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The Bicolor LED

Introduction

In an earlier experiment (The Line Clock), we mentioned the "persistence of vision" effect which makes an LED appear to be on constantly even when it is actually blinking rapidly. In this experiment, we will show that this is necessarily the case, and show how we can use this to advantage in practical applications.

Have you ever paid attention to those multi-color LED displays that show words and pictures moving around in different patterns? Close inspection shows that they are composed of a lot of individual LEDs, and that the display can be red, yellow, or green. Yet, the display doesn't have separate LEDs for each of these colors. Rather, each LED can turn on in any of these colors, while still having only two leads. How is this possible? You'll find out as you demonstrate this very phenomenon in this experiment.

This experiment is designated "optional" because it requires that you purchase a bicolor LED (Radio Shack part #276-012 or equivalent). Sometimes it is known as a tricolor LED. The LED costs a few dollars, and is not likely to be used in future experiments. Nevertheless, it's an interesting experiment and will demonstrate some phenomena that we will use in future experiments.



Preliminary Experimental Procedure

Take a look at the bicolor LED. It has two leads and a translucent, colorless body. This uncolored body allows the device to output any color it is capable of generating.

To understand the internal structure of the bicolor LED, insert it into your breadboard socket, between logic switches S0 and S1, with the shorter lead connected to S0. Don't clip the leads; this is only a brief procedure.

Set both switches S0 and S1 to logic 1, then turn on power to your experimental circuit. At this point, the LED should remain off.

Now, set S0 to logic 0. How does your bicolor LED respond to this? Set S0 back to logic 1. Next, set S1 to logic 0. Now how does the LED respond? Leave S1 at logic 0 and set S0 to logic 0 at the same time. What is the response of the LED? Is this what you expected?

The bicolor LED actually consists of a red and green LED in a single package, connected back to back so that each cathode is connected to the other diode's anode. The shorter lead and flat side typically denote the green cathode and red anode.

With both S0 and S1 at logic 1, both ends of the LED are connected to +5 volts through 1K resistors. Therefore, they're both off. When you switched S0 to logic 0, you actually grounded the green cathode. This turned on the green LED. Then, when you had S0 at logic 1 and S1 at logic 0, you reversed the polarity and turned on the red LED. However, with both switches at logic 0, both ends of the LED were grounded and both LEDs were again off.

Clearly the two diodes in this LED can't both be on at the same time, which is also the logical conclusion. But we can turn either LED on by selecting the polarity of the applied voltage.

Now turn off power to your experimental circuit, put your bicolor LED aside for later use, and continue with the main body of this experiment.



Schematic Diagram

A driver circuit for a bicolor LED.

In order to drive the bicolor LED in both directions, we'll use a pair of DTL NAND gates as the controlling elements. The main driving signals will be the CLK and CLK' signals, which you installed in the last experiment. You'll use S0 and S1 to enable or disable each color separately.

With both S0 and S1 at logic 0, both gates will be disabled and the LED will remain off. With one switch at logic 0 and the other at logic 1, one gate will be enabled and will pass that CLK signal on to the LED, turning on one of the two diodes. This much seems intuitive and reasonable, and in keeping with the brief experimental procedure above.

The question is, what will happen when we enable both NAND gates at the same time? That's what we will find out in this experiment. At the same time, it will tell us whether or not our CLK and CLK' signals really are what we believe them to be.



Parts List

To construct and test the circuit on your breadboard, you will need the following experimental parts:



Constructing the Circuit

This circuit is spread out a bit on your breadboard socket for ease of construction and access. It will require three adjacent sets of five bus contacts on the top half of the breadboard. When you have selected a clear space on the right end of your breadboard socket, refer to the image and text below and install the parts as shown.



Circuit Assembly

Start assembly procedure
















Starting the Assembly

Your experimental circuit for this demonstration will be spread out across the top half of the entire portion of your breadboard socket shown in the assembly diagram to the right. This isn't really essential, but makes the components fit easily into place. Remember to have power and ground connections made from your original test bed circuitry.

Click on the `Start' button below to begin. If at any time you wish to start this procedure over again from the beginning, click the `Restart' button that will replace the `Start' button.

0.3" Black Jumper

Locate a leftover 0.3" black jumper, or prepare a new one using the methods you have used in past experiments. Install this jumper in the location indicated in the assembly diagram.

Click on the image of the jumper you just installed to continue.

0.3" Black Jumper

Locate or prepare a second 0.3" jumper, and install it in the location indicated in the assembly diagram.

Again, click on the image of the jumper you just installed to continue.

4.7K, ¼-Watt Resistor

Locate a 4.7K, ¼-watt resistor (yellow-violet-red) and form its leads to a spacing of 0.5". You may already have a few of these left over from your DTL experiments. If necessary, clip the leads to a length of ¼". Install this resistor in the location indicated to the right.

Click on the image of the resistor you just installed to continue.

330Ω, ¼-Watt Resistor

Locate a 330Ω, ¼-watt resistor (orange-orange-brown). Form its leads to a spacing of 0.5", clip them to ¼", and install this resistor in the location indicated in the assembly diagram.

Again, click on the image of the resistor you just installed to continue.

330Ω, ¼-Watt Resistor

Locate a second 330Ω, ¼-watt resistor (orange-orange-brown) and form its leads to a spacing of 0.5". Clip the ends to a length of ¼" and install this resistor as indicated to the right.

As before, click on the image of the resistor you just installed to continue.

4.7K, ¼-Watt Resistor

Locate or prepare a 4.7K, ¼-watt resistor with a lead spacing of 0.5". Install this resistor in the location shown in the assembly diagram.

Once more, click on the image of the resistor you just installed to continue.

1N914 Diode

Locate a 1N914 diode and form its leads to a spacing of 0.4". You can use one of the diodes left over from your DTL experiments. Clip the ends to a length of ½" if they're not already formed, to make handling easier for these very small components. Note the orientation of the diode in the assembly diagram. You must observe the same orientation as you install this diode in the indicate4d location.

Click on the image of the diode you just installed to continue.

1N914 Diode

Locate or prepare a second 1N914 diode, with a lead spacing of 0.3". Install this diode in the location indicated to the right. Observe the orientation of the diode.

Again, click on the image of the diode you just installed to continue.

1N914 Diode

Locate or prepare another 1N914 diode, with a lead spacing of 0.4". Install this diode in the location indicated in the assembly diagram. Observe the orientation of the diode.

As before, click on the image of the diode you just installed to continue.

1N914 Diode

Locate or prepare another 1N914 diode, with a lead spacing of 0.4". Install this diode in the location indicated to the right. Observe the orientation of the diode.

As usual, click on the image of the diode you just installed to continue.

1N914 Diode

Locate or prepare another 1N914 diode, with a lead spacing of 0.3". Install this diode in the location indicated in the assembly diagram. Observe the orientation of the diode.

Again, click on the image of the diode you just installed to continue.

1N914 Diode

Locate or prepare one more 1N914 diode, with a lead spacing of 0.4". Install this diode in the location indicated to the right. Observe the orientation of the diode.

Once more, click on the image of the diode you just installed to continue.

NPN Silicon Transistor

Locate an NPN silicon switching transistor (2N3904, 2N4124, or similar) and, if necessary, form its leads to a spacing of 0.1". Install this transistor in the location indicated in the assembly diagram. Observe the orientation of the transistor.

Click on the image of the transistor you just installed to continue.

NPN Silicon Transistor

Locate another NPN silicon switching transistor (2N3904, 2N4124, or similar) and, if necessary, form its leads to a spacing of 0.1". Install this transistor in the location indicated to the right. Again, observe the orientation of the transistor.

Once more, click on the image of the transistor you just installed to continue.

Bicolor LED

Locate your bicolor LED and note the shorter lead and flat side of the case. Do not clip the LED leads, but do install it with the flat side (and shorter lead) oriented to the left as indicated in the assembly diagram.

Click on the image of the LED you just installed to continue.

Orange Jumper

Locate or prepare an orange jumper, and insert one end of this jumper in the location indicated to the right. Connect the other end to S0.

Click on the image of the jumper you just installed to continue.

Green Jumper

Locate or prepare a green jumper. Connect one end to location indicated in the assembly diagram. Connect the other end to the CLK signal.

Again, click on the image of the jumper you just installed to continue.

Orange Jumper

Locate or prepare an orange jumper. Insert one end of this jumper in the location indicated in the assembly diagram. Connect the other end to S1.

As before, click on the image of the jumper you just installed to continue.

Green Jumper

Locate or prepare a green jumper, and connect one end to location indicated in the assembly diagram. Connect the other end to the CLK' signal.

Once more, click on the image of the jumper you just installed to continue.

Assembly Complete

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, proceed with the experiment on the next part of this page.

Restart assembly procedure


Performing the Experiment

Step 1. Set S0 and S1 to logic 0, then turn on power to your experimental circuit. Note the condition of your bicolor LED.

Step 2. Set S0 to logic 1, note the effect on the LED, and then set S0 back to logic 0. Next, set S1 to logic 1, note the effect of this on the LED.

Step 3. Leaving S1 in the logic 1 position, move S0 to the logic 1 position. Now how does the LED respond? How do you explain what you see?

When you have made these determinations, turn off the power to your experimental circuit and compare your results with the discussion below.



Discussion

With both S0 and S1 set to logic 0, both NAND gates were disabled and the LED remained off. With only S0 set to logic 1, the green diode was turned on. With only S1 set to logic 1, the red diode was turned on. In each of these cases, the behavior of the LED was intuitive, and matched reasonable expectations. The fact that the CLK and CLK' signals were NANDed with the S0 and S1 inputs, respectively, was not apparent; the diodes appeared to be on constantly.

However, when both switches were set to logic 1, the LED behaved in a new way. Both LEDs were turned on, and the result was a yellow color emitted from the LED. By looking at the sides of the LED, or looking closely at it, you could tell that the light was actually being produced by a red and a green diode. However, from a distance, looking at the top of the LED body, it actually looked like a yellow LED.

The yellow color stems from the fact that the human eye perceives the combination of red and green light as yellow. We observe the same phenomenon with television receivers, which actually produce only red, green, and blue light, and get the rest of the visible spectrum by mixing these primary colors.

In addition to this phenomenon, this experiment has demonstrated that the CLK and CLK' signals really are pulse trains with opposite phases. Thus, these two signals cause the two diodes to turn on alternately, at a fast enough pace that they both seem to be on constantly, even though we know this isn't really possible.

When you have completed this experiment, make sure power to your experimental circuit is turned off. Remove all of your experimental components and put them away ready for use in future experiments.


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