Shabbat Clocks
Most authorities permit a Jew to program a timer, referred to as a Shabbat clock, before Shabbat to perform automatically a prohibited action on Shabbat. For example, it is permitted to attach a timer to a light switch on Friday afternoon so that the light will turn off late on Friday night when people wish to sleep, and will turn on again the next day when people are awake. The underlying rule is based on a Talmudic principle, codified by Maimonides, that permits a Jew to begin an action on Friday even though the action will be completed automatically on Shabbat.
However, Rava in the Talmud prohibits a Jew from adding wheat on Friday to a water mill that will run automatically on Shabbat because the addition of wheat to the mill will cause a loud noise which disturbs the peaceful environment of Shabbat. Rav Yosef disagrees with Rava and permits this. Rishonim disagree as to which opinion is normative. Rabbi Joseph Caro in the Shulchan Aruch permits this action, but Rabbi Moses Isserles (the Ramo) prohibits it absent great need. Accordingly, Rabbis Moses Feinstein and Shlomo Auerbach prohibit programming a radio to turn on during Shabbat, or allowing it to run on Shabbat, not because of the violation of electricity as such, but rather because the noise of the radio violates a separate prohibition.
Some authorities have raised other reasons to prohibit Shabbat clocks in general, but the consensus of many rabbis permits their use.
Read more about this topic: Electricity On Shabbat In Jewish Law
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—Henry Vaughan (1622–1695)