On Fri, 11 Jun 2010 10:56:13 -0400, Tony wrote:

wrote:
I've seen $250 humidity detecting fans (Broan?). I have a timer on my bath
fans, and motion detectors on my hallway lights (installed when my dad was
dying), so I can't believe they can't make a wall switch which detects
humidity. ANyone see one? Couldn't find one with google. Besides the
bathrooms, I'd wonder if I can't have one for the house when I'm away and
don't want to have the heat or cool on just to prevent mildew. I hadn't used
cooling in years and the upstairs duct perspired onto my ceiling.

I bought 2 army surplus one years ago, high quality stuff! I used it to
totally rebuild a donut proofing box because none of the original parts
were available. (the donut proofing box circulates warm humid air
around the raw donuts so they rise without drying out). A fan, a
stainless steel container like in a salad bar with a generic water
heating element installed in it, the humidistat, and a larger container
of water to siphon water in the small "boiler". Sorry I can't recall
where I bought them.

To *measure* humidity, the easiest I've found are the HIH-4000 series
from Honeywell. Vout is very nearly linear against percent relative
humidity (a second-order fit improves it a little at the cost of some
computations). Feed it 5 VDC, read with an ADC, simple.

For a setpoint, though, the GE HS12 or HS15 are cheaper. They must be
driven by AC (no DC bias allowed) and the impedance range is over 4
decades, so it is not just a simple ADC hookup. But just to get a trip
point reading would be fairly straightforward.

--
Rich Webb Norfolk, VA