I did something similar with a little low power transmitter board, and I used this circuit to put it all together:
The resistors load the stereo output of the MP3 Player or PC Sound Card. Mine was used for a sound card line out at 600 Ohms, you can use lower Ohm values for headphone jacks.
(thanks to MRAM for pointing out my error below – this red text show the previous WRONG INFO: I suggest the power blocking diode (any low voltage (6-25v) unit from the junk box should work)
CORRECTED INFO – a power blocking capacitor is suggested by a more skilled contributor, as follows
With the low value of input resistance, a larger electrolytic
would be in order. The input resistance along with the capacitor forms a high-pass filter which if the cap is too small would limit lower frequencies.
As a passive 1st order RC filter, the cutoff frequency would be calculated using the capacitor value and the input resistance of the transmitter audio input as
f=.159/RC
Solving for C you get
C=.159/RF
where C is capacitance, R is input resistance and F is the cutoff
frequency.
In your case, about 4.7 microfarads should take the frequency down to around 50 Hz with an input resistance of 680 ohms. Of course the actual audio circuit of the transmitter would affect this calculation.
Make sure polarity is observed unless you have a non-polarized cap.
(because electret mic modules often receive a low voltage DC input to power the microphone, and you don’t want that wandering up the line into your audio device. Older units had 3 leads – audio, power and ground – but newer units take power and audio in the (+) terminal and separate the two internally.
And Tomi’s circuit linked above, repeated for reference:
Experimental broadcasting for a better tomorrow!
