Data transmission using FM microphone and FM radio

Hi ,

I've FM Mic and a FM radio .
I can use them to send my voice.

But can i use the same setup or slightly modified setup for data transmission ?

Say if i just connect Binary input at mic ?

If not ,then how can i do that ?

I want any four bit code to be transmitted via RF waves !
What others options can be ?

(Please don't suggest ICs that are not commonly available )

Thanks !

With your modest data requirements, you can use a two tone audio generator. No tone means no data. One tone means a data one. The other tone means a data zero. At the receiving end you can make two active band pass filters to detect the tones. Put a rectifier at the output of the filters and then comparators after the rectifiers. Be sure to use a simple RC filter after the rectifiers to get a DC level.

Another method is to use a tone generator with a different frequncy for each of your 4 bit data combinations. At the receive end, use a simple frequency counter.

You can probably send the data as a 1 bit digital word. +50 mV for a "1", -50 mV for a "0". The problem will be if the data rate is too slow or too fast. The microphone input is probably AC coupled, so if you send data at a rate slower than perhaps 200 bits per second, it will not come out like a square wave at the other end. If you send data at over 10 Kbits per second, the 15 KHz lowpass filter in the FM receiver will start giving you trouble.

At the receiver end, a simple ac coupled comparator op amp can recontruct the digital word.