Instead of going with a microcontroller he built the circuit around a 555 timer. What he really needed in this case is a flip-flop but lacking a chip for that he went with the 555 because it has ...

The urge to replicate the venerable 555 in nothing but vacuum tubes is understandable, as it uses little more than a pair of comparators and a flip-flop, circuits [David] has already built vacuum ...