Sensors

The most obvious solution to measuring moisture in the soil is sending current through and then measuring the resistance of it. This however has a major drawback, sending DC currents through the soil will result in affecting the characteristics of it as we have effectively created an electrolyte. There are ways around this, one is trying to measure the capacitance of the soil and one is sending more AC then DC through it. I picked the later and based my work on the electrical theory of the SMX Sensor by EME systems.

Sensor islands

As I wanted to build a project with quite a few sensors and water controlled ports it was fairly obvious from start that building all the sensors one by one and connecting them to the main Arduino would not be feasible. This would create a wiring mess and an obvious lack of ports on the Arduino itself. For this reason I decided to build two sensor controllers and then connect 4 sensors to each controller. This will reduce the number of wires and ports needed on the Arduino quite a bit. I build the sensors into a small plastic box and then connected each sensor via a 2.5mm phono outlet. I wanted to be able to switch the sensors around (as I built them in different lengths).

Probes

The actual sensors going into the soil was a bit of a headache to figure out. Mostly since i wanted to do something pretty and nice to have in the greenhouse. I quite early decided upon buying a few plastic barn yard animals and using them as a base for the sensor probes, this as the idea of a cow and a zebra in the pots seemed quite ok. At the end, the probes became less sexy. I built them using a brass stick (I could get them fairly cheap at the local hardware store, 1m lengths 4mm in diameter) cut into lengths of around 12cm connected to a connection block. I soldered the wires directly onto the brass and then connected to a wire to a 2.5mm headphone male connector (that goes into the sensor island). For cabling I decided upon buying a couple of cheap and long headphone extension cords (10m long ones), cut them into pieces and solder my own connectors to them in various lengths. This after quite a bit of headache trying to learn what sort of professional wiring I should get that would be cheap and flexible enough.