Automatic fan control even in moisture prone conditions. Precisely modulate the speed of ventilation fans in response to changes in ambient temperature. Optimum air circulation is maintained in ventilated areas and buildings with minimum noise and efficient use of energy. A thermistor monitors the air temperature and, as the temperature rises or falls within a preset band above set point, the fan speed is correspondingly increased or reduced between maximum and minimum speed values. Especially designed for agricultural buildings, although of broad application in all ventilated areas, these fan controls are compact units designed to fit on a standard 4x4 junction box. Construction protects the electronic controls against dust and solid state design ensures long life and reliable operation.