How do I make references to my own products (eg. circuit boards) or libraries, without having my post flagged as spam or self-promotion?
The link or mention should be relevant to the question
You must disclose your affiliation. For example: "My web site xyz.com sells boards that do what you want".
Links to external sites are OK
Provide context for links
Links to external resources are encouraged, but please add context around the link so your fellow users will have some idea what it is and why it’s there. Always quote the most relevant part of an important link, in case the target site is unreachable or goes permanently offline.
Note that answers should be, as far as practical, self-contained. An answer that just has a link in it will be useless if the target site goes down, possibly permanently.
Stack Exchange is supposed to be a site with answers to questions. Not links to answers to questions.
In some cases (eg. libraries) this may be impractical. In that case hosting your library on a site that is likely to be around indefinitely (like GitHub) is preferable.
Avoid overt self-promotion
Don't talk about your product / website / book / job too much. Folks will read your answers for their ability to solve a specific problem; if you're good at doing that, then they'll find themselves more interested in who you are and what you're working on. If you respond only to questions where the answer can be something you're selling, they'll assume you're just here to sell.
Don't include links except to support what you've written. Links are not a substitute for including information in your answer itself, and links should always be directly relevant to a part of your answer.
In other words, if every post you make references your web site, and your web site is selling something, or is full of ads, people will think you are only here to drive traffic to your site. Avoid that by making most of your answers stand alone, and only refer to your own site where this is directly relevant.
Avoid appearing to force people to pay for your answer
If you answer a question, and the answer includes a library, and you link to that library (and it is required to implement the answer) then the library must be freely available, and not hidden behind a paywall or requirement to divulge personal information like an email address.
If you solicit a donation for downloading the library, then it must be made clear that the donation is not required. For example: "Enter a donation amount of $0 if you do not wish to donate".
How to solicit donations
On your own website, preferably separate attempts to make money from the information on your personal site. For example, have a separate "Donate" button (eg. using PayPal) which is not required to be used before downloading your library. Or suggest people support you on Patreon or other similar sites.
Of course, you can run your own web site as you see fit. But links from Stack Exchange to a site that does not follow these guidelines may be flagged as self-promotion or spam.