Is it permitted to use a digital baby monitor on Shabbat, if it is set up before Shabbat?
It is permitted to leave a baby monitor turned on for Shabbat near the baby when necessary. Even if one talks near the monitor and the voice is heard in the other room, there is no prohibition as it is considered “Pesik Reisha Delo Nicha Lei” (an inevitable action that the person is not interested in), similar to sensors and cameras which operate almost everywhere when passing by them. The prohibition by sensors is only when a person walks by them with a clear intention to activate them. Therefore, also in this case there is a prohibition only if the adults use the monitor in order to talk to each other, similar to the prohibition of speaking into a microphone on Shabbat (see Igrot Moshe Orach Chaim 3, 55. Tzitz Eliezer Vol. 3. 16, 11) It is important to emphasize that in any case of the slightest need it is better to leave the monitor turned on and not risk a life.