Temperature Difference Sensor and Switch for Solar Water Circulation Pump

ALL MY PROJECTS ARE HERE: https://opencircuits.com/index.php?title=User:Definitionofis

This is using two diodes to sense the temperature of the roof solar collector and the storage tank in the house. The diodes form a H-bridge which drifts +- through to -+ across the op amp inputs, depending on the temperature difference which varies the diodes' forward resistance. I have been running this for seventeen years with the diode on the roof touching the hot water outlet pipe.