A Rav told me (about light sensors in the street) that he once asked R’ Shlomo Zalman Aurbach zt”l and he paskened that “A sensor is a switch”, and it is assur. I asked the Rav what he would do if he realised he is in front of a sensor – would he stay there the whole shabbos – and he said that he would be soimech on the makilim (e.g R’ Shmuel Vosner in Shevet HaLevi).
(My Rav also paskened, based on R’ S Z Aurbach, that if you have a light in front of your house that will go on automatically, you must stay at home the whole shabbos and daven beyechidus!)