Herbert, you can make a humidity sensor by taking a long human hair (over
15 cm long), and suspending it in a tube using a small weight (1 gram is
enough). By polishing the bottom of the weight you can use a 'fork' type
optocoupler to read the height of the weight. I'd do this out of 18mm PVC
installation tube with some holes in it, optocoupler at the bottom and a
plug with a screww in it at the top to adjust the height of the weight.
The weight could be a small piece of aluminium or brass wire of 2 mm
diameter (the hole in the 'fork' is often 2.5 to 5.1 mm wide).

Peter

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.