A switch for temperature differences can be done using electronic diagram below. The electronic circuit can measure temperature in two different places and, depending on the difference between them, to connect or disconnect relay.
Both the desired temperature difference and hysteresis can be set completely independently and do not influence each other. In addition, adjustments are almost linear potentiometers so can be easily equipped with an accurate scale. An LED indicates the relay activity.
As temperature sensors serve two LM335 type integrated circuits. The integrated circuit is equivalent: to a Zener diode, whose voltage stabilization increases by 10 mV / ° K. At room temperature (20 ° C), so the voltage measured 293 ° K-10 mV. ^ K = 2.93 V.
Since the temperature sensors have an input balancing voltage calculated above can be adjusted easily.
Voltages of sensors are compared to each of the op amp IC1. This integrated circuit is connected as an comparator. If Z1's temperature becomes higher than that of Z2, then the LED lights and relay connects. If P1 is turned up to zero, then we can just switch to a voltage higher than the Z1.