Friday, April 23, 2010

FM Transmitter - in Stereo!

A long time back I built a super-simple FM transmitter that worked off 1.5 volts. I used this schematic which you can find all over the internet (this image is from a pdf of a book called "101 Spy Gadgets for the Evil Genius"). It worked well and was easy to build.

Understanding nothing about how it worked, I attempted to plug in my iPod in place of the microphone. While I could recognize the received signal as definitely the same one I was sending, it was very strongly garbled. The iPod transmits a signal with some power, and it was enough so that when I unplugged the AA battery the thing still transmitted. This probably screwed up its operation a bit.

A few weeks ago my EE200 (Intro to Signal Processing) professor mentioned that we could do an extra credit project. The project could be just about anything so long as he approved it. I immediately decided to return to the FM Transmitter project. I quickly found this page that describes how to build one without using a special IC designed for stereo FM transmission. It is relatively simple to build and the page describes how it works in detail.

Here is a photo of it assembled on a breadboard. I tested it by listening to the output through my roommate's portable radio. Once I got everything right (see the below listed quirks) the sound was of very nice quality! It is powered from the wall via an adjustable breadboard power supply from LadyAda of Adafruit Industries.

  • There's a lot of background noise. The website makes a few suggestions about how to improve it, some of which I may try in the future. Then again, I don't think it will be so noticeable when listening to it through my ancient and terrible car radio.
  • Every so often, when I touch something (even the iPod's metal case!) I hear I high-pitched, very clear tone. No clue what causes that. It isn't consistently replicable...
  • Finicky tuning. The signal is only really clear if you tune it just right. If you don't you can hear the signal but it starts to get badly distorted and very noisy.
  • Clarity of signal is best if the iPod is set to about 1/4 volume and the radio is adjusted to liking. I got weird distortion above that.

After this project has been presented I plan to pack it all onto a piece of Veroboard and into a nice enclosure. I will make it either battery powered or car-outlet powered with the addition of a 5v limiter. I will fix the tuning issues (touching the inductor/variable capacitor or getting anywhere near them changes the frequency to which the transmitter is tuned, which means I will have to isolate these parts while still having some mechanism for turning them in place).

While buying a transmitter is cheaper and probably results in better sound quality, it just isn't as cool as building one myself. I'm so excited about this project that for the last hour or so I've been listening to my iPod through the radio even though I could technically play all my music directly from my laptop.