I have noticed that many lights outside homes are activated by a sensor that turns them on at night? Is there a problem on shabbos Friday night?
If the sensors are set in a manner that it is possible to pass by without activating them, and they are only activated if one actually approaches the house, then it would be permitted to walk by. This is because of the general rule that when a particular action might (but might not) cause the result of a prohibited labor, the action is permitted, and one need not be concerned for the potential result — provided that one does not have positive intention of causing the result (davar she’eino miskaven).
However, if there is no way to walk down the street without activating the sensor, and turning on the lights is an inevitable result of walking down the road, this rationale would not apply.
Yet, some authorities rule that because there is no intention to activate the sensors, and there is generally no benefit from them (this assumes that there is adequate street lighting without the lights), it is permitted to walk by even if the sensors will certainly be activated (provided there is no intention to activate them).
This leniency is based on a number of rationales. One is the ruling of the Yerushalmi (Shabbos 13:6) whereby it is permitted to close the door of a house, even when this would inevitably cause a deer to be trapped. The closing of the door, which is not generally defined as an action of trapping animals, is not considered a Shabbos labor on account of the deer (see Rashba, Shabbos 107a; Shevet Halevi 4:97). The same will apply to walking on the street – a person is “only” walking on the street, and the fact that the light will go on does not turn this into an act of labor.
[See also Taz (316:3) concerning closing a box that may or may not contain flies, which is also quoted a possible source for leniency. Even if the box does contain flies, which means that closing it will inevitably trap them, it is permitted to close the box. However, this will not help if one knows that there are light sensors.]
Another rationale is based on the fact that this is not the ‘ordinary’ way of switching on a light (this reasoning is arguable; one can claim that for this type of light, this is the normal way of activation). To this one can add that lights today generally don’t involve any element of “fire” (the bulb is not incandescent and there is no filament) so that there is no Torah melacha involved.
Therefore, it is permitted to walk in the street in spite of sensors that turn on lights, provided that there is no other way to walk. Rabbi Yosef Shalom Elyashiv, shlita, has been cited as ruling that it is permitted to do so when on the way to do a mitzvah–for example, going to and from shul or to the Shabbos meal.
For one’s own home, one must turn off the light before Shabbos, for otherwise one would be deriving benefit from the light that turns on.