I'm trying to make a FM transmitter work properly. I built the following circuit :

I can synchronize with 94.8Mhz channel on any FM radio, I know that because the original sound from that radio is eliminated and I start to hear sound (hollow sound) each time I tap the microphone. I can also hear (sharply) any whisper I make close to the microphone.

The problem is: I can't hear normal human sound and words spoken through the mic, they seem not to pass on to modulate the carrier. I tried to replace the mic and electronic 10uF coupling capacitor but that didn't fix it. Any idea why such situation can occur ?

Oh yes, the microphone is an electric mic:

  • 2
    \$\begingroup\$ What device did you use for the microphone? \$\endgroup\$
    – shimofuri
    Commented Mar 9, 2013 at 9:59
  • \$\begingroup\$ As above ideally include the microphone part number you're using. But if you don't have a datasheet / part number increasing R1 to 10K is something that can't do any harm and worth a try to see what difference it makes. \$\endgroup\$
    – PeterJ
    Commented Mar 9, 2013 at 10:02
  • \$\begingroup\$ @gutto: I told you when you tried to edit to not use two different accounts. It causes confusion and you shouldn't have two accounts anyway. Flagging for moderator attention. \$\endgroup\$ Commented Mar 9, 2013 at 17:53

1 Answer 1


Probably the audio signal going into the base of Q1 isn't strong enough. Microphones vary widely in the strength of the signal they put out. This circuit is clearly meant for a electret microphone, which generally produce larger signals than most other common types. If you have a dynamic or condenser (unlikely) mic, then the signal will be a lot weaker. Make sure you really have a electret mic.

If it still doesn't work well with the proper mic, try adding a little amplification. From the general looks of this circuit, I'd say that the microphone signal shouldn't be more than 100 mVpp, probably significantly less. You can experiment by using the audio output of a radio or preamp or whatever other source you have lying around. Put a resistor divider on the output to attenuate it. Start with 50 mVpp and see how that works, then adjust the level from there by listening to the FM radio and also checking how far the signal spills into adjacent bands.


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