It is important to emphasize that when the Sages prohibit an action because of the appearance of transgression, the prohibition is absolute, and applies even if the act is performed in private. As it is stated in the Talmud, “Wherever the Sages prohibited an action due to the appearance of prohibition, it is prohibited even in the innermost chambers” (Shabbat 64b).

An example of an action that is prohibited due to “the appearance of transgression” is having a television turn on by means of a timer, even if the television is muted. The reason is that an onlooker might see the television on and think that it was turned on in a prohibited manner on Shabbat.

In addition to the prohibition against performing certain actions on Shabbat due to “the appearance of transgression,” the Sages also prohibited certain acts whose performance is not seen by anyone, but which produce noise and may therefore mislead people and arouse suspicion.

An example of this is a washing machine that is operated automatically by a timer. Despite the fact that activating the machine with a timer is not a desecration of Shabbat, it is prohibited because one who hears the noise of the washing machine does not know when and how it was turned on, and he might suspect that the device was activated by its owner on Shabbat. This is also one of the reasons it is prohibited to operate a radio by means of a timer.

That said, anything that people are used to seeing turned on automatically may be used in that manner. For example, lighting, sprinkler systems, and the like may be operated by means of a timer. Similarly, noisy appliances such as air conditioners, which generally turn on and off automatically after being set ahead of time, may be operated by means of a timer.