FIGURE 4. Verify proper settings in the MP3
files you use by checking their properties.
FIGURE 5. Basic breadboard wiring diagram for the Musicbot. The two
continuous rotation servos that move the bot are powered from their own
4xAA battery pack. The servo power supply and the Arduino share a
common ground. Use double-ended long header pins to attach the servos
and other cabling to the breadboard.
Goldwave. Audacity is free. Goldwave is paid
software; either an outright lifetime purchase,
or a less expensive annual license.
Both allow you to record, edit, and export clips to MP3
format. A slate of effects let you modify the sound — add
echo and reverb, create a robotic voice, or modify the pitch
Here are the steps for processing an existing sound file
to MP3 in Audacity. It’s not complicated, but it does involve
a number of commands that you must select in the proper
Note: Before you can create an MP3 file in Audacity,
you must first download a copy of the LAME MP3 encoder
program. Instructions are provided in the Audacity help. (In
Audacity, choose Help->Quick Help, then in the main page
click on Index; next, find and click on LAME MP3.)
3. Choose File->Export. In the Export File dialog box,
choose MP3 Files in the Save as type list.
4. Click the Options button, then select the following:
Bit Rate Mode: Constant
Quality: 160 kbps
Channel Mode: Joint Stereo
then click OK.
5. Provide a name for the clip. The Arduino MP3
player code in this month’s installment assumes a
specific filename format — tracknnn.mp3 — where
nnn is a three-digit number starting at 000. For
FIGURE 6. A self-contained capsule speaker incorporates its
own amplifier, speaker element, and rechargeable battery.
1. Open an existing WAV or other sound clip
(see Figure 3). If prompted, choose the
option “Make a copy of the files before
2. Examine the Project Rate setting in the lower
left corner. Set it to 44100, if it’s not already.
Bit Rate Compression Ratio File Size Per Minute* Quality Similar To
64 kbps 22:1 480K AM Radio
96 kbps 15:1 720K FM Radio
128 kbps 11:1 1 MB TV Sound
160 kbps 9:1 1.2 MB Tabletop CD
192 kbps 7:1 1.4 MB Hi-Fi CD
*Calculated using this standard formula: (Bit Rate 60) / 8.
SERVO 05.2012 41