“The Doorbell” is one of the first exercises in Robert Faludi’s book “Building Wireless Sensor Networks”. An XBee is attached to an Arduino to make wireless communication between the two boards possible. When the button is pushed on the button-board, it sends a signal (“D”) to the other board. This board, the bell-board, receives this signal and responds with the sound of a buzzer. The code you need for the Arduino’s can be found on Robert Faludi’s website: faludi.com/bwsn_book/code.