This is a direct solar differential temperature relay to control a fan or pump. The first TL431 creates about a 5V reference voltage for the two matched thermistors. At the same temperature, the voltage at their junction is half of the 5V reference. A change in temperature between the two sensors causes the junction voltage to rise. The second TL431 controls the relay and turns on when the voltage exceeds 2.5 volts. That about 5V reference, which is divided by the two thermistors, can be lowered by the 20K pot. The insures a greater temperature differential must be reached before the relay turns on. When the relay is not on the circuit has a very low standby current of 5ma, ideal for battery systems. On it can consume 40ma, depending on relay. Pot should always be adjusted to some resistance. Near zero resistance, relay may trip due to small variation in thermistors. Pot resistance may be increased for higher differentials.
All parts are fairly common and can be scrounged from old electronics. The 12V relay should be at least 375 ohms, preferably higher. TL431 variations can be found in most old wall wart supplies. Anything with a 431 on the case is likely one. Construction cost is less than $5. Most resistances are not critical as noted. The resistor across the LED shunts low currents and prevents ghosting. The LM431 always draws some current and this prevents a dull glow.
Parts List
(2) 10K resistive temp sensors
(2) LM431 TL431 adjustable zener
(1) 12V relay 375 ohms or greater coil
(1) red LED
(1) 1-4.7uF, electrolytics to prevent false trips
(1) 470 ohm resistor
(2) 1.5K resistor
(1) 47K resistor can be any value from 22K to 68K
(2) 150K resistor, can be any two matched from 100K to 160K
(1) 20K pot, 50K could be used with slightly different range
