Question: May one place a baby monitor (intercom) in a baby’s room on Shabbat in order to be able to hear if the baby cries and to be able to adequately care for his/her needs?
Answer: At first glance, it would seem that the baby monitor is similar in the way it works to a microphone which has already been prohibited by the greatest luminaries of the past several generation for several reasons, including the fact that it is a vessel designated for making noise whose usage on Shabbat is subject to a rabbinic prohibition. Indeed, Maran zt”l writes lengthily in his Responsa Yabia Omer (Volume 1, Chapter 19) to prohibit the usage of a microphone on Shabbat for several reasons.
Nevertheless, there seems to be more room for leniency regarding a monitor placed in a baby’s room, for an infant is considered to be “an individual ill with a non-life-threatening illness” even when he is healthy (since a baby’s condition is very sensitive and he is constantly in need of his parents’ care); thus, there is room for leniency regarding a rabbinic prohibition not being performed by the parents and which is only a result of the raising of the baby’s voice. Although the parents’ voices are heard on the intercom as well when they enter the baby’s room in order to care for him and speak there, nevertheless, this is still permissible since they have no intention for their voices to be heard and they gain no benefit from this in addition to the fact that there is no physical action. Thus, regarding a rabbinic prohibition, there is room for leniency.
On the other hand, some claim that the usage of a microphone or baby monitor on Shabbat is not only a rabbinic prohibition but is actually a Torah prohibition, for the sound waves create an electrical function whereby sparks of fire are emitted via electrical conduction. Several great luminaries from the previous generation rule likewise. Nevertheless, Maran zt”l disagrees with this opinion, for after having spoken this matter over with his great friend, Hagaon Harav Shlomo Zalman Auerbach zt”l (who was known to be an expert in the Halachot regarding electricity), the latter informed him that it is clear that the opinion of the aforementioned luminaries is completely incorrect and one’s speech does not create any kind of sparks and only causes an increase in the sound waves which has no connection to the forbidden works of igniting or extinguishing a flame. All experts in the field of electricity point to an increase in the sound waves but no fire (electricity) is increased or decreased as a result. The reason why some Poskim wrote this way in previous generations is because the mechanics of electricity were not yet clearly understood by all.
Thus, it is clear that using a microphone on Shabbat is only a rabbinic prohibition and regarding our situation which is a necessity for an infant and where no physical action being performed, one who is lenient and uses a baby monitor on Shabbat surely has on whom to rely.