Adafruit has announced the availability of a new temperature sensor and fan controller they have created in the form of the EMC2101 I2C. Priced at just $5.50 the fan controller and temperature sensor ...

In either case you want your case fans spun up, so which sensor do you tie them to? With Fan Control you can combine sensors with a function and then use the output of that as your source. So for ...

IIRC the extra pin on a 4 pin motherboard header may report if the fan has totally stopped, like a failure line, to tell the mobo to shut the CPU off. I've got an old 4 pin fan here from a '94 vintage ...