mind. However, one could easily substitute a microphone
and op-amp for the player, creating a wireless microphone. The small size, digital tuning, and crystal-controlled
transmitting stability make this chip transmitter a natural
for this purpose.
Audio Signal Generator
As small as it is, the Butterfly’s processor is overkill for
this application. With all that extra processing power
available, I chose to add a simplistic audio signal generator
to aid in testing the transmitter. Although one could use
the D-to-A converter or PWM to generate a signal, I chose
to generate a square wave by toggling a spare output bit
high and low at a 1 kHz rate. A simple RC filter removed
many of the high frequency components, essentially
turning the square wave into an attenuated triangular
wave. This signal was then fed into the audio input ports
on the transmitter. Having a stable audio signal facilitated
testing the rest of the project. Being able to create the
signal from within the project itself was both fortuitous
and convenient.
It is desirable to turn the audio signal generator on
■ FIGURE 1. The Butterfly Broadcaster schematic. Currently
transmitting on 102.1 (MHz, FM radio band). An optional
audio signal generator produces a 1 kHz tone. Following
filtering and attenuation, it can be used as an audio source
for setup and testing.
and off as needed. When on, a small amount of the 1
KHz signal can leak into the transmitter even when it is
disconnected from the audio inputs. It should be off,
therefore, when the unit is connected to a real audio
source. Pushing the joystick inwards usually stores the
new frequency in memory for later use. However, if the
frequency is set to 108 MHz (the top of the FM radio
band), the unit toggles the audio signal generator on and
off when the joystick is pressed inwards.
Construction Concepts
Figure 1 is the schematic for the project. Given the
small number of components involved, one can lay this
circuit out on a breadboard in a matter of minutes. I
soldered a row of header pins to the FM transmitter
module to allow me to insert it directly into a breadboard,
although wires could be use as well. The audio input
connections can be tied to whatever jack best fits your
needs. A mini-stereo jack is ideal, but recall you will need
a matching cable to plug into your MP3 player.
A short wire is soldered directly to the antenna pad
on the module. The crystal and power supply filtering
capacitor for the transmitter chip are already provided on
the module. Be sure to order them separately if you order
the chip without the board for your own design. There
are two pull-up resistors for the I2C clock and data lines.
The Butterfly is easily programmed using its on-board
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May 2008