Fun With Crystal Oscillators: Part 2

In Fun With Crystal Oscillators: Part 1 we discovered how to use a single component to generate an RF signal. In part 2 we will add one more part to create a low powered AM broadcast station.

Creating a transmitter that just outputs an AM carrier wave can be useful. I demonstrated in part 1 how to send morse code by keying the device (done by applying voltage and removing it). Often you will want to do a little more.

To broadcast voice or music (or even data) you will need to modulate the AM signal. Amplitude Modulation works by changing the output power of the carrier wave which will creates audio. This graphic below (courtesy of Wikipedia) demonstrates what happens to the AM signal.

The top most portion of the graphic shows a carrier wave. The middle graphic shows an Amplitude Modulated wave. These fluctuations in the wave allow audio to be heard on an AM radio receiver.

To easily modulate the signal generated by a crystal oscillator, an audio transformer can be used.

This design uses a 1000 ohm to 8 ohm audio transformer. These can be purchased in many places and can probably be found at your local Radio Shack. The 1000 ohm side will have three wires and the 8 ohm side will have two. Simply ignore the middle wire.

The + and - audio can be input via a monaural audio headphones jack. Again, these jacks can be found in a lot of places and also at Radio Shack.

As the audio is input into the transformer, the power output will increase and decrease, causing the modulation.

Once you hook up your circuit, plug the audio jack into a MP3 player, and tune your radio to 1000 kHz (or whatever frequency your crystal is outputting). You should hear music playing! If you don’t hear any audio, you may need to reverse the transformer so audio is input into the 8 ohm side and the power in the 1000 ohm side. My old iPod works in the way the circuit is diagrammed above. My current smartphone needs the reverse.

Here’s the circuit on the breadboard:

Credit goes to SciToys.com for this idea. Read up for more details and other interesting projects!

Read more:
Fun With Crystal Oscillators: Part 1
Fun With Crystal Oscillators: Part 3