We’re all familiar with IVRS systems that let you access information using a touch-tone telephone. [Achu Wilso] built his own version which uses a cellphone, microcontroller, and computer.
The cellphone is monitored by an LM324 op-amp with an attached 555 timer chip. When a call comes in the voltage on the headphone output goes high, activating the timer circuit. If it goes low and does not go high again for about 25 seconds the call will be ended. Each incoming touch tone acts as a keepalive for the circuit.
An MT8870 DTMF (touch tone) decoder chip monitors the user input. An ATmega8 microcontroller grabs the decoded touch tones from that chip, and pushes them to a PC via USB. The PC-side software is written in Python, using MySQL bindings to access database information. eSpeak, the open source speech synthesizer software is used to read menu and database information back to the caller.
Not a bad little system, we wish there was an audio clip so we could hear it in action.