Have a look at my blog 'Paynters Palace' and search for posts with the 'I2C' tag - you'll find quite a bit of useful code, I think.
This project shows how to connect two Arduinos using SPI
This project shows how to connect two Arduinos using I2C and how to pass arbitrary data types between the two units
I agree with @Offer that using UART is the simplest solution to connect the devices.
I just want to suggest two alternatives:
Use an external analog to digital converter IC. Using the Arduino only for analog conversion is a bit over powered. E.g. the ADS1115 has a resolution of 16 bits and there's already a library, so usage should be easy.
The ESP8266's ...
As I see it, the most straight-forward way to connect 2 Arduinos is using their built-in UART (aka Serial port, TX/RX pins) communication.
There are many tutorials on this on Google, and a similar explanation can be found in this question:
Serial communication arduino mega and D1 Wemos Mini
However, there is a slight difference.
Arduino Mega has built in ...
The Arduino UNO WiFi Rev2 defaults to an internal analog reference of 0.55V where the classic (non-"Every") Nano defaults to the external VCC voltage as a reference, which is usually around 5V, often a little lower because of the diode it uses between USB and "5V".
Either you need to be using the same voltage reference by selecting a ...
Well had a bit of luck after spending all day on this.
Arduino UNO (sender): Like mentioned by chrisl, use Serial.write and not Serial.print
UNO Wifi (receiver):
So, instead of mySerial.read I used mySerial.readString.
This is most likely a obvious mistake to almost everybody here; ...