I designed a simple sensor to measure and log light levels and it has mixture of open-source and not-so-open-source hardware components.
Do I need to be worried about how I describe the device or its design? I'm asserting that the device is open source hardware and will publish all the design files (BoM, code, schematics, gerber files, etc) under a CC-SA license. I consider the device to be open source hardware, but do I need to be careful how I describe it to others? Perhaps I don't want to assert wholesale open source status, but don't really want have a drawn-out qualified statement discussing each component either. Is there a bit of a tightrope act here? ... can't be unique issue - after all, most Arduinos and other devices are based around components that are proprietary, right? No one is making chips from scratch.
For additional detail, Sparkfun components (Arduino Pro Mini, microSD card socket BoB, and Dead-On RTC) make up most of the components (I'll attribute).
Open source hardware is an interesting thing. Open source software is much more easily defined. Can I see the source code, if yes it is open source. In the software world I believe you could call something open source even if it uses libraries or drivers that are closed source, but that usually needs to be clearly stated. I don't think you would get in any legal trouble unless you leaked the source code for a closed library and claimed it was open source, but that would first require you to find that carefully guarded source code.
To abstract this to hardware I would give three requirements.
Publish a schematic / connection diagram
Publish a board layout or other production layout document
Publish a parts list
You could never be expected to make each part on the parts list yourself, so don't worry about whether they are open source; it doesn't matter. If you can buy it as one entity it goes in the parts list. For hardware it is assumed that what you are open sourcing is how you put these available parts together, not everything from the ground up.
From a strictly legal view I would think you wouldn't have anything to worry about unless you are giving out proprietary information from others.
I don't see anything to stop you publishing details of your circuit design, layouts, masks etc, as long as it's your own work. In the likely case that your circuit uses some commercial components you would just specify them by type / part number. As long as you don't reverse engineer the components you've bought in and publish proprietary information about their design, I don't see why there would be any problem.
Why don't you describe it as "open source using some commercial hardware components" then in the parts list specify the manufacturer and partnumber of any commercial components.
The manufacturer will be pleased because you are advertising his parts.
The person using your device has been warned there are commercial components and by looking at the parts list can decide if he is happy with what is being offered.
As long as you don't reverse engineer the components you've bought in and publish proprietary information about their design,
No, in the UK it is perfectly legal to reverse engineer anything for the purpose of compatibility.
Also there is no legal bar to publishing proprietary information providing you were not employed by the company who's information it is in which case there might be a gagging order in your contract of employment.
I was sent on a legal course by one company that employed me and that is what they told us.
I am not sure what you mean by:-
not-so-open-source hardware components
Are you talking about something like an RF module? That is not a bar to making your design open source. It is the design that is your property no matter what parts it uses. If those parts are on the open market then you are free to do what you want with your design.
In general there are two ways to protect IP (Intellectual Property). These are patents and trade secrets. In the case of a patented device they have already disclosed how it works, so there is no danger of you doing so. There is legal danger in building your own and that is the point of the patent.
Trade secrets on the other hand are things the company tries to keep secret, but should they get out by legal means (reverse engineering, independent discovery, etc.) there is nothing they can do about it.
The best ways to get in trouble for disclosing information would be to steal information from a company and disclose that, or break a signed NDA (non-disclosure agreement) with a company that had entrusted you with information. As a typical or even atypical hobbyist these things are very unlikely, so I don't think you have much to worry about. Also in the grand scheme of things no one is going to take you to court unless you really piss them off or have a LOT of money. If either of those things is the case you should have a good lawyer by now.
There is legal danger in building your own and that is the point of the patent.
Only if you try and sell them.
A patent only gives the holder the right to take legal action to recover any losses that a patent infringement has cost them. If you only make one then that is the cost of one.