I'm doing a really simple project with an Arduino Nano and found a problem. The circuit is a simple temperature sensor and a tension read from an analog pin everything showed on a Nokia 5110 LCD.
When the Nano is connected to the USB everything works and the reads are perfect.
When I install the circuit on my car taking the 12V to the Vin and the GND from the centraline everything goes bad: I certainly know that the 12V is obviously not stable and oscillates a lot (between 12V and 14V).
What can be the solution? Should I add some ref or should I regulate the input tension to maintain it stable to a certain quantity?