I got my first home made computer in 1981. My uncle Arthur gave it to me. It was incredibly powerful and back then, it was science fiction.
My own projects were things like extra RAM on a ZX80 and then an external keyboard. I think this was in the early 1980s so I was about 10-12 IIRC, but these were projects that other people had designed of course, not my own. Maplins used to do a load of things like that for the early computers.
I have done plenty of Atari projects with the ST, TT and Falcons, still working on a few for my Falcon even today.. Or rather even these days.
My first ever true PC that I built myself was not that long ago really... I had bought a knackered 386 SX and based on that, I bought a Mobo, CPU, and re-built it, but soon afterwards built another from scratch and that as a 486 DX25 that got upgraded up to a 586/DX100 and then ever since I have built them all.
My mozt upgraded system, is my Atari Falcon. It started life as a 16Mhz 68030 CPU with only 1MB RAM and a 65MBHD, and it is currently a 120MHZ 68060 with 16MB+512MB RAM running off a 30GB SATA SSD and its running MiNT and this allows it to run basic Unix Apps alongside proper Atari Apps ( Kind of like being able to run Linux Programs in windows without emulation, or more closer to wine really ) and the graphics are now ATI 9200 running from one of its 4 PCI slots ( Another add on ).