
There are two issues here:  copyright and patent.

The magazine article's contents are copyrighted, and may not be reproduced,
translated, etc., without the copyright holder's permission.  However, this
does not cover the *ideas* expressed, only the form of expression.  Copying
their circuit diagram or PC-board pattern is copyright infringement.  But
it's unlikely that they could stretch copyright far enough to claim that
the circuit design itself is copyrighted.  So long as you draw your own
diagrams and lay out your own boards, copyright shouldn't be an issue.

Patents are different.  The author does *not* have to give you any warning
that the design is covered by patent (although it would be sensible for
him to do so).  In fact, it's possible that *he* was infringing on someone
else's patent without realizing it.  Ignorance of the patented status is
not a defence against infringement, although it might reduce the damages
a court would award.

However... unless there was something seriously novel about the circuit,
almost certainly it is "obvious to one skilled in the art" and therefore
unpatentable.  Routine engineering is not patentable; patents (in theory)
cover only inventions, ideas that are genuinely new.

Caution:  I am not a lawyer.  Consulting a professional would be wise if
significant amounts of money are at stake.