motion censors

Question

1. I have a tenant that is not religios(ham etc.). Leading to his entrace at the back of my house is a light which works by motion censor. My tenant repeatedly tells me that he really appreciates the light(it's extra dark going back there).Am I "placing a stumbling block" in front of him by causing him to activate this light on shaabos? 2. are motion censors a problem on shaabos being that it is definately going to turn on(obviously i'm only asking in a situation of benefit that i am happy with)?