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Why cant I make FM transmitters? Answered

So I am very angry at this point. I've made over a dozen FM transmitters and they NEVER work.  The list includes http://makezine.com/projects/super-simple-fm-transmitter/, http://www.buildcircuit.com/simple-steps-for-making-fm-transmitter/, https://www.instructables.com/id/The-Ultimate-FM-Transmitter/. These are just a few. I use all the right parts and I make all the right coils just like they say. What is wrong with me or my circuits?????

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Downunder35m

4 years ago

a) How did you test the transmitter?

b) Define "never work"

A transmitter can be really simple or really complicated but they all to the same: transmit on a selected frequency.

I simply assume, without more details at hand, that you put the transmitter together but could not hear anything on the radio?

If so, did you consider testing the output frequency of the transmitter?

Was an antenna used and if so was it matched to the frequency?

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HavocRCDownunder35m

Reply 4 years ago

A) Just by tuning the pot and trying to hear it on the radio. I don't have an oscilloscope to check for frequencies.

B) I can't ever get audio to play over the FM radio from the transmitter.

Yup, exactly.

Yes, but I can't really test it. I don't have a oscilloscope!

Yes, and no. I just did what all the instructions said. A 5-10 inch antenna.

I guess what you are telling me is to get a tester, figure out the frequency, then use a formula to determine a antenna length?

Thanks!

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Downunder35mHavocRC

Reply 4 years ago

Problem with discrete transmitters is that the tuning will be affected by the way it is build.

Try to borrow a scanner and let it scan from around 50MHz to 250MHz.

It is the only way I know to check without an oscilloscope.

If you can upload an image of your transmitter, both side of the circuit board.
Quite often the layout and lenght of the connections are the culprit if a trnasmitter does not what was expected.

Same story for the components, if tolerances are too big the frequency will shift.

Take the circuit from your first link as an example:

It is a copper board with the whole copper surface as ground - this keeps some unwanted interference away.

The parts are soldered directly and with quite short legs - no wires poking out anywhere.

If you build this on cardboard and use wires fro the ground connection the circuit is almost doomed to fail and only with some serious tweaking it might work.

Also having it in a metal enclosure with just antenna connection going out helps a lot.

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mpilchfamily

4 years ago

Making a transmitter is not a simple thing and troubleshooting issues requires some expensive equipment. Problem is even the little things make a huge difference. Something as small as the board layout, size of the holes through the board and any air gap that may exist between the solder joint and board can adverslty affect the transmitter.