Nuts and Volts - May 2008

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.