Voltage Divider

see also:

http://hyperdramatik.net/mediawiki/index.php?title=Voltage_Divider_what%3F

https://www.kobakant.at/DIY/?p=6102

Voltage Divider

External voltage divider connected to Arduino

Internal voltage divider inside Arduino

You can use the following line of code to turn on the internal pull-up resistors inside the Arduino's ATmega328 chip:

pinMode(A0, INPUT_PULLUP);