I have four of this soil moisture sensors: http://www.dx.com/p/soil-humidity-moisture-detection-sensor-module-blue-black-white-200142#.U9lkrfkhCbg Individually they work perfect. However, i wired all of them to the arduino board to have readings from different points of the garden.
I have a peace of breadboard with the electronics connected ones near the others, connected to the sensors. Then, a 6-wires cable send the power and signal to the arduino. This cable only use one 5v and one GND wires to power the four sensors. At the breadboard, all the 5V pins are wired together, and the GND too. But each out signal from the devices goes to a different signal wire in the cable.
The problem i detected is that the third sensor in the array does not returns "high" when iput it into water (for test purposes), EXCEPT is any of the others sensors also shows a "high". I don´t know if you see what i mean. If i put only sensor 1 into the water, the ligh of the board turn on. If i repeat this action for the other sensors, one by one, all of them works well except the third in the array. If i repeat the experiment but with two sensors inthe water each time, the third sensors also returns "high" when i put it into the water together with any other sensor...
I checked the wiring and the sensors (changing the sensor from one board to the other), and i have the same behavior.
There is not a code problem, because the board of the sensor works independently of arduino or any other device.
Do you imagine what could be the problem? Thanks!
P.D. I am out of home and i am not hable to post a picture of the wiring, buti think i explained correctly