Our house has an outdoor PIR motion detector that turns on a couple of
lights. It works fine most of the time, but it fails when the weather
has been rainy for a couple of days. The symptom is that the monitor
LED flashes when it sees motion (so the PIR detector is still working)
but the lamps never come on. At first glance, it looks like the circuit
still thinks it is daylight (there's a CdS photocell to detect ambient
light).

My guess is that either the photocell has become moisture-sensitive, or
the circuit board has built up enough crud to provide a low-resistance
path across the photocell when it gets damp. Has anyone seen this
behaviour and fixed it before?

If the board is just dirty, cleaning it and perhaps adding some
conformal coating to the board should fix it. If the photocell is bad,
it will have to be replaced. Or should I just toss the whole thing and
replace with new? It's mounted high above the garage door, so it's
somewhat of a pain to get at, particularly when it is raining (and when
the weather is good, it works fine).

It's a Heath/Zenith model. There are a couple of mode switches on the
bottom as well as a sensitivity adjustment, so the case is not sealed.

Dave