CHAPTER 7

Sound ProjectsAN ARDUINO BOARD CAN BE used to both generate sounds as an output

and receive sounds as an input using a microphone. In this chapter we

have various “musical instrument­type” projects as well as projects that

process sound inputs.

Although not strictly a “sound” project, our first project is to create a

simple oscilloscope so that we can view the waveform at an analog input.

Project 18OscilloscopeAn oscilloscope is a device that allows you to see an electronic signal so

that it appears as a waveform. A traditional oscilloscope works by

amplifying a signal to control the position of a dot on the Y axis (vertical

axis) of a cathode­ray tube while a time­base mechanism sweeps left to

right on the X axis and then flips back when it reaches the end. The result

will look something like Figure 7­1.

Figure 7­1 A 230 Hz sine wave on an oscilloscope.

These days, cathode­ray tubes have largely been replaced by digital

oscilloscopes that use LCD displays, but the principles remain the same.

This project reads values from the analog input and sends them over a

USB cable to your computer. Rather than be received by the Serial

Monitor, they are received by a little program that displays them in an

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oscilloscope­like manner. As the signal changes, so does the shape of the

waveform.

Note that as oscilloscopes go, this one is not going to win any prizes for

accuracy or speed, but it is kind of fun and will display waveforms up to

about 1 kHz.

COMPONENTS AND EQUIPMENT

This is the first time that we have used capacitors. C1 can be connected

either way round; however, C2 and C3 are polarized and must be

connected the correct way round or they are likely to be damaged. As with

LEDs, on polarized capacitors, the positive lead (marked as the white

rectangle on the schematic symbol) is longer than the negative lead. The

negative lead also often has a minus sign (­) or a diamond shape next to

the negative lead.

Hardware

Figure 7­2 shows the schematic diagram for Project 18 and Figure 7­3 the

breadboard layout.

Figure 7­2 Schematic diagram for Project 18.

Figure 7­3 Breadboard layout for Project 18.

There are two parts to the circuit. R1 and R2 are high­value resistors that

“bias” the signal going to the analog input to 2.5V. They are just like a

voltage divider. The capacitor C1 allows the signal to pass without any

direct current (DC) component to the signal (alternating current, or AC,

mode in a traditional oscilloscope).

R3, R4, C2, and C3 just provide a stable reference voltage of 2.5V. The

reason for this is so that our oscilloscope can display both positive and

negative signals. So one terminal of our test lead is fixed at 2.5V; any

signal on the other lead will be relative to that. A positive voltage will

mean a value at the analog input of greater than 2.5V, and a negative

value will mean a value at the analog input of less than 2.5V.

The diode D1 will protect the analog input from accidental overvoltage.

Figure 7­4 shows the completed oscilloscope.

Figure 7­4 Project 18: oscilloscope.

Software

The sketch is short and simple (Listing Project 18). Its only purpose is to

read the analog input and blast it out to the USB port as fast as possible.

LISTING PROJECT 18

The first thing to note is that we have increased the baud rate to 115,200,

the highest available. To get as much data through the connection as

possible without resorting to complex compression techniques, we are

going to shift our raw 10­bit value right 2 bits (>> 2); this has the effect of

dividing it by four and making it fit into a single byte.

We obviously need some corresponding software to run on our computer

so that we can see the data sent by the board (Figure 7­1). This can be

downloaded from

www.arduinoevilgenius.com(http://www.arduinoevilgenius.com).

To install the software, you first need to install some software called

Processing. Processing is the natural partner for writing computer

applications that communicate with an Arduino. In fact, the Arduino IDE

is written in Processing.

Like the Arduino IDE, Processing is also available for Windows, Mac, and

LINUX and can be downloaded from

www.processing.org(http://www.processing.org).

Once Processing is installed, run it. The similarities with the Arduino IDE

will be immediately apparent. Now open the file scope.pde, and click the

Play button to run it.

A window like Figure 7­1 should appear.

Putting It All Together

Load the completed sketch for Project 18 from your Arduino Sketchbook

and download it to the board (see Chapter 1). Install the software for your

computer as described previously, and you are ready to go.

The easiest way to test the oscilloscope is to use the one readily available

signal that permeates most of our lives, and that is the hum from the

electrical service. Home electricity oscillates at 50 or 60 Hz (depending on

where you live in the world), and every electrical appliance emits

electromagnetic radiation at this frequency. To pick it up, all you have to

do is touch the test lead connected to the analog input, and you should see

a signal similar to that of Figure 7­1. Try waving your arm around near

any electrical equipment and see how the signal changes.

The signal shown in Figure 7­1 is actually a 215 Hz sine wave supplied by a

smart phone function generator application.

Sound GenerationYou can generate sounds from an Arduino board just by turning one of its

pins on and off at the right frequency. If you do this, the sound produced

is rough and grating. This is called a square wave. To produce a more

pleasing tone, you need a signal that is more like a sine wave (Figure 7­5).

Figure 7­5 Square and sine waves.

Generating a sine wave requires a little bit of thought and effort. A first

idea may be to use the analog output of one of the pins to write out the

waveform. However, the problem is that the analog outputs from an

Arduino are not true analog outputs but pulse­width modulated (PWM)

outputs that turn on and off very rapidly. In fact, their switching

frequency is at an audio frequency, so without a lot of care, our signal will

sound as bad as a square wave.

A better way is to use a digital­to­analog converter (DAC). A DAC has a

number of digital inputs and produces an output voltage proportional to

the digital input value. Fortunately, it is easy to make a simple DAC—all

you need are resistors.

Figure 7­6 shows a DAC made from what is called an R­2R resistor ladder.

Figure 7­6 DAC using an R­2R ladder.

It uses resistors of a value R and twice R, so R might be 5K and 2R 10K.

Each of the digital inputs will be connected to an Arduino digital output.

The four digits represent the four bits of the digital number. So this gives

us 16 different analog outputs, as shown in Table 7­1.

TABLE 7­1 Analog Output from Digital Inputs

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Another way of generating a particular wave shape is to use the Arduino

analogOutput command to generate the wave shape. This uses the

technique of PWM that you first met back in Chapter 4 to control the

brightness of LEDs.

Figure 7­7 shows the signal from a PWM pin on the Arduino.

Figure 7­7 Pulse­width modulation.

The PWM pin is oscillating at about 500 times per second (hertz), with

the relative amount of time that the pin is high varying with the value set

in the analogWrite function. So, looking at Figure 7­7, if the output is only

high for 5% of the time, then whatever we are driving will only receive 5%

of full power. If, however, the output is at 5V for 90% of the time, then the

load will get 90% of the power delivered to it.

When driving motors with PWM, the physical inertia of the spinning

motor means that the motor does not start and stop 500 times per second

but is just given a kick of varying strengths every five­hundredths of a

second. The net effect of this is smooth control of the motor speed.

LEDs can respond much more quickly than a motor, but the visible effect

is the same. We cannot see the LEDs turning on and off at that speed, so

to us it just looks like the brightness is changing.

We can use this same technique to create a sine wave, but to do this, there

is one problem. That is that the default frequency that Arduino uses for its

PWM pulses is around 500 Hz, which is well within the range of audible

frequencies. Fortunately, we can change this frequency in our sketch,

making it much higher and outside the range of our hearing.

Figure 7­8 shows two traces from an oscilloscope of a 254 Hz sine wave

being generated by writing out successive values from an array.

Figure 7­8 Oscilloscope trace for sine wave generation.

The array contains a series of values that, when used to set the value of

analogWrite, one after the other produce the effect of a sine wave.

The bottom trace shows the raw PWM signal, with the pulses bunched up

for the peaks and troughs of the sine wave and more spread out for the

parts of the sine wave in the middle. The top trace shows that same signal

after it has been passed through a low­pass filter that chops off the high

PWM frequency (63 kHz), leaving us with quite a well­shaped sine wave.

Project 19Tune PlayerThis project will play a series of musical notes through a miniature

loudspeaker using PWM to approximate a sine wave.

If you can get a miniature loudspeaker with leads for soldering to a

printed circuit board (PCB), then this can be plugged directly into the

breadboard. If not, you will either have to solder short lengths of solid­

core wire to the terminals or, if you do not have access to a soldering iron,

carefully twist some wires round the terminals.

COMPONENTS AND EQUIPMENT

Hardware

To try to keep the number of components to a minimum, we have used an

integrated circuit (IC) to amplify the signal and drive the loudspeaker.

The TDA7052 IC provides 1 W of power output in an easy­to­use little 8­

pin chip.

Figure 7­9 shows the schematic diagram for Project 19, and the

breadboard layout is shown in Figure 7­10.

Figure 7­9 Schematic diagram for Project 19.

Figure 7­10 Breadboard layout for Project 19.

R1 and C1 together make a low­pass filter that will filter out the high­

frequency PWM noise before it is passed on to the amplifier chip.

C2 is used as a decoupling capacitor that shunts any noise on the power

lines to ground. This should be positioned as close as possible to IC1.

The variable resistor R2 is a potential divider to reduce the signal from

the resistor ladder by at least a factor of 10, depending on the setting of

the variable resistor. This is the volume control.

Software

To generate a sine wave, the sketch steps through a series of values held in

the sine array. These values are plotted on the chart in Figure 7­11. It is

not the smoothest sine wave in the world, but it is a definite improvement

over a square wave (see Listing Project 19).

Figure 7­11 A plot of the sine array.

The setup function contains the Evil Genius’s magic incantations for

changing the PWM frequency.

The playNote function is the key to generating the note. The pitch of the

note generated is controlled by the delay after each step of the signal

within the playSine function that playNote calls.

Tunes are played from an array of characters, each character

corresponding to a note and a space corresponding to the silence between

notes. The main loop looks at each letter in the song variable and plays it.

When the whole song is played, there is a pause of 5 seconds, and then the

song begins again.

LISTING PROJECT 19

The Evil Genius will find this project useful for inflicting discomfort on his

or her enemies.

Putting It All Together

Load the completed sketch for Project 19 from your Arduino Sketchbook

and download it to the board (see Chapter 1).

You might like to change the tune played from “Jingle Bells.” To do this,

just comment out the line starting with char* song = by putting // in front

of it, and then define your own array.

For a longer­duration note, just repeat the note letter without putting a

space in between.

You will have noticed that the quality is not great. It is still a lot less nasty

than using a square wave but is a long way from the tunefulness of a real

musical instrument, where each note has an “envelope” in which the

amplitude (volume) of the note varies with the note as it is played.

Project 20Light HarpThis project is really an adaptation of Project 19 that uses two light

sensors (LDRs): one that controls the pitch of the sound the other that

controls the volume. This is inspired by the Theremin musical instrument

that is played by mysteriously waving your hands about between two

antennas. In actual fact, this project produces a sound more like a bagpipe

than a harp, but it is quite fun.

COMPONENTS AND EQUIPMENT

Hardware

Figures 7­12 and 7­13 show the schematic diagram and breadboard layout

for the project, and you can see the final project in Figure 7­14.

Figure 7­12 Schematic diagram for Project 20.

Figure 7­13 Breadboard layout for Project 20.

Figure 7­14 Project 20: light harp.

The LDRs, R4 and R5, are positioned away from each other to make it

easier to play the instrument with two hands.

Software

The software for this project has a lot in common with Project 19 (see

Listing Project 20).

LISTING PROJECT 20

The main differences are that the period passed to playSine is set by the

value of the analog input 0. This is then scaled to the right range using the

map function. Similarly, the volume voltage is set by reading the value of

analog input 1, scaling it using map, and then using it to scale the values

from the sine array before outputting them.

LDRs have different ranges of resistance. So you may find that you need

to tweek the values of the variables ldrDim and ldrBright to get better

ranges of pitch and volume.

Putting It All Together

Load the completed sketch for Project 20 from your Arduino Sketchbook

and download it to the board (see Chapter 1).

To play the “instrument,” use your right hand over one LDR to control the

volume of the sound and your left hand over the other LDR to control the

pitch. Interesting effects can be achieved by waving your hands over the

LDRs.

Project 21VU MeterThis project (shown in Figure 7­15) uses LEDs to display the volume of

noise picked up by a microphone. It uses an array of LEDs built into a

dual­in­line (DIL) package.

Figure 7­15 Project 21: VU meter.

The push button toggles the mode of the VU meter. In normal mode, the

bar graph just flickers up and down with the volume of sound. In

maximum mode, the bar graph registers the maximum value and lights

that LED, so the sound level gradually pushes it up.

Hardware

The schematic diagram for this project is shown in Figure 7­16. The bar­

graph LED package has separate connections for each LED. These are

each driven through a current­limiting resistor.

Figure 7­16 Schematic diagram for Project 21.

COMPONENTS AND EQUIPMENT

The microphone will not produce a strong enough signal on its own to

drive the analog input. So, to boost the signal, we use a simple single­

transistor amplifier. We use a standard arrangement called collector­

feedback bias, where a proportion of the voltage at the collector is used to

bias the transistor on so that it amplifies in a loosely linear way rather

than just harshly switching on and off.

The breadboard layout is shown in Figure 7­17. With so many LEDs, a lot

of wires are required. Make sure that the bar­graph LED module has the

negative LED connections to the left of the breadboard as it appears in

Figure 7­17. If it is not labeled, then test it out using one of the 270 Ω

resistors and the 5V supply from the Arduino.

Figure 7­17 Breadboard layout for Project 21.

Software

The sketch for this project (Listing Project 21) uses an array of LED pins

to shorten the setup function. This is also used in the loop function, where

we iterate over each LED, deciding whether to turn it on or off.

LISTING PROJECT 21

At the top of the loop function, we check to see if the switch is depressed;

if it is, we toggle the mode. The ! command inverts a value, so it will turn

true into false and false into true. For this reason, it is sometimes referred

to as the marketing operator. After changing the mode, we reset the

maximum value to 0 and then delay for 200 ms to prevent keyboard

bounce from changing the mode straight back again.

The level of sound is read from analog pin 0, and then we use the map

function to convert from a range of 0 to 1023 down to a number between

0 and 9, which will be the top LED to be lit. This is adjusted slightly by

extending the range up to 0 to 11 and then subtracting 1. This prevents the

two bottom­most LEDs from being permanently lit owing to transistor

bias.

We then iterate over the numbers 0 to 9 and use a Boolean expression

that returns true (and hence lights the LED) if i is less than or equal to the

top LED. It is actually more complicated than this because we also should

display that LED if we are in peak mode and that LED happens to be the

peakValue.

Putting It All Together

Load the completed sketch for Project 21 from your Arduino Sketchbook

and download it to the board (see Chapter 1).

SummaryThis concludes our sound­based projects. In Chapter 8 we go on to look at

how we use an Arduino board to control power—a topic always close to

the heart of the Evil Genius.

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