FM Transmitter audio problem

Discussion in 'The Projects Forum' started by vm12, Mar 26, 2013.

  1. vm12

    Thread Starter New Member

    Mar 26, 2013
    2
    0
    I built a small FM transmitter from this site (http://www.talkingelectronics.com/projects/FM-Bug/FM BUG.pdf) and I removed the electret microphone and would like to add a line-in for use with an mp3 player/ computer. I used a shielded audio cable connected to ground with a DC blocking cap but My transmitter keeps on decreasing in power through the antenna as I watch it with my RF power meter.

    I have not found any circuits on google for a line-in, most are all FM bugs with a mic.

    Could someone explain what I can add to stop this?
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    To connect the MP3 player to the circuit shown:

    In addition to removing the mic, you should remove the 22k resistor, which only supplies power to the mic.

    Then, connect each of the stereo leads to a 27k resistor and tie the other ends of the resistors together and to the coupling capacitor.

    Connect the stereo ground to the transmitter ground.

    Don't expect too much. The transmitter will drift and it will be difficult to adjust the drive from the MP3 player to get proper audio.

    Good luck.
     
  3. vm12

    Thread Starter New Member

    Mar 26, 2013
    2
    0
    Thanks for the help! I got it working. Yes these things are very unstable, but I have been getting good results from not being near the device when on and proper layout over a ground plane with copper clad.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    You are welcome.
     
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