Basics Pull-up / Pull-down resistor Understanding why and how use pull-up and pull-down resistors was mandatory to build my first Arduino based circuit.
Basics NPN transitor in application How to use NPN to control large current which an output pin of an Arduino board cannot manage by itself
Basics Ohm's law in application The Ohm's law is the first rule to understand to be able to connect e.g. a LED to a power supply.
