Adjusting a Timer on Shabbos
by Rabbi Meir Maryles
Question: If a light or an air conditioner is set to turn off at a later time by means of a mechanical timer, may one adjust the settings of the timer on Shabbos to cause it to turn off earlier?23
Discussion: Setting a timer to turn a light off is an indirect act of turning the light off. Nevertheless, it cannot be permitted as a grama.24 This is because lights and appliances are typically set to turn on or off by means of timers, even during the week. And, as cited above from Achiezer, when performing a melachah through an indirect act is normative, it is not considered grama, and the act is forbidden.25
Question: What about the opposite situation, where a light that is off is set to later turn on, and one wishes to adjust the timer to prevent it from doing so?
Discussion: In this instance, one is not turning the light off by means of a timer but, rather, preserving its present state by preventing the timer from turning it on. Mishnah Berurah (277:11) rules that if wind is fanning a fire and one obstructs it to prevent the fire from being fanned, one is not considered to be extinguishing the fire, and it is permitted even in circumstances where grama is prohibited. Therefore, some Poskim argue that one may adjust the pins of a mechanical timer to prevent it from changing the present state.26 However, many Poskim rule that even this is forbidden on Shabbos.27