Here is a Millivolt Source i built for Calibration in the early days. It uses only CMOS Digital and Mixed Chips from Intersil and CD40xx Series. Digital Potentiometer – del50003
This input module converts J, K Thermocouple and 4-20 mA Inputs to 0-2V Full Scale. These can be used for any voltage/current inputs too. The RTD module can be modified
This is a RTD Pt-100 Transmitter, It can Convert Thermocouple mV or RTD mV to 4-20 mA Current Transmitter. The output is a current source and not a current sink.
These are the circuits and boards of a Mini Temperature Transmitter for a Platinum hundred ohms temperature Sensor. The customer wanted a two wire system, this needs low power opamps
The easiest way to measure temperature is with a glass diode like 1N4148 or a metal Transistor like 2N2222. These have a small temperature range, probably -5 to +120 is