Automatic Plant Irrigator Circuit Diagram . The circuit presented here waters your plants regularly when you are out for a vacation. The circuit comprises a sensor part built using only one op-amp (N1) of quad op-amp IC LM324. Op-amp N1 is configured here as a comparator. Two stiff cop-per wires are inserted in the soil containing plants. As long as the soil is wet, conductivity is maintained and the circuit remains off. When the soil dries out, the resistance between the copper wires (sensor probes A and B) increases. If the resistance increases beyond a preset limit, output pin 1 of op-amp N1 goes ‘low’. Automatic Plant Irrigator Circuit Diagram This triggers timer IC2 (NE 555) configured as a monostable multivibrator. As a result, relay RL1 is activated for a preset time. The water pump starts immediately to supply water to the plants. As soon as the soil becomes sufficiently wet, the resistance between sensor probes decreases rapidly. This causes pin 1 of op-amp N1 to go ‘hig...