Air flow detection apparatus

An air flow detection apparatus useful in operating an accessory item in an HVAC system comprising:

• a sensor printed circuit board assembly having a first thermistor adapted to be exposed to an air flow and associated with a first pair of heating resistors, and a second thermistor connected in parallel with the first thermistor adapted to be hidden from the air flow and associated with the second pair of heating resistors;
• and
• a control printed circuit board assembly interconnected with the sensor printed circuit board assembly and having a voltage regulator circuit for applying a DC voltage source to a power regulator circuit and a sensor monitoring circuit interfaced therewith,
• whereby the power regulator circuit enables the first thermistor and the second thermistor to be heated to a target temperature greater than a highest specified operating ambient temperature and maintains the temperature of the second thermistor regardless of the actual temperature, and the sensor monitoring circuit responds to the cooling of the first thermistor in the presence of air flow to output a signal operating the HVAC accessory item,
• wherein the power regulator circuit includes a differential amplifier outputting through a first transistor to control the temperatures of the first thermistor and second thermistor.