May one cross at a sensor-controlled traffic light on Shabbat / Yom Tov. I.e. - the light turns green only when it senses there is a pedestrian? Would it matter if it would be dangerous to cross ...
Would there be a problem with leaving a recorder on from before Shabbat or Yom Tov, and thereby recording a lecture given on Shabbat itself? There is a wealth of literature on the subject of Melacha ...