A micro-transmitter is so small and sensitive, it have only 8 easy-to-get parts and can be assembled in one hour. (6/3/2006)
This is the basic fm trasmitter that I built. In theory, according to electronics, it shouldn't work but works fine and is very sensitive. It can transmit the signal up to 45 yards (about 40 meters) . A sensitive FM radio can pick up the signal up to 100 yards (80 meters) but I can't see any reason for it.
This kind of microphones are also called "spy microphones" but knowing that the battery is bigger than the actual circuit, I can't see any application like that, anyway, this circuit should be used as educational application only.
Here is the circuit of the fm micro transmitter:
It doesn't have special parts and the frecuency is selected by the coil.
The coil is a 24 AWG wire, it have 5 loops with a diameter of one centimeter. It can be easily build using a pencil.
Here is the circuit assembled in the construction board.
This circuit can be tested without problem before soldering the parts. This is a photo taken I was testing the circuit.
After the circuit was tested and verified that works, I build my FM transmitter
Now, the last step, is to adjust the frecuency. With a toothpick you can make wider or narrow the coil until the frecuency is between 89 Mhz to 104 Mhz.
It takes a while to setup and tune up, but is not too hard to make it work. As variable capacitors are not easy to buy, the solution is to choose the frecuency with the coil. Once it is tuned, hot glue can be used to keep the coil fixed, so the frecuency will not change.
This microphone cannot be used close to water, the body or near metals because it may cause the frecuency to change.
This problem can lead to another useful circuit: Metal detector. If you use a bigger coil and changing the frecuency, you can build a metal detector. That will be explained later.
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