View source for 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.

[[File:Water_temp_difference_sensor_and-on-off-switch_for_solar-pump.jpg]]

The copper pipes transfer temperature changes with a lag and the volume of flow is small so the off/on power cycling to the pump is less frequent than you might surmise.
Also, my pump can run without harm at low voltages, and variable speed. So a sharp slope in the voltage changes by size of temperature difference is not needed for my pump. 

2025: I wanted a steeper slope of voltage changes for instantaneous off/on so I added a pre-amplifier:
[[File:water_temp_difference_preamp_NoRelay.png]]

Alternative configuration with a relay: [[File:Water_temp_difference_sensor_and-on-off-switch_for_solar-pump_relay.png|200px|thumb|left|relay version]]