They also only implement serial sending (so the Arduino receives) so far, although they mention the next step would be to build the other side of the connection.

An Arduino Uno provides the processing power and drives the serial monitor. A joystick and a Hitachi H48C accelerometer are mounted on a breadboard and wired to the Uno.

Connect the Arduino to your computer using the USB cable. Open the Serial Monitor in your IDE to see the values from the sensor – this will also help you to calibrate your plant monitor.

He uses a cheap, $12 sensor with an Arduino and a Radiation shield and monitors the data on his computer.