One can ask a non-Jew to do an issur d’rabbanan for the sake of a mitzva or great need (SA OC 307:5). According to most opinions using electricity (other than incandescent lights) is such a prohibition. Some claim that Rav Shlomo Zalman Auerbach even said that really there is no prohibition but minhag Yisrael is to be machmir.
So far as the extra weight question is concerned see “The Shabbat Elevator” by
Rabbi Yisrael Rozen (on-line). As for the sensor, if the non-Jew enters with the Jew and is first I would imagine that the Jew did not do anything (and the non-Jew did it for himself). See also “When May I Ask a Gentile for Help on Shabbos? Part II” by Rabbi Yirmiyohu Kaganoff (also on-line).