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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)
Source: http://www.josepino.com/?fm_microphone Tags: electronics
circuits
homemade
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.
Taken from http://www.josepino.com
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.
... someday.
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08/27/09 - 00:19:36 Adolfo from Sonora, Mex wrote: can somebody give me some PCB layout so i can mount it on a very small surface?
thanks...
jams12160msn.com
08/30/09 - 11:21:38 anil from India wrote: hmmm.. An SMD version..!!!!. great tutorial .. BTW i'm working on a 3V version similiar to this. shal post once completed..
regards, anil
09/20/09 - 10:10:28 secret from india wrote: does this work?? please make a video.
09/20/09 - 10:14:46 karthik from SECRET wrote: please make a video how to make it.i am not confiden.does ths work in india....please respond.why i am asking means I AM A 11YEAR KID
10/07/09 - 12:16:31 jon from india wrote: WHAT IS 24AWG WIRE MEANS?? WHERE CAN I GET IT IN INDIA?
IN WHICH SHOPS? HELP
10/07/09 - 16:49:20 José Pino wrote: AWG stands for American Wire Gauge, and it is a standard used for denoting wire conductor diameter.
You can get this wire from a piece of unshielded twisted pair cable for network or telephone wires.
10/25/09 - 12:05:02 Tutu Zulu from Zambia wrote: Fine. I have these other components apart from a 6.8pf capacitor? I will try 5pf and I will let you know if it works
11/04/09 - 10:56:57 yehu from Israel wrote: Hi,
thanks for transmitter circuit.
can you tell me which instrument you use to test the frequency (as sown in picture)?
best regards
Yehu
11/04/09 - 17:23:51 José Pino wrote: It is a TV with digital FM tuner. I use it to measure frequencies.
11/05/09 - 07:08:33 yehu from Israel wrote: Thanks. Where can I find kind of this instrument low price?
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