Probably the best free set of plans you can get, come from none other than Bob Boyce himself. You can get his electrolyzer plans here:
Bob Boyce Electolyzer Plans

Here is a free hydrogen Course, with dry cell plans included:
Free Hydrogen Course

Thats about the extent of free hydrogen generator plans. That is why we include some of the best how-to hydrogen hho videos. Most hydrogen systems are very simple to build, and being able to see a video as its being built is worth more than plans, in most situations.