A recent episode of Stargate Atlantis prompted me to think about how would could design equipment that's intended to function far into the future. The episode required stuff to function 48,000 years after construction, but perhaps we could be less optimistic.
Say 1000 years.
Note, the requirement is not that the equipment function *for* 1000 years, but that when it is turned on, 1000 years from now, that it will work.
It seems to me that semiconductors are out due to effects of difusion and radiation.
But how about thermionic valves? They're not very reliable, but do they age when not in use? Would they hold a vacuum over that time?
Obviously electrolytic capacitors are a no-no, but can resistors and capacitors be made stable enough that they'd work?
Would it help to enclose the entire circuit in a vacuum tube? Again, could the tube sustain the vacuum over such a period?
An energy source is a problem. Perhaps a cell where acid is added (how?) at the appropriate time?