Some time ago I developed this modules as add-on for cheap comercial radios. The modules were programmed each with different tones and then inserted in the radios in order to distinguish the talkers.
The board size is small enough to fit the mic unit and most of times inside a portble.
This module was developed for active_low PTT signal input, and active_high PTT signal output.
Basically, you signal the PTT to the module. The module do the TX, insert the tones and wait for you to release the PTT. Then the module insert again the tones before releasing the transmitter.
Operates on 3.3V or 8.2V. This is a simple project using AT80C851 and Bas51, SMT Board done in TraxMaker. The signal level was enough for me with the 100K output resistor, without amplifier.
I have developed also a simple Dtmf decoder with LCD and RS-232 output as add-on for the base station receiver. Of course you can use any DTMF decoder including that some VHF/UHF radios have the decoder built-in.
Maybe I will post later the decoder part.
Happy DIY.
if you Find the problem pleze tell me!