Melody,
Are you certain you want to build and install this? Stating that you don't know what a humidistat is, how to obtain one or how to obtain a basic exhaust fan gives me the impression that you might be better off purchasing what you need and/or considering a different approach.
The exhaust fan approach works well only when the outdoor humidity is acceptable. Even then, you are exhausting indoor air which you may have paid to heat or cool. Whenever your exhaust system is running, you need to keep a window cracked open somewhere to relieve negative pressure in your house. Otherwise, you can greatly increase the radon levels in your home since the negative pressure will draw radon from the basement floor (this is especially bad if you have a sump pump or you have cracks in the basement floor or the basement walls.)
If your windows are open, then your device will give you the same inside humidity as the ambient (outdoor) air, which may not be desirable. If you are running AC, then you are getting the air dehumidified automatically by the AC and you probably don't need your device. If you are running the furnace, then you can also run a $100-$150 dehumidifier, which has automatic controls. If needed, you can also use a dehumidifier in a problem room while running the AC, if needed. (Usually, low humidity levels are the problem in furnace season.)
You seem intent upon reinventing the wheel but you don't sound like an experienced inventor. Buy what you need and avoid a lot of hassle. I know how to acquire the materials and build what you want, but I'd never consider wasting my time when the $100 dehumidifier would work better and be a much easier solution. If the initial cost was a concern, then I'd look for a dehumidifer in a garage sale, or better yet I'd grab one of the many that sit on the curb on trash day. One half hour cleaning the coils will usually restore one to excellent operating condition.
A dehumidifer uses more electricity per hour than an exhaust fan, but the dehumidifer is going to run a lot less than a fan set to run automatically by a humidistat which isn't aware of the ambient humidity. When you consider the heated or air conditioned air which is wasted by the exhaust fan, then the dehumidifer is going to be much less expensive to operate.
FYI - my next door neighbors have a humistatically controlled basement exhaust system that was installed by a basement waterproofing company. It is extremely rare to walk by the house and not hear it running. They paid a small fortune to have it installed (about $1000 !) and they are continuing to pay dearly as it runs even when the furnace or the AC is running. That's like leaving a front and back window open when running the furnace or the AC.
Good luck, Gideon
=======
My house suffers from too much humidity. The wood floors my DH installed (his trade) are beginning to buckle. So the situation has now become somewhat critical.
As I understand it I should build able to build basically an exhaust fan that is controlled by humidity.
But I have no clue exactly how to go about it. I intend initially to fit it into a window (similar to the way a window air conditioner is installed in a window). I don't know where to obtain the fan (get a bathroom exhaust fan and rip the guts out?) and I definitely don't know where to get the humidity control.
Any help?
Take care, Melody