Do bear in mind that if you resign as a director while remaining as a shareholder, you will be considerably adding to the responsibilities of the other directors - where a company have shareholders who are not directors, it is necessary for the directors to exercise much more caution and prudence in running the company than would otherwise be necessary - on the basis that the directors have a particular duty of care to any shareholders that do not have a direct presence in the boardroom, so to speak. Any failure to properly observe this duty of care may have nasty implications for them if anything goes wrong in the future.
Because of this, it should be only fair to the other directors to discuss with them your plans to resign, in advance of so doing.