Color Mixing with Red, Green, Blue LEDs

Posted 2024-06-10 08:34:50

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Abstract

This is a good project for someone who is interested in both electronics and color vision. The equipment needed is on the expensive side, but if you continue studying electronics, you can use it again and again.

Objective

The goal of this project is to learn some basic principles of color perception by experimenting with various combinations of colored lights. For controlled light sources, you will build an electronic circuit to control the current for three separate #LEDs: one red, one green, and one blue.

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Introduction

An LED (light-emitting diode) is a special kind of diode that produces light (see Figure 1).

When current flows through the diode in the forward direction, some of the current is converted into light of a specific color (i.e., wavelength). The color of the light depends on the material from which the semiconductor is made. LEDs are available in many different colors.

As the current through the LED increases, the brightness also increases. Typically, the recommended current for an LED is 20 mA or less. Above this value, the lifetime of the LED will be decreased significantly. Far above this value, the LED will fail catastrophically, like a flashbulb.

To keep the LED current at a reasonable level, LEDs are typically connected in series with a current-limiting resistor, as shown in Figure 2.

The voltage drop across an LED is about 2 V (except for blue or white LEDs, where the voltage drop is about 4 V). In the circuit in Figure 2, the voltage drop across the resistor will be 9 − 2 = 7 V. Using Ohm’s law, the current, I, through the resistor will be V/R = 7 V/1kΩ = 7 mA.

Figure 3 shows you how to use Ohm’s Law to calculate what size resistor you need to limit the current through the LED to the desired value. The voltage drop across the resistor will equal the supply voltage minus the voltage drop across the LED (or, VS − VL). You can then use Ohm’s Law to calculate the resistance, R, needed to produce a desired current, I:

So, if the supply voltage is +7 V, what resistor would you need for a 15 mA current? R = (7 − 2)/0.015 A = 333Ω. The closest standard resistor value would be 330 Ω. For more details, and a set of online calculators, see the LED references in the Bibliography section (Hewes, 2006; Ngineering, 2003).

For this project, we’d like to vary the output of each of three different LEDs over a continuous range. Obviously, it would be a major inconvenience if we had to keep changing the current-limiting resistor for each LED! The circuit shown in Figure 4 is a different approach to solving our problem.

The large triangle marked “LF411” in the center of the schematic is an operational amplifier, or op-amp, for short. An op-amp has two inputs and one output. (It also has positive and negative supply inputs, typically +/− 15 V, which are usually not shown in schematics.) In the schematic, the first input (marked +, and called the non-inverting input) is connected to a potentiometer. The second input (marked −, and called the inverting input) is connected to two circuit elements. It connects to the cathode of an LED, and to a 330 Ω resistor that connects to ground. The output of the op-amp is connected to the anode of that same LED. So the output of the op-amp is fed back to the inverting input, after first passing through the LED. Op-amps are almost always used with some type of feedback.

What does the op-amp do? For a good basic understanding of how op-amps work in circuits, you just need to understand two simple rules (Horowitz and Hill, 1989, 177):

“The output attempts to do whatever is necessary to make the voltage difference between the two inputs zero.

“The inputs draw no current.”

These rules are simplifications—for example, the inputs do draw some current. For the LF411 op-amp used in this project, 0.2 nA. You won’t be able to measure that with your DMM! So these op-amp rules, as the authors state, are “good enough for almost everything you’ll ever do,” (Horowitz and Hill, 1989, 177).

What do the op-amp rules mean for our LED control circuit? Let’s examine the inputs to the op-amp to figure it out. The potentiometer connected to the non-inverting input provides a voltage, ranging between 0 and +5 V, depending on how far the knob is turned. Simple enough. OK, what about the inverting input? This is the feedback connection from the output. So, by rule 1, the op-amp output should do whatever is necessary to make the voltage at the inverting input match the voltage we set with the potentiometer.

Let’s consider three examples:

with the potentiometer knob all the way down (0 V),

half-way up (+2.5 V), and

all the way up (+5 V).

Example 1 is the easiest. With 0 V at the non-inverting input, there should be 0 V at the inverting input (rule 1), so the op-amp does nothing. No current flows through the LED.

With the potentiometer half-way up, the non-inverting input of the op-amp sees +2.5 V. How does the op-amp match that at the inverting input? It does it by passing current through the LED. How much current? Enough current so that the voltage at the inverting input equals +2.5 V. Where does the current go? The current flows to ground through the 330 Ω resistor (by rule 2, we know that the inputs draw no current, so the resistor is the only path to ground). By Ohm’s Law, we have I = V/R = 2.5 V/330 Ω = 7.6 mA. So with the potentiometer half-way up, the op-amp output will provide 7.6 mA of current to the LED.

With the potentiometer all the way up, there is now +5 V at the non-inverting input. By the same analysis, we conclude that the current through the LED is now 15.2 mA. Perfect! The circuit has just the right range for a typical LED. The LF411 can supply 15 mA without a problem, so the behavior we expect from our op-amp rules is exactly the behavior that we get.

You can see from Ohm’s Law that the LED current increases in direct proportion to the voltage from the potentiometer. Do some more calculations for more potentiometer settings to convince yourself that this is true. Then build the circuit and see how it actually performs. You’ll be able to control the LED with good precision and repeatability. What colors will you be able to produce from red, green, and blue?

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

Ohm’s law

LED (light emitting diode)

Integrated circuit

Operational amplifier (op-amp)

Photoreceptor

Rods

Cones

Wavelength

Spectrum

Perception

Questions

If you wanted the output current through the LED to max out at about 10 mA, what resistor would you use in place of the 330 Ω resistor in the LED control schematic?

The voltage drop across an LED is about 2 V for red and green, and about 4 V for blue. What voltage would you expect to measure at the LF411 output (pin 6) when the potentiometer is half-way up (+2.5 V)? When the potentiometer is all the way up (+5 V)?

What combination of intensities from red, green, and blue LEDs makes yellow light? Orange light? Purple light? Magenta light? White light?

Materials and Equipment

The following electronic parts are available from Jameco Electronics:

Powered breadboard, part #1942163

Jumper wire kit, part #19290

Digital multimeter. See our multimeter tutorial if you do not know how to use a multimeter.

LF411Cn operational amplifier (6), part #23018

High-output red LED (2), part #333526

High-output green LED (2), part #334473

High-output blue LED (2), part #2210896

Linear taper 10 kΩ potentiometer (3), part #29082

330 Ω resistor (3 required, minimum order quanity is 10), part #690742

Red knob, part #265051

Green knob, part #265084

Blue knob, part #265077

Red #22AWG solid hook-up wire, part #36856

Black #22AWG solid hook-up wire, part #36792

Heat shrink tubing, part #71871

Notes on electronics parts:

The powered breadboard is expensive, but well worth it if you plan on doing more electronics experiments in the future. You could also check with a local university to see if you can get access to an electronics lab with a powered breadboard to do your project.

The LF411CN IC is a static-sensitive part. Only three are used in the project, but six are specified so that you’ll have spares in case of a mishap.

One extra of each color LED is specified.

You may prefer to use audio taper potentiometers instead, see the Variations section for a suitable Jameco part number.

Tools:

Power drill

5/16″ and 1/8″ drill bits

Center punch (hammer and sharp nail will also work)

Small file for deburring

Small (precision) screwdriver (for potentiometer knob set screws and for safely removing ICs from solderless breadboard)

Wire cutter/stripper

Needle nose pliers

Soldering iron

Solder (use 60/40 rosin core)

Optional: hair dryer (for shrink tubing)

Miscellaneous:

Safety glasses

Rubber band

Empty tunafish can (washed; watch out for burrs)

Small cardboard box

One slab of paraffin wax (available at grocery store in 1 lb. boxes, 4 slabs/box)

Experimental Procedure

Global Connections

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.

Variations

Careers

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