The circuit described here lets you control a cellar drainage pump so that it turns on when a preset liquid level is reached and turns off again when a different preset level is reached. The author investigated several approaches to the problem. Commercially available pumps equipped with float switches are not suitable as they are sometimes so powerful that there is a danger that their suction can start to cause movement in the ground beneath the building. A more reliable approach is used here. A simple circuit determines the level of water using a pair of suitably-spaced electrodes and then pumps out a preset quantity of water. The author has used this circuit over a period of ten years in a sump (a pit dug in the cellar) to detect the presence of any groundwater less than a set level below the cellar floor.
The circuit can be used in two situations. 1. It can be installed in a sump to keep the groundwater level more than a set distance below the cellar floor. In this case a pumping cycle can be designed to reduce the level by say an inch (perhaps a gallon). Because of the small change in level there is no risk of causing movement of the ground below the building.
2. When the heating or the boiler in the cellar must be emptied, for example to replace the sacrificial anode, the water can be drained into a tank and pumped from there to the garden: using this pump control circuit means that the process does not have to be closely monitored.
The circuit has been kept very simple, in the interests of giving good reliability. Gates IC2.A and IC2.B form a bistable flip-flop whose state is flipped by the two electrodes: and this is all done using a single, low-cost, CMOS IC. Power switching is done by a relay, which can equally well be used with either 12 V or conventional 230 V / 115 V pumps. The author uses both types: a 12 V marine pump as the primary pump and, as a backup in case of failure, a conventional pump. The latter is only activated when the water level reaches a higher threshold, which does not occur unless the primary pump has failed. The 12 V system is powered from a car battery (12 V, 70 Ah) which is trickle-charged. Two relays are hown in the circuit diagram, corresponding to two positions for relays on the printed circuit board with different pin configurations. Only one of the two possibilities is fitted. The three electrodes are made from solid copper wire with a cross-section of 1.5 mm to 2 mm with the PVC insulation stripped back at the ends. Electrode EL1 acts as a ground, EL2 sets the switch-off level and EL3 sets the level at which pumping is triggered. The circuit changes state when a current flows between these electrodes, which happens when water comes into contact with them. These currents, although tiny, lead to electrolysis of the electrode material, and so the electrodes have to be replaced every year or so. The amount of exposed copper on EL1 (the ground electrode) should be twice as long as that on the other two electrodes.
If an AC powered (115 V or 230 V) pump is used, particular attention must be paid to the galvanic isolation of the power supply, to the selection of an appropriate relay, and to the insulation of all wires carrying live voltages. Circuits at live potential should only be installed by suitably qualified personnel.