The first part gets students started with Arduino while introducing basics principles of digital systems (Analog to digital converter, binary system, PWM mode, serial data communication, using sensors, voltage dividers ..).
The second part visits basics electronics components that used in digital circuits. Transistors, diodes, transformer, voltage regulator, AC to DC.
The third part is an introduction to logic gates and logic circuits. We visit gates, S-R latch, flip-flops, binary counter, shift-registers.
The last part covers some basics configurations of op-amp. Even if they are analog devices they are the basics block of digital circuits.
The idea was to cover the basics principles of digital electronics from a large scale (Arduino) to a small scale (basics components and chips) for a small budget. In the second part Arduino is used as a protoboard that provides HIGH or LOW. So there is no need to invest in more sophisticated protoboard since it is a simple introductory class.