Note 2022: although still very useable, nowadays ofcourse it is very simple to make this with just a moisture sensor baord and a relay board from say Aliexpress or banggood (don’t use the standard moisture sensor itself that comes with the module, as that is crap)
A short while ago I published an automatic irrigator for garden soil based around an op amp and like usual, when pondering about it, one comes up with improvements.
One of the biggest issues with the previous design was that it does not have a sensor for a low water level, with the risk of your pump running dry if the reservoir is empty. Submersible pumps generally do not like that at all. But there is a simple solution.
S1 is a level switch that is controlled by a magnet or a floater and that closes when the water level is getting dangerously low. Connect the switch with a series resistor, parallel to the to the moisture probe, or, in layman’s terms: to the wires of the moisture sensor.
If the soil is dry and the pump starts pumping, as soon as the level will get too low, the switch will close and the mositure sensor will suddenly be ‘seen’ by the op-amp as having a very low resistance. The Op amp will therefore ‘think’ that the soil is wet enough and stop the pump.