Just made this altimeter for my quadcopter, pretty proud of it! It uses a PIC12F675 to read a Motorola MPX4115A pressure sensor. It then transmits that data serially to an Arduino through a bit banging routine. I should cite this page [http://jbgizmo.com/page27.htm
] as the source for the circuit. I wrote my own code in C, instead of using his.