Can one adjust a socket timer (picture below) on Shabbat to keep one's hotplate or lamp on for longer, etc?
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R’ Moshe Feinstein (Igros Moshe OC 4:60) writes that one may only use timers on Shabbos for one’s lights. Firstly, timers are akin to instructing a non-Jew to perform a melacha on one’s behalf which is prohibited. Additionally, it isn’t respectful for Shabbos. As people always had non-Jews come in to their houses to light and extinguish their lights, lights remains an exception.
Nonetheless, most poskim disagree. The Chazon Ish (OC 38:2), R’ Ovadia Yosef (Yabia Omer 3:17), R’ Shlomo Zalman Auerbach (Minchas Shlomo 1:11:8), R' Eliezer Waldenberg (Tzitz Eliezer 1:20:9) and R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 13:23) write that one may set timers for other electrical appliances, too.
This wouldn’t apply to appliances that make a noise, though. Thus R’ Shlomo Zalman Auerbach (Minchas Shlomo 2:20) writes that one wouldn’t be able to put a radio on a timer. R’ Moshe Feinstein (Igros Moshe OC 4:91:5; YD 3:47:4), R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 2:110) and R' Eliezer Waldenberg (Tzitz Eliezer 1:20:9) holds that timers are muktza and must therefore not be adjusted on Shabbos.
R’ Shlomo Zalman Auerbach (Minchas Shlomo 1:11:8), R’ Ovadia Yosef (Yabia Omer 3:18:2) and R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 13:25) hold that timers aren’t muktza, however. Thus, while one isn’t allowed to adjust the timer to switch on or off earlier than planned, one may adjust the timer to prevent or delay the timer from switching the appliance off by pulling pins out (Minchas Shlomo 1:11:8; 1:13). Likewise, one may adjust the timer to prevent or delay an appliance that is off from coming on.