The heart of the circuit is the LM35 temperature sensor which is factory-calibrated in the Celsius (or Centigrade) scale with a linear Degree->Volt conversion function. The output voltage changes with temperature from 550mV (-55oC) to 1500mV (150oC). This greatly simplifies the thermostat designed only need to provide a precision voltage reference (TL431) and an accurate comparator (A1 of LM358) in order to construct a complete thermostat.