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?

 

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.

 

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.

 

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.

You are welcome.