In other words I would give you the set point and at that temperature it would turn on the fan and when it drops below the set point it turns off the fan.. the temperature would have to be measured using a contact type measuring device like a thermocouple or thermistor.
Mail from IS Brazil
See Craig's thermostat circuits
Try this One Chip solution from Maxim Semiconductor - MAX6641 temperature sensor and fan controller -
"Accurately measures the temperature of its own die and the temperature of a remote pn junction. The device reports temperature values in digital form using a 2-wire serial interface. The remote pn junction is typically the emitter-base junction of a common-collector pnp on a CPU, FPGA, or ASIC."
Some of my circuits are here Industrial Process Control Circuits