Arduino: A Quick-Start Guide, Second Edition (2015)

---

Figure 23. How raster scan works (Image created by Ian Harvey)

After the last line was drawn, the electron beam moved back to the top and drew the next image. Depending on the TV standard, this process happened 50 to 60 times per second, and it was fast enough to create the illusion of motion. (Actually, most TV sets needed two passes to draw a single image using a mechanism called interlacing, but for our purposes that’s irrelevant.)

Moving the electron beam across the screen isn’t sufficient. You have to somehow encode the information you’d like to draw. Therefore, you have to change the electron beam’s intensity while it traverses the screen. Due to a chemical reaction, the TV screen will glow in different colors when the electron cannon hits it with different intensities. For a monochrome signal, you need to generate the voltages explained in the following table.

Voltage:	0.0V	0.3V	0.6V	1.0V
Color:	SYNC	Black	Gray	White

A voltage of 0V represents the SYNC signal. It tells the TV set that a new line of the image begins. All the other voltages represent different colors. To draw a white dot, you have to set the voltage to 1V.

All we have to do is create a couple of different voltage levels. That doesn’t sound too difficult, but unfortunately, the Arduino has only analog input pins. It cannot emit analog output signals. At least not directly, but in the next section you’ll learn how to do it.

Building a Digital-to-Analog Converter (DAC)

The Arduino doesn’t natively support analog output signals, so we need to find a way around this limitation. The solution is a digital-to-analog converter (DAC).^[77] As the name suggests, such a circuit turns a digital input into an analog output. You can find DACs in a lot of consumer devices—for example, in your MP3 player. It turns your digitally encoded music into an analog sound wave.

One of the most important characteristics of a DAC is its resolution. In our case we need to generate four different voltage levels to create a video signal. To encode four voltage levels, we need two bits—that is, our DAC has a 2-bit resolution. The following table shows how we could map the four binary input values to their voltage levels.

Binary input:	00	01	10	11
Output voltage:	0.0V	0.3V	0.6V	1.0V

We can use two of the Arduino’s digital pins to control the DAC’s input value, but we still have to find a way to generate different voltages depending on the pins’ values.

There are several ways to achieve this, but one of the easiest is by using a binary-weighted DAC. It has the following characteristics:

· You need a resistor for every input bit.

· All resistors have to be in parallel.

· Resistor value for bit #0 is R. For bit #1 it’s 2R, for bit #3 it’s 4R, and so on.

Let’s say we use the Arduino’s digital pins D7 and D9 to control the DAC’s input value. In the following figure, you can see our DAC’s circuit. You have to add the 470Ω and 1kΩ resistors yourself, but you get the 75Ω resistor for free, because it’s part of your TV set’s input connector.

In principle, the binary-weighted DAC is a voltage divider^[78]—that is, it turns an input voltage into a smaller voltage. The output voltage depends on the resistor values—75Ω, 470Ω, and 1kΩ, in our case. If you set both input signals to 0V, the output voltage will be 0V, too. That’s how we can create the SYNC signal.

Calculating the output voltage for the remaining three combinations of input values isn’t rocket science, but the theory and formulas of voltage division are beyond the scope of this book. Just to give you a feeling, the following figure shows how to calculate the output voltage when you set D7 to 0V and D9 to 5V.

The following table shows the corresponding output voltages for all possible combinations of pin values.

Pin D7	Pin D9	Output Voltage	Color
0V	0V	0.0V	SYNC
0V	5V	0.3V	Black
5V	0V	0.65V	Gray
5V	5V	0.95V	White

Now you should see why we’ve used a 470Ω and a 1kΩ resistor. The value 1000 is roughly 470 times 2, so the resistor values follow the rules of a binary-weighted DAC. Also, these two resistors (combined with the TV set’s 75Ω resistor) produce the output voltages we need. Note that the output voltages don’t exactly match the specification, but in practice the small differences are negligible.

Connecting the Arduino to Your TV Set

Even with a digital-to-analog converter in place, we still have a problem: the Arduino doesn’t have an RCA jack—that is, you cannot plug an RCA cable into an Arduino. We could attach an RCA jack to a breadboard and connect it to the Arduino, but there’s an easier solution. We’ll modify an RCA cable and connect it directly to the Arduino.

First, you have to cut off one of the cable’s connectors using a wire cutter. (See Learning How to Use a Wire Cutter, to learn more about wire cutters.) Then remove about three centimeters of the cable’s outer insulation. Be careful, because the insulation isn’t very thick. Use the wire cutter to cut it slightly and then remove it by pushing it slowly toward the cable’s end. The cable should look like the following image.

As you can see, there’s a mesh of wires below the outer insulation. Bring back the mesh into wire shape by rubbing it between your thumb and forefinger, so you can solder it to a solid-core wire later. The result should look like the following image.

The cable usually also contains an inner insulation made of plastic. Use the wire cutter again to remove the inner insulation. In my experience, it’s best to put the insulation between the wire cutter’s blades and then turn the cable slowly and carefully, increasing the pressure while turning the cable. Be very careful that you don’t accidentally cut the signal wire! After you’ve cut through the whole insulation, you can easily remove it. You should now see the cable’s signal wire, and your cable should look like the following image.

Finally, we have to connect the two resistors to the RCA cable’s signal wire, and it’s not sufficient to simply knot them together. You have to solder them. While you’re at it, connect the RCA cable’s ground wire to a piece of solid-core wire, so you can easily attach it to the Arduino. The following image shows what it should look like.

That’s it! You’ve turned an RCA cable into a binary-weighted DAC that you can can connect to your Arduino to generate your own video signal. Plug the 470Ω resistor into port D7, the 1kΩ resistor into D9, and the ground wire into one of the Arduino’s GND ports. You can see the final circuit in the following image.

Using the TVout Library

Okay, the hardware’s done, but how do we bring it to life? We could try to write our own library to emit video signals, but I have to admit that I didn’t tell you the whole truth. To generate a clean and stable video signal, you not only have to output different voltages, but you also have to make sure that you emit your signals according to a very accurate schedule. The timing has to be so accurate that you have to implement it in assembly language!

Don’t worry! Of course there’s a library for that. The TVout library^[79] not only generates crystal-clear video signals, but also comes with a lot of utility functions for drawing geometric shapes. On top of that, it supports different fonts in several sizes.

Note that the TVout library doesn’t support every Arduino board. For example, it won’t work on the Arduino Leonardo or the Arduino Due. Check the TVout’s website for a list of compatible hardware.

Download TVout,^[80] unzip it, and copy the contents of the zip archive to the libraries folder of the Arduino IDE. Then restart your IDE.

The library comes with a few examples. The most important ones are DemoNTSC and DemoPAL. In principle, it’s only one example that demonstrates all of the library’s features, but it comes in two flavors: NTSC and PAL. This is necessary because there are different standards for analog TV. NTSC and PAL are two very popular ones. They don’t differ much, and modern TV sets are usually capable of working with both. Still, your TV set might be pickier about its input. If you’re living in the United States, you’ll probably need the NTSC demo; in Europe, PAL is the way to go.

Compile and upload the sketch to your Arduino; then connect the Arduino to your TV set’s composite input using the RCA cable. You should see an impressive demo showing TVout’s capabilities. At the end it even shows the inevitable rotating 3D cube.

The library’s standard example shows nearly all of TVout’s functions in action, so it’s a good idea to have a look at the code. Still, the best way to learn how to use the library is to write your own code. In the next section, you’ll create a graphical thermometer that displays the current temperature on your TV screen.

Building a TV Thermometer

To build our TV thermometer, we’ll use the TMP36 sensor again, so we’ll combine the circuit we created in Increasing Precision Using a Temperature Sensor, with the circuit we created for generating the video signal. You can see the result in Figure 24, Circuit of the TV thermometer.