Fixing module that randomly turns on
Credit: Steve Bloom - newsgroup post
The WS467 wall switch (and I suspect it's
variations, as well) does indeed "randomly"
turn on.
Specifically, under the correct conditions,
a WS467 can "glitch" on due to a power spike
from a large bank of magnetic ballast
fluorescents on the same circuit, large
motor, etc. And filter caps across the 78566
chip, resistor change in the "button" line
nor MOVs do not help.
The solution is actually quite simple, once
it is figured out. The 78566 chip in the
WS467 contains 2 unused pins (pin #8 and pin
#9) who's function is unknown to me.
However, manipulation of pin #9 can cause
the WS467 to turn on the light.
After discovering this, I have since tied
pin #9 to -v and the "random" light turnons
have stopped. My personal preference is to
install a 10K 1/8-1/4w resistor across the
top of the ic between pin #9 and pin #18. A
hard wire will probably be ok, but not
acceptable standard practice when dealing
with bidirectional I/O pins.