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Shabbat Parashat Pinchas| 5770

Ask the Rabbi: Sensors on Shabbat

Question: We are increasingly being exposed to movement sensors, related to security systems, internal and external light systems and the like. Is it permitted to pass by such sensors on Shabbat when one knows that his motion will be detected?


Answer: [The following is an adaptation of a responsum found in one of Eretz Hemdah’s books of sh’eilot u’teshuvot, Bemareh Habazak IV, 40, featured in our Hebrew weekly Torah dispatch, Hemdat Yamim, a few weeks ago.] 

One must distinguish between cases, depending on what results from his passing by and the different ways that one activates the electric devices. Certainly we cannot discuss every possibility and “before the ink dries” there are likely to be new technologies, but we will address some major applications.

It is forbidden to step on a mat or pass by a sensor that directly activates the opening of a door (Shemirat Shabbat K’hilchata 23:53). (In a footnote, he does raise the possibility that in certain cases, one might just stop a flow of light that keeps the device deactivated, but the above is the bottom line.) In these cases, one should wait for a non-Jew to activate the opening of the door and follow him in.

Sensors that are part of fire alarm systems are not usually affected by normal movement in the room but by smoke that makes its way toward the system. 

Regarding motion detectors on alarm systems that are used to notify that someone has entered the room, the best thing is to cover the system before Shabbat or have it work on a timer so that it is not picking up the movement during times of the day that people are meant to use the area. However, we are aware that this is not always possible.

It is important to know approximately how the system works. The system has a part that sends waves and a part that receives waves. There is then a part that analyzes any differences between that which was received during different times based on the movement of objects. The system can involve, among other things, the activation of a notification light and/or a sound alarm. (The alarm will be timed to not sound during times when people are expected in the building or room.) If the light is deactivated, there is much less of a problem because according to the standard approach to electrical devices, the connection of electrical circuits does not by itself involve a Torah violation. Under these circumstances one can more easily apply the concept of p’sik reishei d’lo nicha lei. In other words, the person who is detected by the sensor does not intend to affect the electrical circuits and even if he is certain to do so, he does not benefit from this outcome. According to many authorities, this is permitted regarding rabbinic prohibitions.

In the case where a light on the system will go on, the matter is much less clear, as this can involve a Torah prohibition. Yalkut Yosef (Shabbat V, p. 216) leaves the matter as an unsolved question whether one is allowed to walk in a place where a light will go on when he passes. Although a Shabbat prohibition results, the opinion of the Rashba, permitting closing the door of a house where a deer is inside (and getting inadvertently trapped), is relevant. Some explain that this is so because the action the person does is not related to the object of the melacha (in his case, the deer), in which case it would be permitted unless he intended for the result. The situation is similar for one entering a building and tripping a light. Yalkut Yosef cites Rav Wozner as saying that a person simply walking is even less of a direct act than closing a house’s door, which is an act of trapping under many circumstances. One could make the claim that our case is worse, as usually the people going into the area are those who operate the system, who might have in mind at times to check the system.

Thus, one should detach all lights. However, if he failed to do so and not being able to enter the area would cause an embarrassing situation or an inability to enjoy Shabbat on a basic level, he has a right to rely on the lenient approach.

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In memory of

The Rishon Letzion

Rav Mordechai Eliyahu ztvk”l


This week’s Hemdat Yamim is dedicated in loving memory of
R' Meir ben
Yechezkel Shraga Brachfeld

Hemdat Yamim is endowed by
Les & Ethel Sutker of Chicago, Illinois in loving memory of
Max and Mary Sutker and
Louis and Lillian Klein, z”l.

Dedicated in memory of
Mina Presser bat

Harav David and Bina

on the occasion of her yahrzeit, 24 Tammuz
and members of her family who perished in the shoah
Al Kiddush Hashem


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