Q. Can one use a voice baby monitor or room intercom, if it was left on before Shabbat began, and the controls are taped?
A. On question 1127 regarding why we prohibit using a microphone on Shabbos, and we permit speaking to a person wearing a hearing aid, we wrote: Igrois Moishe (O.H. 4: 85) explains that microphones became prohibited by most Poskim because they involve “hashmoas kol” or an activity that is publicized and creates awareness to all that a prohibited melocho is possibly being transgressed, which is not the case with a hearing aid.
Also, he adds, only a small amount of people in need require hearing aids, thus it is a “milsa delo shechiach” or an uncommon occurrence, that our sages usually do not prohibit.
On question 1126, we quoted an additional reasoning that in reality the melocho is not done by the hard of hearing, but by the one who addresses him, and he is “eino mechaven” (does it without intention). Igrois Moishe maintains that it is not an unavoidable melocho (psik reisha) prohibited also when done without intention, since the hard of hearing may not even be listening.
Regarding a baby monitor Poskim disagree. Some are stringent, since unlike a hearing aid, it also involves “hashmoas kol” (Teshuvos Vehanhogos 1: 230, Ma’ayanei Shlomo 41, Vayaan Dovid 1: 69 and others).
However, other Poskim find more room for leniency regarding baby monitor, since an infant is considered to be “an individual ill with a non-life-threatening illness”, because even when he is healthy he is constantly in need of his parents’ care.
Hacham Ovadia Yosef’s grandson Horav Yaakov Sasson rules that one who wishes to be lenient regarding a baby monitor has upon what to rely. He notes that Hacham Ovadia (Teshuvot Yabia Omer 1:19) concludes, based on discussion with Rav Shlomo Zalman Auerbach, who had a very sophisticated understanding of electricity and Halacha, that no Torah prohibitions are violated when using a microphone. Thus, he maintains that the question regarding the use of baby monitors involves only a Rabbinic prohibition, and in need one can be lenient. Maase Choshev (2: 6), Divrei Sholom (6: 128) and others are also lenient.
Horav Shlomo Miller’s Shlit’a opinion is to be stringent as mentioned above, due to the prohibition of “hashmoas kol,” (Ma’ayanei Shlomo 41) unless in case of real need.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a