There is a National sensor that works like that (more or less), the LM35 I
think.

Your problem is to detect the -2.2 mV/K signal from the sensor which will
have to be calibrated individually. Connected directly to a 10 bit A/D at
5V supply you can only read about 1 deg. C. Maybe this is enough for you,
else there are schemes to make the sense transistor an amplifier too. They
are complicated to get working other than in ones (i.e. calibration
problems).

Peter

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