If an electric alarm clock goes off on Shabbat, one may not turn it off, because doing so involves the use of electricity. If the noise is disturbing, the clock may be wrapped in blankets and moved where it will not be heard. If there is no way to minimize the noise, and the ringing is so loud that it is difficult to rest, one may turn off the alarm using a shinui. This follows the principle that a shvut di-shvut is permitted for the sake of a mitzva (as explained in the previous section).
Before the alarm actually goes off, one may press a button to deactivate it. Similarly, one may move an analog clock’s hands in order to delay the time that the alarm will go off. However, one may change the time to an earlier time only for the sake of a mitzva or when there is a great need (above, 6:7). One may not change the time if it involves typing words or digits or any other use of electricity.
One may wear an electronic watch that displays the time. Even if there is a computer inside, one may wear it, since its primary purpose – displaying the time – is permitted (below, 22:8). However, if one knows that he is likely to end up using the computer, he may not wear this watch on Shabbat. Of course, one may not wear a watch that requires him to press a button to see the time, as it is muktzeh.
One may not wear a watch that measures the room temperature and displays it, because the movements of the wearer cause the watch to work. The claim that one is not interested in this feature is patently false; if this were the case such watches would not be made, and if they were made, no one would buy them. However, if the watch detects the temperature but does not display it unless one presses the appropriate button, one may wear the watch. The reason is that the measurement is done via grama, and it is a case of a psik reisha where the person is indifferent toward the result (see above, ch. 9 n. 3).
Some are stringent and avoid wearing a solar-powered watch or an automatic quartz watch that is powered by movement. They are concerned that whenever the wearer moves his hand or enters a well-lit place, he causes the watch to recharge. Others are lenient on condition that the watch would be able to function for a few days without being charged, so that the charging that takes place on Shabbat is not really necessary. One who wishes may be lenient, and one who is stringent should be commended.
A digital photo frame, which stays on all week and cycles through a slideshow of family pictures or scenic views, does not need to be turned off before Shabbat. This is because everyone knows that it is automatic and is on nonstop throughout the week.
. See SSK 28:20, 22 and Yeĥaveh Da’at 2:49, which state that if one need not press an electric button to see the time, one may wear it on Shabbat, and even if it has a calculator built in, it is not considered a base for a forbidden object (basis le-davar ha-asur; Tzitz Eliezer 6:6; Orĥot Shabbat 19:43). Regarding a thermometer and the like, the relevant principle is the one described above in section 14, namely, that causing the activation of sensors that have no present purpose is permitted, but is forbidden if they have a present purpose. Regarding the adjustment of an alarm clock to an earlier or later time, see SSK 28:33; Orĥot Shabbat 8:90-91.
. Those who are lenient maintain that when there is no need to charge the battery, doing so is considered mitasek, as he is moving his arm for entirely different purposes, and the clock is recharged incidentally and without any purpose. This is even more lenient than a psik reisha de-lo niĥa lei; in a psik reisha, one plans to do a particular activity, but in the present case, moving one’s arm is not even an intentional activity. Moreover, there are some who maintain that a psik reisha de-lo niĥa lei is permitted if the activity is rabbinically prohibited, and certainly according to those who maintain that electricity is a rabbinic prohibition, in which case the present example is a rabbinic prohibition on a rabbinic prohibition, as explained above (9:2). Moreover, it is possible that his movements will not generate electricity because the internal battery is already full. Thus, wearing it is a davar she-eino mitkaven that does not reach the level of a psik reisha. See SSK 28:28. However, when the battery needs to be full for Shabbat or even Sunday, he wants it to recharge, and it is therefore forbidden. According to those who maintain that electricity is forbidden by Torah law, one should be concerned that this is a violation of Torah law. According to those who maintain that electricity is forbidden by rabbinic law or on account of Boneh, there are grounds to say that even if the watch cannot continue for much longer on its own, one may still wear it, as carrying the watch or exposing it to light merely adds to the current, which is not forbidden, especially when it is done unintentionally. However, if the watch has already stopped, it is forbidden for anyone to activate it, as stated with regard to winding up the spring of a watch in SA 338:15 and MB ad loc. 15 (see also Ĥelkat Yaakov 1:75; Yabi’a Omer 6:35:8; Tzitz Eliezer 9:20). See Orĥot Shabbat 26:50.