>
> Early street-lamp photo-cell units were notorious for failing after
> only a few years.  My limited experience with these failures is that
> the failures were caused by excessive power dissipation within the
> photo-cell while driving the relay coil in low to moderate light
> levels.  Seemed to be much more of a problem in northern areas
> because of the slow change from bright to dim and vice-versa.


I took one apart, and was rather surprised to see about a 1" diameter CDS
cell, and a relay, in series.
No electronics at all.. I was wondering about dissipation, but I assumed
they had such things under control.

It certainly couldn't get any simpler :)
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