A polite suggestion: Use Google to search previous postings on this forum. This subject has been beat to death here...

The coil isn't specified in your drawing. What coil do you propose to use?

There are lots of ways you can prevent interferring with other people's reception. The best way is just to find a quiet spot in the FM band to transmit on. Further, you have to limit the output power so that your neighbors cannot pick up your transmissions.

Every subject gets recycled here at some point. I can't even count how many threads are about lighting an LED, probably the most simple thing anyone will ever do in electronics.

I for one don't mind new threads on old topics. The old ones on this subject get so long, and so many contributors and the S/N levels are rediculous. I don't have the patience for some of them. At least the OP came with something in mind. Every day, there are numerous posts that say, "I have no idea what to do. Please do it for me." That's the threads the annoy me.

Sorry, I don't have any suggestions. I'll look at the circuit later, I'm not sure about how the coil is connected. Something doesn't look right. Look at Audio Guru's transmitter. Those coils are correct, and winding isn't hard at all. Probably easier to wind than to find a suitable one.

PS: The center tap on that coil is superfulous, and might prevent the circuit from working. My guess is the author had a junk coil lying around with a center tap and connected it to the most convenient place, and somehow got the thing to work. If you can't find a suitable coil, either wind one or grab a handful from a busted radio and experiment.

The original schematic shows the antenna connected to +9V instead of to the tap on the coil.
I don't know why the OP is using input transformers.

The circuit will have extremely poor performance because is is too simple:
1) Its frequency will change if something moves near its antenna and as the battery voltage runs down.
2) This FM transmitter does not have pre-emphasis so it will sound muffled when played on an FM radio. Like your stereo with its treble tone control turned all the way down.
FM radio stations use pre-emphasis (treble frequencies boost) and all FM radios have a corresponding de-emphasis (treble frequencies cut) to reduce hiss but allow audio frequencies to be perfect.
3) Its value for C4 is so high that frequencies above about 1kHz will not be transmitted.
4) It is designed for a very low sensitivity microphone so its input will be severely overloaded by the signals from your PC.

I fixed an FM transmitter similar to this one:
1) I added an RF amplifier transistor at its output so that its antenna is not connected to the oscillator's tuned circuit and its frequency does not change when something moves near its antenna.
2) I added a low-dropout voltage regulator so that its frequency does not change as its battery voltage runs down.
3) I added pre-emphasis so that it sounds perfect when played on an FM radio.
But like this one it is mono, not stereo.

There is an excellent FM stereo transmitter kit in Australia called The Micromitter. It was featured as a project in Silicon Chip magazine about 5 years ago. It uses the same IC as most MP3 to car radio transmitters use.

Just buy an FM modulator designed for IPods, MP3 players, and such. They are cheap and likely will work a lot better than anything you will build, especially if you are not familiar with RF layout requirements.

To get FM stereo you need a stereo modulator circuit as part of the FM transmitter.

Thanks for the reply audioguru. Here's a few proposed modifications based on your suggestions:
1) Remove the joint to the +9V line on the antenna. I think this was a drawing error by the original designer.
2) Run a 7809 regulator from the +12V line on a molex inside my PC to power the cirucit.
3) Change C4 to a smaller value - will 5nF do the job?

The two things I'm not so sure on are the RF amp transistor on the on the output and the pre-emphasis. I've pretty much completely guessed with the transistor.

http://i32.tinypic.com/2gwys9k.gif

crutschow - I'm doing this because I enjoy these kinds of projects, not just the end result. Thanks though.

Hi Burningmace,
Your changes to the transmitter circuit will stop its oscillator from working.
the coil must be from the collector of the oscillator transistor to +9V and the RF amplifier must be capacitor-coupled to it and be biased with a resistor.

Here is my FM transmitter circuit:

If you want to transmit stereo you might build a kit such as this FM Stereo Transmitter Kit | Distributed By MCM | FMST-100D (FMST100D).

Originally Posted by crutschow

If you want to transmit stereo you might build a kit such as this FM Stereo Transmitter Kit | Distributed By MCM | FMST-100D (FMST100D).

It uses the horrible-performance obsolete BA1404 IC that was replaced 6 years ago with much better ICs.