Find an RGB color chart and describe how the color changes with R₁, increasing from 100 to 1 kΩ. (c) Find a value of R₁ that could be used to achieve a khaki color approximating RGB hex code C2BD23, RGB (194,189,35).

FIGURE 4.89