Updated: Nov 3, 2021
The Daily Halacha Moment - Melachah on Erev Shabbat that will Continue on Shabbat 🕒
״כל השונה הלכות בכל יום - מובטח לו שהוא בן העולם הבא״ (נידה עג ע״א, מגילה כח:)
“Anyone who studies Halachot every day is guaranteed that he is destined for the world-to-come” (Megilla 28b, Niddah 73a)
May one perform a melachah on erev Shabbat that will continue on Shabbat?
For example, may one put something on a timer to activate on Shabbat?
One may perform a melachah on erev Shabbat even if the melachah will continue to be performed by itself on Shabbat. For instance, one may turn on a sprinkler on erev Shabbat, even though it will continue to water the grass on Shabbat itself. 
Even objects which make a lot of noise when in use may be running on Shabbat if they are set before Shabbat. For instance, one may place clothing in the dryer before Shabbat, even though it will continue to run on Shabbat. 
Similarly, one may set a timer from before Shabbat which will turn on an item on Shabbat.  Similarly, one may set an alarm clock on erev Shabbat for Shabbat morning in order to wake up the next day even though it will make a lot of noise. 
. Shulchan Aruch 252:5; Chazon Ovadia, Shabbat, vol. 1, p. 134. See also in Halachah Berurah, vol. 13, p. 512 who adds that one may also put a cast on erev Shabbat or cream on for healing purposes, even though it will continue to heal on Shabbat.
. Chazon Ovadia, Shabbat, vol. 1, p. 146; ibid., vol. 5, p. 240; ibid., Yom Tov, p. 72 in footnote 157; Yechaveh Daat 3:18; Yalkut Yosef, Shabbat, vol. 1, book 1, p. 415; Halachah Berurah, vol. 13, p. 518.
. Chazon Ovadia, Shabbat, vol. 1, p. 140; ibid., vol. 6, p. 105; Yabia Omer, 3:11 & 10:26. See also in Shevet HaLevi 1:47.
. See in Igrot Moshe, O.C. vol. 4, 70:6 who is stringent if people will be able to hear the alarm from outside of one’s room. However, most poskim are lenient. See in Shevet HaLevi 1:47; Shemirat Shabbat Kehilchatah, chap. 28, footnote 66; Ohr LeTzion, vol. 2, 39:3; Halachah Berurah, vol. 13, p. 527
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