Stand alone digital thermometer is very easy to build – especially when microcontroller has an ADC interface built in. Tuxgraphics has posted a new cute thermometer project which uses two NTC temperature sensors. They are cheap and quite precise for most hobby projects. One problem that they are non linear. But this what microcontroller is for. Some basic semiconductor physics and math and characteristics is straighten.
Device reads temperatures from two sensors – inside and outside. Both temperatures a displayed on standard HD44780 compatible display and additionally information is sent to PC via I2C interface. Why I2C and not UART? It helps to save the hardware like MAX232 and multiple capacitors. Having connectivity with PC is great – you can always call a small GUI to see current temperatures or by running cronjob you can collect a long term statistics or even publish on the web.
We recommend EasyEDA for circuit design and PCB prototype