We can use the circuit below to maintain the level of water in the header tank within prescribed limits. A 3HP submersible bore pump which has a high starting current is controlled by this circuit, necessitating a solid-state relay sufficient to take the starting load. The upper and lower water sensors in the tank are connected with two Darlington transistors, Q1 and Q3, in conjunction with Q2 and Q4. Q2 and Q4 act as a NOR gate and have common 5.6Kohm load resistor. The output of the NOR gate drives Q5 which activates relay RLY1.
Initially, both sensors will be open circuit when the water level is low, the NOR gate output will be high and the relay will be turned on. As a result, the normally closed (NC) contacts of the relay to open and disconnect the lower sensor. However, to keep the relay closed, the upper sensor will still be open circuit and the NOR gate output will be high. To operate the solid state relay RLY2, the normally open (NO) contact of the relay will be closed to run the pump.
The circuit remains on this state until the water reaches the top sensor which will then drop the output from the NOR gate to 0V. This will disable the RLY1 and the pump is stopped.
In practice the upper level sensor is just below the overflow from the tank and the lower sensor about half way up the tank. The contacts of the sensor are simply two stainless steel screws about 25mm apart and screwed through the poly tank walls. Neutral-cure silicone sealant protect the wiring junctions on the side of the tank. [Source: siliconchip.com.au]