If you want to save the value of the variables during power off, the best way is to use EEPROM. This is a nonvolatile memory that will help save the value of variables and device settings. The I2C connection makes the use of EEPROM AT24C128 very simple.

Library and sketch: https://drive.google.com/open?id=1l99...

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Useful links:

EEPROM 24C128: http://ali.pub/1w3yh5
Display 1602 + expander ports: http://ali.pub/1vb6ja
Display 1602: http://ali.pub/1uo1ww
Micro SD Module: http://ali.pub/1tkx36
Arduino Mega 2560: http://ali.pub/1ny34r
Cheapest Arduino Pro Mini for crafts: http://ali.pub/1ny2z4
Breadbord: http://ali.pub/1ny3ae
Lintels: http://ali.pub/1ny3la
Quality crosspieces: http://ali.pub/1ny3j5

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