Here it's configured as a comparator, and works better than an opamp because the IC 555 has built in opamps which are at par with any single opamp and also the output of a 555 IC is able to sink sufficient current in order to drive a relay without a transistor driver stage.
The above features particularly makes the above design very simple, low cost and yet too effective with its functions.
The pin#2 here becomes the sensing pinout of the IC, and is held at ground level via R2 which must be calculated as per the desired soil humidity triggering threshold.
The points A and B can be seen fixed inside the soil which needs to be monitored for the intended automatic watering from the water pump.
As long as the points A and B senses some level of humidity corresponding to a resistance value which may be lower than R2, the IC 555 output is held low, which in turn keeps the relay deactivated.
However as the soil tends to get dryer, the resistance across the probes starts getting higher and at some moment of time it becomes higher than R2, creating a potential below 1/3rd supply voltage at pin#2 of IC555.
The above situation instantly prompts pin#3 of the IC to become high, triggering the connected relay.
The relay activation switches ON the water pump which now starts pumping water to the particular area of the soil via a distributing water channel.
As this happen, the soil gradually gets wetter and as soon as the predetermined level is reached, the probes immediately sense the lower resistance and revert the IC ouput pin#3 to a low again switching OFF the relay and the water pump consequently.
C1 ensures a slight hysteresis in the operations ensuring that the relay triggering is not sudden or abrupt, rather it switches only after sensing a genuine response from the soil conditions.
The above explained automatic plant irrigation circuit was successfully built and tested by Mr. Ajay Dussa.
The following images show the prototype unit and the PCB design built by Mr. Ajay.
All resistors are 1/4 watt 5% CFR
R1 = 10K
R3 = 2M2
R4 = 100K
R2 = 1M preset or cermet
C1 = 1uF/25V optional for creating delay effect on the relay
Relay = 12V, 400 ohm SPDT
Supply input = 12V/500mA DC