This simple four channel temperature meter can be connected directly to your computer COM port and doesn’t require additional power supply. Check you computer back if there are any com ports before you start.
Temperature meter is based on ATtiny15L microcontroller which has built in ADC. Power for this circuit is taken from com port ER and RS signal lines. High level of these lines is from 6 to 12V and can supply up to 5mA of current. As L series microcontrollers are low power this is more than enough.
As temperature sensors there are four 103AT thermisters used. They give pretty good precision at room temperature (~0.3Cº). Author provides firmware for ATtiny15L and VBA project that logs temperature data to excel file. [via]
COM port powered temperature meter – [Link]