- Joined Apr 21, 2022
Does anyone have a reliable circuit for a Short range FM transmitter please?, i just need to cover about 40ft in range.
AG,I fixed a simple FM transmitter circuit with 4 steps:
1) The mic preamp did not work when the 9V battery was new at 9.5V and also when the battery was run down to 6V.
A re-design with a 5V low-dropout voltage regulator fixed it.
2) The radio frequency changed as the 9V battery voltage ran down.
Connecting the RF oscillator to the 5V regulator fixed it.
3) The radio frequency also changed when something moved toward or away from the antenna.
Adding an RF amplifier between the oscillator and antenna fixed it.
4) The audio received with an FM radio was muffled with no high audio frequencies.
Adding pre-emphasis (treble boost like all FM stations use to match the de-emphasis in all FM radios) to the mic preamp
But it is illegal because it is powerful enough to cause interference to many neighbors' FM radios.
A kit for an FM transmitter used an attenuator at the antenna to reduce the range.
Here is my circuit:
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