Question: Can we leave a baby monitor in our baby’s room on Shabbos so that we can hear her when she cries?
Answer: Contemporary poskim give different reasons for why one can’t use microphones on Shabbos.
R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 2:17), R’ Eliezer Waldenberg (Tzitz Eliezer 3:16:11; 4:26) and R’ Moshe Sternbuch (Teshuvos Vehanhagos 1:230) write that one can’t use a microphone on Shabbos even if it has been switched on before Shabbos just like one can’t leave a radio or television playing on Shabbos. There is an issur derabanan of avsha milsa, allowing sounds to be made on Shabbos that will give people the impression that one is performing a melacha (See Shabbos 18a; Eruvin 104a; Rema OC 252:5). Thus, R’ Moshe Feinstein (Igros Moshe OC 4:70:6) wrote that one shouldn’t set very loud alarm clocks to go off on Shabbos.
R’ Moshe Feinstein (ibid. OC 3:55; 4:84) gives other reasons why one mustn’t use a microphone on Shabbos, even if it was switched on beforehand. He explains that when one speaks loudly into the microphone, the electric current increases accordingly, and one mustn’t operate anything electrical on Shabbos. While it is difficult to understand exactly how electricity works, he compares the act of amplifying one’s voice to the melachos of kosev, (writing) boneh (constructing) and makeh bepatish (the finishing act).
Nonetheless, R’ Moshe (ibid. 4:85) permitted people to wear hearing aids, if necessary, on Shabbos. As it isn’t clear what the prohibition is, we don’t forbid using them in case of great need.In conclusion, one shouldn’t use a baby monitor under normal circumstances on Shabbos. Only under extenuating circumstances such as with an unwell child should it be used, though when doing so, adults should be careful not to talk when in the room.