The Polly system needs to know whether it's dark or light, of course, and this is achieved by a little circuit with an ORP12 light dependent resistor measuring the light level. This is mounted in a plastic box with a translucent lid, fixed to the inside of a window.
This week the plastic succumbed to the effects of 21 years of sunlight and burst into a shower of dust. A true hardware failure!
With the aid of a hot glue gun I've installed the board in a new box and remounted it on the window. I suppose I ought to make a note in my diary to replace it again in the 2030s.
The electronic and software parts of the system continue to perform well, and as I type the uptime is 199 days, and counting.
P.S. You might think that when I wrote "a little circuit" above, that seems a bit vague. Well, to be honest I've no idea what's in there, and there are no surviving notes from 1993. Probably a 741 op-amp as a voltage comparator, I guess.