Not many details on the humididstat. I used a cross grain cutoff from a spruce top, so it was about 8" long and maybe 1/2" wide, by 1/8" thick or so. I used Super glue to stick a strip of tinned brass stock, say 1/8" wide to the center of one surface running the length of the strip. One end was glued to a base block. At the other end I set up a contact with an adjusting screw where it would touch the metal strip as the humidity rose and the strip bent. This was hooked up to a battery that operated an old relay. When the relay was triggered by the contact it turned on a light that heated the air up in the box where everything was housed. As the air warmed up the humidity went down, when it got low enough the light went out I could put wood or parts that I was working on in the box to dry them out. A the time I was working in a basement shop that was impossible to dehumidify in the summer. So long as stuff lived in the box most of the time it stayed dry enough.
That setup was based loosely on the humidity gauge that my violin making teacher had. She took a strip about 2" wide off one edge of a sheet of plywood (so it was 8' long), and milled off one of the surface plies. The uneven construction caused it to bend with changes in humidity. She mounted it on one wall of the garage she used as a shop, so that one end would move up and down with changes in humidity. Whenever the local radio station announced the R.H., she'd check it, and mark the number on the wall at the end of the pointer if had not already been done. Eventually she had a pretty accurate gauge. Trevor Gore has talked about making gauges on this principle by gluing strips of wood together.
One issue with this is the one I ran into with my relay trigger: when the humidity goes much lower than what it was when the strip was glued on the shrinkage sets up a lot of stress in the cross grain strip. Eventually it cracks, and the thing doesn't work any more. Recently one of my students gave me something that might solve that problem. It's called an 'Old Woodsman's Weather Gauge'.
It's a twig, about 18" long, that has been cut from the trunk of a small tree, with a short section of the trunk wood. The bark has been peeled off. For reasons that will become apparent I know this one is some sort of hard wood. In life the twig grew out perpendicular to the trunk; parallel to the ground. That's important, too. In use, you nail the mounting section from the trunk to the wall, and the free end of the twig rises and falls depending on the R.H. When the humidity is low the twig points up, and when it's high it points down.
This works because there's 'reaction wood' in the twig. 'Cold creep' would make a twig that's is parallel to the ground droop, as you see in 'weeping' willow or cherry. To prevent this the tree builds up 'reaction wood' that has fibers that run at an angle to the length of the twig. This fights that tendency to droop, but also causes the reaction wood to change in length much more than normal wood; about 10-15 times as much. In the living tree this is not an issue, since the wood is saturated with moisture anyway. It can cause problems for us when we dry the wood out, though. At any rate, softwoods grow reaction wood on the lower surface of a branch, while hardwoods grow it on the top. That's why I knew this was a hardwood twig; it bends up as it dries out because the upper surface shrinks more due to the reaction wood. Although a band of very heavy reaction wood can cause a split in a piece this is abnormal: the cell structure is pretty well tied in, and, in this case, the whole twig can move to relieve the stress.
In theory, then, all you need to do is find a nice twig, set it up with the contacts you need, and calibrate it, and you could have a pretty accurate humidity gauge. In practice; who knows?