789. Why are Shabbos clocks OK?
Electronic timers—or “Shabbos clocks”—are extremely prevalent today. We use them to turn lights, air conditioners and other appliances on and off for our comfort and convenience throughout Shabbos. But do they comply with the halachos of Shabbos?
There are certain poskim who issue a flat-out prohibition on the use of all timers, claiming that it is similar to the issur (prohibition) of amira l'nochri (telling a non-Jew [to do prohibited work for a Jew] on Shabbos), for the apparatus is "doing work” for us on Shabbos. If these timers would have been invented in times of Chazal, some of these authorities maintain, they too would have prohibited them. However, it is still a widespread custom, supported by a majority of contemporary poskim, to set timers for Shabbos.
Yet, there may still be another issue with the use of timers on Shabbos: Noise-making devices are generally forbidden on Shabbos (even if they were set before Shabbos, like an alarm clock*) and timers are often used on devices that emit sound (like air-conditioners, fans or humidifiers).
Many contemporary authorities argue, however, that timers used on air-conditioners and similar power-driven appliances for home use emit sound that is generally not considered “noisy”—called white noise. This type of background hum is not of concern on Shabbos.** There is an additional heter (dispensation) for makom hefsed (in cases of loss). This is even more apropos in modern society, where the inability to use an A/C or similar appliance set with a timer on Shabbos would entail considerable discomfort.
Another justification is that a Shabbos clock is designated for Shabbos use; it can be assumed that it was set before Shabbos began and there is no appearance of chillul Shabbos. In fact, some computerized timers are programmed way in advance to function only on Shabbos, or even have a built in year-round memory for Shabbos and Yom Tov times. The fact that Shabbos clock use is so widespread adds considerable weight to this argument.
* Earlier poskim permitted the use of noisy (mechanical) clocks on Shabbos, and therefore clocks are not part of the prohibition of noisy machines. However, the reasoning behind the original leniency is that general clock-maintenance entails setting and winding clocks on a twenty-four hour schedule, so that it can always be assumed that a ticking (and chiming) clock was set before Shabbos. This is not so with alarm clocks when the time for setting it is usually right before retiring for the night.
** Certain home appliances, such as a dishwasher, involve other possible issurim—prohibited work on Shabbos—beyond the level of noise emitted by the machine. The use of such timed devices on Shabbos is beyond the scope of this one-minute halachah.