Can I open source something that has a patent.?

No, II don't think so. But contact the owner, they may grant you a license for a nominal fee.

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1. You can legally show people how to build the item -- for example by providing schematic diagrams etc. -- but only if you give away the plans for free. You cannot legally profit from selling the plans.

2. Concerning a kit of parts: You can legally sell a kit of parts, provided that none of the parts is proprietary. The kit must contain parts only, with no plans or directions for assembly of the finished device.

3. No one can legally sell a device that they build from this set of plans. They can offer the device at no charge, but they can't sell it.

In conclusion: The basic principle for patents is: Fair use by private individuals, but no commercial profit.

Information of patent is available to the public. To use the patent in a product, you have to ask permission from the patent holder. Otherwise, you are infringing the patent. Patent right belongs to the first person that is granted the patent. Once a patent is granted, it is difficult to get the patent even you have the idea before existing patent holder.

It might be an urban myth but I frequently hear that big companies engage in "dog-in-the-manger" patenting so as to keep inventions off the market if those inventions would take away, market share. An example is the idea that oil companies own all of the good patents on electric cars and that pharmaceutical companies own all of the good patents on "cures". You might not be able to do what you want to do, without infringing the patent. Maybe a lawyer will post an answer.