This was the very first electronic circuit I assembled 40 years ago. Now, in 2020, I have designed it properly.
I hope it would help someone in the next generation to learn the basics of electronics more easily.
It is licensed under Creative Commons Attribution-ShareAlike (CC-BY-SA) license, which means you can use it for anything you want to, as long as you give me appropriate credit. In practice it means you keep the "https://pintergabor.eu" link printed on the PCB, and do not delete it from the documentation either.
As it is impossible to make only a few samples of these small cards, I have plenty of bare PCBs available in the shop for ridiculously low prices.