The humble beginnings...

It is the first logic circuit I've ever made, it was time when I acquired the interest in electronic circuits and when I got seriously interested in ways digital / logic circuits work. I started from the very bottom, from transistors and resistors: learned how the transistors work and how to connect them to create logic gates. And then, how to connect basic logic gates to yield a more complicated functions.

This decoder was the first thing I came up with as a learning example.

The circuit uses Resistor-Transistor Logic.