That brings back memories. I used the parallel printer ports of two MS-DOS computers to run my graduate thesis experiment.

I remember a little program called "aacircuits" for drawing those ASCII-art schematics, still useful for sticking a schematic fragment into a comment block in a program.

But I also remember almost burning the lab down. What is almost always lacking from descriptions of projects to control things with PCs is any consideration of safety, including what happens when electronic components or software fail. For instance a failure mode of relays -- mechanical and solid state -- is to get stuck in the ON position, leading to a run-away system.

I think that some sort of introduction to fail-safe design should be part of basic education.

I seem to recall one could damage a computer easily by interfacing through the parallel port without isolating correctly... Doesn't look like this guide covers the pitfalls.

There's a current limiting resistor and a back-emf protection diode across the relay coil, that's nothing fancy but it ought to be enough.

Ew. This guide is missing any information about isolation and safety requirements :(