Water pump control for solar power system

In most small solar power systems using a boiler it is required that the water circulation pump is not switched on until the temperature of the collector (the solar panel) exceeds that of the water in the vessel. Here, a two-sensor monitor is presented that enables this condition to be met. One sensor is fitted on the collector, the other on the water vessel. The control shown here has two adjustments: one for the temperature difference at which the pump starts to operate, and one for the temperature difference at which it is switched off. Although these settings are independent, the switch off level must be lower than the switch-on level. Calibration in degrees Celcius is simple because the gradient of the voltage at the wiper ofthe potentiometers (orpresets) that set the orr/off ternperatures is exactly 0.1 V °C^-1.