Analyzing the circuit shown in the figure we see that the difference in relation to the others is in the use of a PNP transistor to amplify the current of the sensor element. When the temperature rises, the base current of the transistor increases and with that its resistance decreases. the result is an increase in voltage at the input of the inverter gate 4093. When a sufficiently high value for switching is reached, the output goes down and the transistor q2 is polarized in order to conduct the current. Under these conditions, the motor is powered, providing the desired ventilation for the system. Again, we use a 500mA transistor, but higher current transistors can be used. The adjustment of the trigger point is made in the trimpot P1. The sensor can be any common general-purpose diode. Observe the reader that in all circuits we always use a 1 k ohm resistor in series with the sensor diode. This component prevents that, in case of an accidental inversion of the diode at the time of assembly, it is not polarized in the direct direction without any current limitation. This would cause an intense short-circuit current that, passing through the transistor, would cause it to burn.
