FM Transmitter Experiments


I have often worked on projects requiring short-range high quality wireless audio. Bluetooth is an obvious choice these days, but it has some problems outlined here. The ubiquity and decent quality of FM radios makes FM transmission an attractive choice, since you don’t need to build your own receiver. I’m not really interested in high power pirate radio broadcasting (less than 100 feet is generally enough for my needs), so hacking mini FM transmitters (usually intended for cars) is an attractive option. This page collects my experiences. Share the airwaves. Only transmit on empty frequencies, or better yet frequencies with no neighboring stations either. (You will get better range that way, and avoid interfering with other users of the spectrum.)

Pyle FM Transmitter

Pyle FM Transmitter closeupIn 2013 I found an extremely cheap ($8 – $14) transmitter with built-in MP3 player, the Pyle PMP3B2 (B2 suffix indicates blue color. R2 = red, etc). It seems to be discontinued now but similar ones are easy to find. I have a hunch they are all similar inside. Here are some things I found during testing and using these for art installations.

  • Informative multi-line display (though dim and low contrast)
  • Tunable to any FM broadcast frequency via +/-  pushbuttons
  • Frequency memory survives power loss
  • Frequency stability is fine (No drift in hot/cold temperatures.)
  • Sound quality is fine, with typical FM stereo separation and frequency response
  • Built-in MP3 player: If media is found on the SD card slot or USB port then it auto-plays and auto-loops the files with no need for user intervention. It restarts playback successfully after power loss, but it resumes the previous playback position instead of starting from the top. The SD/USB media must be formatted for Windows, not Mac. (If files are added on a Mac, be sure to empty the trash before you eject the disc, otherwise trashed files will still play!)
  • If no SD card or USB drive is inserted, the signal from the 1/8″ stereo jack will be transmitted. It accepts line-level signals and there is no annoying auto-off circuit.
  • The transmitter is intended to be powered by 12v DC from a car. It has no internal battery. (I tried to run it on a 9v battery and it worked, but I could hear some interference and transmission range was reduced.) The main board requires 5v, delivered by a separate dc-dc power supply board inside the cigarette lighter plug.  If you want to use your own 5V power supply, you can easily remove the dc-dc converter and apply power directly to the main board.
  • The antenna connection is labeled with a handy “A” on the main board, and it’s trivial to extend it to a more efficient length. (Look online for antenna length calculators like this one to tune the antenna to your desired frequency.) That’s the extent of the “hacking” required to extend the range of the transmitter. With a half-wave dipole antenna I got a few hundred feet outdoors. With a quarter-wavelength whip I got about 50ft indoors.
  • Reliability: On one transmitter, the LCD kept flickering. I noticed that the flex cable that connects it to the board was not fully soldered. I carefully cut the foam tape behind the LCD and re-soldered the connection. It was fine. Out of 6 units I modded, one has failed mysteriously, transmitting a silent carrier instead of the input I fed it. After a few power cycles it “fixed” itself. The other five units have been used for months without problems.

Taking apart the Pyle PMP3B2 FM Transmitter:

pyle apart 1

1) Pry out the plastic hinge pins.

pyle apart 2

2) Peel the plastic face off to reveal two screw holes. Remove the screws.

pyle apart 3

3) Flex the front face to pop the clips that hold it on.

pyle apart full tall

Opened up, showing the main board on top, USB board on left, and 12v-5v converter on right.

pyle apart 4

See the “A” label on the row of connections in the upper left? That’s the antenna. Solder your extended antenna there.

pyle-circuit-board

The business side of the main board.

pyle-circuit-board-LCD-connection

The flex cable behind the LCD. If you have display problems, look here.

Adafruit Stereo FM Transmitter – Si4713

adafruit FM transmitterThis transmitter board requires a microcontroller (Arduino, etc) to run, but it may be a promising candidate for installations if the audio quality and range is decent. I haven’t tested it yet.

The Belkin Tunecast series

belkin tunecast IIThe Belkin Tunecast II was the first transmitter I hacked, using this instructable several years ago. Even after the hack it was cumbersome at best. (It’s no longer available, but later revisions have similar hacks.)

  • It was important to disable the auto-off circuit that detects silences, otherwise the transmitter would turn off at unexpected times.
  • You had to hold a button combination to turn it on, and it reset to 88.1 MHz whenever power was interrupted, so it was way too “fiddly” for art installations.