● B● ●
BROU●
ATDCTASETERR FLY
BY JAY CARTER
MP3 players are great when
you are wearing earphones,
but wouldn’t it be nice to
listen through your home or
car sound system? Now you
can! Just plug your player
into this project and it will
broadcast the music to any
nearby FM radio.
The key component to this project is a thumbnail-sized
FM radio transmitter on a chip — the NS73M — by the
Niigata Seimitsu company. Just add a microcontroller and
you can be on the air. No coils, no tricky alignment procedures, just pure digital bliss. Photo 1 shows the transmitter
chip pre-installed on a small, experimenter friendly circuit
board. This module is perfect for breadboarding. While many
“wireless microphone” designs exist, low cost units often
use a simple RF oscillator. The transmission frequency may
be hard to set, and the units tend to drift off frequency
easily. With the NS73M transmitter chip, the frequency is
digitally selected with a microcontroller. The frequency is
rock solid, being controlled by a crystal and a phase locked
loop (PLL). Drifting frequencies are a problem of the past.
AVR Butterfly ATMega169
Processor
The ATMEL AVR Butterfly microcontroller board
(shown in Photo 2) is perfect for this project. It includes
an ATMega169 processor, six-character LCD display, five-position joystick, and a piezo-electric beeper, all put to
good use in this project. The entire package is the size of
a credit card, and costs about $21. Its built-in RS-232 serial
port is used for programming, while its 4 MB DataFlash
and thermistor go unused in this application.
You will use the joystick to select the frequency to
transmit on, which is displayed on the LCD. This makes
34 May 2008
■ PHOTO 1 (shown above). The FM radio transmitter
on a chip, the NS73M, pre-assembled on a SparkFun
Electronics breakout board.
frequency selection trivial. The processor stores the
frequency in its internal EEPROM, recalling it when the
device is turned on in the future. For repeated use, just
turn the device on and hit a pre-selected memory button
on the radio. It can’t get any easier than that!
Although the Butterfly board includes a three volt
coin cell battery for normal operation, it is best to use
an external battery or power supply. The battery will run
down quickly if the transmitter is used extensively on its
2 m W RF output mode.
I2C Communications
Connecting the transmitter module to the Butterfly
board is simple as both run on three volts and no level
converts are required. The transmitter can communicate
using either a two wire I2C protocol, or a three wire
protocol. I chose the I2C interface, and tied the
transmitter’s clock and data lines to those on the
Butterfly’s USI port, with additional 4.7K pull-up resistors
on each line. The transmitter’s latch pin is tied to ground,
while the “IIC” mode pin is tied to three volts. A short
wire is attached to the antenna terminal. Power and audio
round out the connections. The transmitter is designed for
