What differentiates the circuit shown in the figure is the use of two inverter gates to trigger the transistor that controls the fan. In this way, the drive is made using an NPN transistor instead of a PNP transistor. The adjustment of the trigger point is made in the trimpot and again the sensor is a common diode for general use. We observed that, as the diode has a certain thermal capacity, its activation or response to temperature variations requires a certain time. For higher powers, transistors such as the TIP31 and power Darlingtons can be used. With 9 V or 12 V power, it is possible to use power MOSFETs. These drive transistors must be equipped with heat radiators.
