I2C Pressure Sensor
The BMP085 (Datasheet) from bosch sensortec is a simple, easy to use, pressure sensing device. It speaks I2C and includes a temperature sensor. Because of it's tiny size of 5.0mm x 5.0mm x 1.2mm it fits nice into the Freerunner. I glued mine into the empty space between the AUX key and the bluetooth antenna.
The sensor needs just four wires: power (3.3V, GND) and I2C (SDA, SCL). The wiki page about I2C describes where to get them.
An experimental kernel driver is available from gitorious. It searchs for the chip at I2C address 0x77 and exports a sysfs interface where you can read the temperature and the pressure as well as adjust the internal oversampling.
--Cmair 19:10, 30 September 2009 (UTC)