Based on the newest design of the GBW Soil Moisture Sensor, we add a second "sensor probe" so that it can take readings at two different levels.
The center probe is the one actually read by the arduino, the two outer probes supply 5vdc and are powered only when taking an actual reading. The probe on the left, being the shortest, willpower up and the center probe read to get the level for the top of the soil, and then the right probe will be powered up, and the middle probe read, and that will be the level for the soil at the bottom of the pot.
The Dual Level sensor also uses an Arduino Pro Mini to act as an I2C slave device, turning this sensor into an I2C sensor. This allows us to make it self contained, and able to reply with two separate readings. It also means that a Sensor Module can support many more than 16 sensors on it's analog ports, up to 100 I2C devices can be used at one time.
DIY Instructions to follow shortly...