Help modifying an FM transmitter circuit

hi, I had found this diagram for a small FM transmitter, and I was wondering if there would be a relatively simple way to change the mic for an audio jack so I can plug it into my mp3 player to listen to songs on my car audio...

I looked at some of the transmitters on this site, but I think I'm going to go with this one, as I find it simple, and only need like 10 feet range (not even probly...)

Is there a way I can modify this circuit to have an audio jack rather than a mic.?

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Thanks for any help

rip out the mic, connect the base of the transistor to a capacitor. Now connect the other end of the capacitor and the -ve to your MP3 player speaker output.

k, so the +ve of the speaker output just goes to the other place that the mic connected? also, any certain capacitor in particular?
Right?

Also... do you think this could cause any damage to my mp3 player? (i don't think it can... just checking...)

-=EDIT=-
like this?
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Since the 15k resistor won't have the current of the microphone in it, increase its value to 27k.

A 0.22uF coupling capacitor from the MP3 player is fine. You should connect both stereo outputs from the MP3 player each to a 47k series resistor. Connect both 47k resistors to the coupling capacitor and a 0.1uF capacitor can be used. Connect the grounds of the MP3 player and transmitter together with the shield of audio cable and the audio signal inside it.
It is not necessary to connect the MP3 player's output "backwards". Just connect its output to the coupling capacitor added to the transmitter and connect the grounds together.
Keep the output level of the MP3 player very low because the transmitter's input is very sensitive.

That FM transmitter is so simple that its radio frequency will change all over the place if you get near it, if the battery voltage changes or if the temperature changes.
Since it doesn't have pre-emphasis (treble boost) like radio stations have and radios have the opposite, reception from it will sound muffled without treble frequencies. :lol:

ummm... could you perhaps draw up a quick diagram of that? sorry, I'm quite new to electronics like this and I'm having trouble following that.

Also, "connect the grounds of the mp3 player and transmitter together with the shield of the audio cable and the audio signal inside it" I'm not sure what your saying there... and what ground from the mp3 player?

would this one be more interference proof?

**broken link removed**

and if it would, could you tell me how to make it plug into my MP3 player?

Hi Draco,
Your 2nd FM transmitter circuit has all problems the same as the other one.
I added a 10k resistor to this diagram of the 1st one:

Thanks alot for the diagram, greatly appreciated.

What will the new resistor do?

Anyway, I'm really short on cash, and I'm doing this more for the learning experience than actually using it (although I probly will use it...). Also, I've read that you have to use ceramic capacitors for transmitters like this... just checking if thats true...

I'm also just curious as to why the freqency would change all over the place so easily

draco_james said:

What will the new resistor do?

Click to expand...

It attenuates the input audio signal so that the level on the MP3 player won't have to be turned so low.

I've read that you have to use ceramic capacitors for transmitters like this... just checking if thats true...

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Yes, for the RF capacitors such as C1 and C3. The audio coupling capacitor and the trimmer can be plastic.
Also, it won't work if built on a breadboard. These VHF circuits must have very short wires.

I'm also just curious as to why the freqency would change all over the place so easily

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The tuned circuit of L1 and C2 and anything capacitively coupled to them determines the tuning radio frequency:
1) The antenna. Anything near it changes its capacitance.
2) The battery voltage. The transistor's capacitance changes with voltage so that it performs frequency modulation. When the battery's voltage changes then so will the transistor's voltage.
3) Temperature. The transistor, coil and capacitors characteristics change with temp changes.

My FM transmitter project works perfectly. I used an RF buffer amplifier transistor to isolate the RF oscillator from the antenna. I used a low-dropout voltage regulator so that the 9V battery's voltage can drop to less than 6V before the RF frequency is affected. I used low distortion circuitry and pre-emphasis so that the sound is superb.
It goes more than 2km though, which is too far for your application. The day after another guy built it, he saw the RF cops in their truck with the antennas on top slowly go down his street. I told him they were looking for me!

Ah, I see, I knew about the breadboard, but thanks for that, and the other info.

Just one more question... for the sheild around the audio cable right where it comes out of the audio jack, what is the sheild? like do i buy that, or is it just like a metal tube or something?

draco_james said:

for the shield around the audio cable right where it comes out of the audio jack, what is the sheild? like do i buy that, or is it just like a metal tube or something?

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Strip the outer insulation off an RCA to RCA cable and you will see the outer shield surrounding the cable that is usually braided copper strands. Cut the cable and you will see the shielded conductor running down the center which is also copper strands.
In your case you could purchase a stereo shielded cable with a plug that fits the jack on your MP3 player and two RCA plugs on the other end. You could connect two RCA jacks at the input of the FM transmitter that you connect to the 47k resistors and to ground. Or you could cut off the RCA plugs and connect each inner conductor to each of the transmitter's 47k resistors and connect both shields to the transmitter's ground. :lol:

I've read that you have to use ceramic capacitors for transmitters like this... just checking if thats true...

Click to expand...

Yes, for the RF capacitors such as C1 and C3. The audio coupling capacitor and the trimmer can be plastic.
Also, it won't work if built on a breadboard. These VHF circuits must have very short wires.

Click to expand...

Ceramic capacitors are good for RF work. I also got a transmitter working on a breadboard, but he says don't use one because the breadboard seems to alter the (or create) capacitance in some sections of the circuit, which in turn alters the frequency, and even the entire operation of the circuit. And avoid wires and long tracks as much as possible (except for the antenna).

thanks alot for the help, when I am able to get all the required parts and I build it, I'll let you know how it goes... (the one store in my town that sold these types of parts WAS radio shack, and when they changed to "The Source by CC" they stopped stocking them

Hey audio, will your transmitter have the pre-emphasis so as to have good sound quality? I did what draco_james is going to do, and it worked, accept for the change all over the place in frequency, and the sound quality was crap.

Ive always wanted to build one that does work so that I can play my mp3 CD player.

Hi Zach,
Of course my FM transmitter project has pre-emphasis so it sounds superb! :lol:
Its received audio frequency response is flat to 15kHz, even better than some old FM radio stations.
I even have it selectable for the different amount of pre-emphasis in other parts of the world.

The Americans started broadcasting FM in the world, and decided to use a huge amount of pre-emphasis treble boost to reduce hiss in the noisy radios of the time. When the Europeans started broadcasting FM years later, audio equipment was improved and had more and better high audio frequencies that caused over-modulation if the pre-emphasis amount was high. So they decided to reduce their pre-emphasis amount and the US is stuck with theirs.

Therefore FM radios in different parts of the world are also different. Play a European FM radio in North America and it sounds too bright. Play a US radio in Europe and it sounds dull.

cool, im gonna definetately build that :lol:

Hi Zach,
Mine is mono and powerful enough to cause trouble.

I saw a Stereo Micromitter made by Maxell on sale in a store, for less than half the cost of the Australian kit. I was tempted to get one and remove its RF attenuator to see how far it goes. :lol:

Hi again, I haven't been able to get parts to build anything yet, cause the last place in town to sell components was Radio Shack, which became The Source, and no longer sells components... anyway, because of the delay, I was thinking I might build your transmitter instead Audioguru, but I was wondering how I'd go about making the range small enough to be legal (and how to change it to use audio input from MP3 :wink: ).

Also, just wondering if you could give me an estimate of how much it cost to get the parts needed to build it (and what is the box with I O on it... I'm really new to this, I know the basics of circuit diagrams and stuff from school, but haven't really made any cicuits myself before)

draco_james said:

I was thinking I might build your transmitter instead Audioguru, but I was wondering how I'd go about making the range small enough to be legal

Click to expand...

Adding an emitter resistor to Q3 should reduce its range.

and how to change it to use audio input from MP3

Click to expand...

I removed the mic and R1 to test it. I made an input attenuator with two 47k resistors from each channel of my music source to a 3k resistor to ground. Of course the grounds were also connected together.

could give me an estimate of how much it cost to get the parts needed to build it

Click to expand...

I have no idea how much it costs. $10.00, $15.00? I already had the parts.
Look them up and order them online at www.digikey.com or www.newarkinone.com .

what is the box with I O on it

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The LM2931A5.0 is a low-dropout 5V voltage regulator IC that allows the circuit to continue working well when the battery voltage drops to about 5.6V. I = its input and O = its output.

I'm really new to this, I know the basics of circuit diagrams and stuff from school, but haven't really made any cicuits myself before{/quote]
Circuits that operate at these very high frequencies need a pcb or a very tight Veroboard layout. Perhaps it would be best for you to make a kit as your 1st project. :lol:

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meh, I'm sure I can figure something out, and thanks a bunch for the info

by the way, do the caps have to have a specific voltage rating, tolerance etc? or for tolerance is it just lower is better?

also, could you tell me which caps have to be ceramic in the diagram you posted?