draco_james said:
What will the new resistor do?
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...
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
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!