Abstract: A controller (30) for operating a pump associated with a liquid supply system. The controller includes a pressure unit comprising a housing (34a, 34b, 34c) having a liquid inlet (98) and a liquid outlet (39, 130). A biased (42) diaphragm (40) within the housing acts against liquid pressure within the housing between the inlet and the outlet. A control circuit (50) which includes a Hall effect sensor is mounted on the housing and is responsive to movements of the diaphragm (40) via a magnet (80) associated with the diaphragm. Thus the Hall effect sensor generates signals related to pressures within the housing from which a rate of pressure change is determined. A threshold pressure value at which the control circuit (50) is operative to switch on the pump is varied in dependence upon the rate of pressure change.