Q. Can a defendant under house arrest wear his ankle monitor (it constantly checks and transmits to police his whereabouts), when permitted to go to shul on Shabbos (no eruv)?

A. On question 1488 in regards to if a father of a newborn baby is allowed to wear the hospital ID on the bracelet in a place where there is no eiruv, we wrote: “Although, Igrois Moshe (O. H. 1: 111), ruled that in principle one is permitted to carry a watch on Shabbos because it is considered an article of clothing, and is similar to a ring with a signet, however, many Poskim disagree and the minhag today is to be stringent. (Chelkas Yaakov 2: 97 et. al. See also Igros Moshe Y.D. 2: 47 on moch dochuk). Shemiras Shabbos Kehilchoso (40: 32) maintains that one should not wear an ID bracelet where there is no eruv.
However, Chashukei Chemed (Shabbos 52b) in regards to hospital ID bands rules that in need, one may rely on the Igrois Moshe, especially when we dealing with a karmelis.
Horav Shlomo Miller’s Shlit’a opinion is similar to be lenient in need, when the ID band will remain on the hand of the father, until the discharge of the baby. The Rov does distinguish between a watch, where the item carried has an ongoing operative working purpose of functioning and showing the changing time, it could stop working and thus become a maso, as opposed to the ID bracelet, that just serves by being there, and thus maintains its malbush status.”
Shulchan Aruch (O.H. 28: 19) permits a prisoner to carry his chains. Piskei Teshuvos (ibid n.188) allows using ankle monitors, as long as one is careful not to activate them by going out from the permitted areas. Horav Shlomo Miller’s Shlit’a opinion is similar.
(See question 890 in regards to immersing in a mikva with them, and also question 1412, in regards to wearing a red string on the street during Shabbos).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a