Comment by floxy
20 hours ago
Hmm, why not? For the astable configuration, you could use a 100F capacitor with R1 = 10 Meg and R2 = 7.5 Meg, for a 55 year period. Base current for the Threshold NPN will come from the Trigger PNP (and hopefully temperature drift matches OK). Other than maybe the 100F capacitor might have some variation in capacitance and leakage current over the course of 55 years ;-)
https://www.digikey.com/en/products/detail/tecate-group/PBLH...
Once you get above a couple meg, the oils on your fingers start affecting the resistance. Better hope no-one touches with your 55 year counter
at 1% initial precision of just eg. LM555 itself, you're looking at +-0.5 year just from the chip's contribution. If you ever find 100F 1% precision capacitor and 0% precision resistors, you may maybe hit +- 1 year if the other conditions are perfectly predictable, which they are not.
For your chosen capacitor, manufacturer doesn't even bother providing the tolerances. :) Good luck with that.