On Sat, 17 Sep 2016 07:34:24 -0700, Larry Jaques
wrote:



Those only last a couple years, yeah? I've replaced quite a few
photosensors over the years in PIR lamp fixtures.

Didn't know that! We'll see, I guess.

The failure rate is ghastly, and invariably they short. Some last a
year, some 20, but in most cases, I've just turned the light on at
night & off in the morning. Often, I've moved to a PIR, and once the
sensor shorts, I just let it shine (5 min timer) the few times I pass
it in the day.

The marginally-designed Chinese 117-volt switch-on-when-dark modules
may fail, but CdS photocells are very reliable. The CdS photocell is
just a photoresistor that varies from a low resistance (a few K) in
presence of light to a high resistance (megohms) in total darkness.

The Chinese dark switches in lights probably use a triac to switch AC
line voltage. Triacs can get zapped by transients, and they'd fail
short when that happens. The remedy is to use higher-voltage triacs,
but that'd cost another few pennies. Designers and mfrs of traffic
controls never use less than 600-volt triacs, and they are protected
from transients with tranzorbs and MOVs.

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