[ars]

Microprocessors with tiny features are a chokepoint, but I suspect you could make a primitive one if you really worked at it, i.e. go back to 1960's technology, such as the AL1 or 4004.

Even going to 1925 and teaching how to make a MOSFET would help.

[Tade0]

Or disregard silicon entirely and use vacuum tubes combined with relays.

Not particularly micro, but a processor nonetheless.

[6510]

Batteries from wooden crates with newspapers, metal rods and is enough to do telegraph. That it is slightly harder ensures serious use.

If the drinking water is far away at the top of a mountain it isn't so convenient to throw your garbage in it, take a dump in it or float the dead in it.

[raverbashing]

BJTs are possibly easier. And can get you there

(a TTL 4004 would be an energy drain and would be hot, but it would work). I think CRAY used ECL

[fragmede]

nandgame and nand2tetris are important works of teaching for this reason. On the off chance I get thrown back in time to the exact right time for it to be useful, I'll be prepared!

[datavirtue]

I saw someone build a 4004 on a piece of plywood about ten years ago.