This temperature sensor circuit is to control a small fan controller. An LM76 as temperature 'estimator' used in this project. To make it a little more interesting, an LPH7653 display to show the heat sink temperature and fan status.
This projects used an AT90S8535, but as the schematic shows, a lot of the controllers pins are left unused. You may want to select a smaller AVR. The choice will mainly be determined by the amount of program memory needed.
The use of the LM76 and the LPH7653 display is explained in the I2C chapter . Again, the MAX232 is used only to generate the (negative) display contrast voltage. The fan is switched with an N-channel Fet. Use a logic-level Fet that can handle the fan starting current and that has a low Rds-on, otherwise it might get hot!
The display shows the temperature in large format digits. When the fan is switched on, the display shows a slowly rotating fan:
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