Here are some useful articles on electronics and microcontrollers.
Digital I/O expansion methods.
Find out how to use the AD9833 to generate sine, square and triangle waves at 0.1Hz resolution.
Arduino Nano ISP: How to program an ATmega328P using an Arduino Nano as the ISP programmmer. One common problem: Programming a sketch into the chip without a reset control - solved here.
Arduino Voltage reference: Both the Uno and Nano have a built-in voltage reference: Find out how to make it more accurate. In addition find out how to use it to measure the supply voltage 1V8 ~ 5V5
Arduino EEPROM: How to use it and How to preserve the life of EEPROM. Two examples sketches to save multiple values to EEPROM.
A tutorial on using the ADS1115 precision 16 bit ADC for low power use.
Learn how to use the TP4056 properly. There's a right, and a wrong way, to use it to safely charge Lithium Ion batteries.