There are many versions of DTMF decoders outside the Internet, but all are doing pretty the same - decoding dual tone codes in to digits, that can be later interpreted by microcontroller. This device uses an integrated decoder chip MT8870DE which decodes all 16 DTMF tone pairs in to 4 bit code.

AVR Microcontroller reads these codes and outputs them to LCD as characters. DTMF receiver stores last 234 received digits in EEPROM that can be viewed by scrolling an LCD character line. DTMF Decoder also can be connected to PC via RS232 interface. So each time TDMF signal is received it sends digit to Terminal. Program is written by using free WinAVR GCC compiler tools.

This project is developed by Bernard Debbasch and is a winner in Philips design contest 2005. it is really cool and flexible answering machine which allows many things including Media storage, internet access where messages can be sent in form of SMS or e-mails, interfacing to phone line, DSP routines. Answering machine(TAM-TAM) also supports multiuser interface where up to four users can have custom settings.

The whole machine is built around LPC2138 ARM7 RISC microcontroller. Project gives a good demonstration of DSP functions including FSK demodulation, voice playback and record, DTMF tone generation and detection. Concurrently there are also running IP stack and File system management tasks. And still, whole system gives good level of performance. Code blocks are written so that they may be easy reused in other projects. All project files are available for download.