I have probably linked to this before on HN, but hey, love is blind: http://www.kilian-leonhardt.de/relaiscomputer/kurz6.htm http://www.kilian-leonhardt.de/rela

I have probably linked to this before on HN, but hey, love is blind: http://www.kilian-leonhardt.de/relaiscomputer/kurz6.htm http://www.kilian-leonhardt.de/relaiscomputer/kurz6.htm It's a 4-bit computer made out of 171 relays. It's got different 8 commands, an input register and 2 CPU registers, one of which is the accumulator. There's memory for 16 instructions (dip switches!). As output it uses LEDs on the registers, so you can for example program it to multiply 2 4-bit numbers, and read out the accumulator at the end. It clicks away at 10 instructions per second. The guy who built it also made a 1500 relais computer which is much more capable. You truly don't need much to build a computer - just a basic building block and enough ingenuity.