FM Microphone.
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:


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.




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.
... someday.
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