Please note: this is NOT a problem with code or project.
Connecting up a display to Arduino (5V,GND,TX,RX) messes up analog readings. Video demonstration: https://www.youtube.com/watch?v=tAP4Q5bPM0A
In the video, I am repeating analogRead(0) over each few seconds and printing results over to Serial. As soon as the display is disconnected from Arduino, the reading stabilizes and becomes correct. When display is reconnected, the reading of a temperature sensor fails again and is fluctuating within an error that translates to +-5 celcius.
Any suggestions how to fix this are welcome. This may be difficult to solve.
Also, external 5V was supplied rather than USB power which didn’t change the behavior.