Can anyone help....I want to build a solar pond heating system with a 12v pump controlled via two thermostats that will monitor the temp in the solar water butt & also the pond 9two zones). I want the circuit to switch on the 12v pump when the temp in the solar tank is higher than the pond temp & off when below. Cheers all !

 

Maybe this will do. Diodes work quite well as temp. sensors, can add more if more signal is desired. As leads might be long, advisable to add RC filter to comparator inputs, 4.7k's & 4.7μF, give or take. Might need to change motor drive if motor current is over 3A. For small temp adjustment, vary one of the 2.2k's.Water-proof the diodes. With TIP127, can increase 560 to 2.2k.