About 10 years ago I decided that I wanted an accurate digital altimeter and variometer that was compact enough to be worn on the wrist. No satisfactory commercial products existed at the time so I designed and built a prototype that I still fly with. While some of the more recent altimeter/varios that have come out show a higher degree of sophistication and integration than my old design, this unit still offers pretty good performance at relatively low cost for those who don't mind some tinkering.
For those folks I've written a description of building the altimeter and variometer components based on my design - along with a brief discussion of the theory of how the damn things work. Also included is a description of a very simple audio-only variometer I recently built that's based on the same design. This unit is still about as small as anything on the market (and is certainly cheaper) and may be the best bet for those looking for a quick and easy vario project.
Here are the schematics for altimeter and variometer sections of my original design, and for the complete audio-only vario . While I've included most of the details I can think of, if you have any questions or have any trouble downloading any of this, email me and I'll see what I can do.