ARM Freephone Control

This project is built around an ARM microcontroller connected to the public telephone network. You can send commands to the microcontroller by allowing the phone to ring a specific number of times. A yellow LED lights up for 30 seconds after you let the phone ring three times and then four times on two successive calls. As the microcontroller doesn’t answer the phone, you can send commands to it from anywhere in the world, free of charge.