Here is an example of a more useful homemade EV3 UART sensor. It is an IMU that uses an MPU-9150 breakout board that can be bought cheaply on ebay. I am usinh a cheap Arduino Nano clone.

The sensor supports 4 modes: Temperature, Compass, Gyro and Acceleration. The last three are all 3D, and have X, Y and Z values. you can program the sensor to do interesting things like on-board fusion of the gyro and acceleration data.

The USB cable in the picture is used for programming the Arduino Nano, and for diagnostics. When used in anger, the device is powered from the EV3.

The EV3UARTEmulation library is used to implement the EV3 UART functionality and the code for driving the MPu-9150 is based on that on the Arduino playground page.

This is the Arduino code for the sensor:

and here is the leJOS sensor driver class: