Turns water valve on whenever the signal from the moisure sensor is low or vice-versa. 

The whole circuit was designed to be connected to a 12V computer power supply.

The circuit is maninly constituted of two parts, a voltage divider suppling a maxium power of [email protected] to the moisture sensor. The output signal from the moisture sensor goes through a NPN NOT logic gate, which is then attached to the water valve (represented by a resistor). The induction effects of the water valve weren't taken into account. Experimentally, it didn't seem to cause much damage. The resistor in the NOT gate must be of a high rating or a parallel combination of several small power resistors. 

The water valve used has a maximum power rating of [email protected] turning off at nearly [email protected].