Q. Is there any heter for using an electronic key card in entering a hotel room during Shabbat, if there is no light that goes on or if the light is covered?

A. The use of electronic key cards has become common worldwide for different reasons. They range from basic better security or simply to avert the need of collecting the keys from guests after they depart, therefore they are here to stay and they present a serious challenge for the Haredi traveler.
Many Poskim maintain that using an electric device that does not produce heat is only a rabbinic prohibition. (See Hachashmal Behalacha Ch. 5, Yabia Omer, O.H. 7:36, Minchas Yitzchok 3: 23. et. al. However, Chazon Ish, O. H. 50:9, maintains that completing a circuit constitutes a biblical prohibition). Therefore electronic card keys may not be used at least by rabbinical law, regardless if the turn on a light or no, and are therefore muktza on Shabbos.
There are a number of options available. The preferred, from the point of view of the Halacha, is to tape or place a magnetic strip before Shabbos over the lock’s receptacle or socket, where the latch bolt enters on the door’s frame, thus avoiding the door from locking when closed. You will need to secure valuables in a safety box or find another mechanical way to secure and lock the door. (Possibly a chain lock).
The next option is to ask a Gentile to open the door for you. Melochim Omnayich, (Horav Y. Zilberstein Shlit”a 48, p. 525), mentions two ways to allow one to ask a non-Jew to open the door on Shabbos. First, he suggests leaving sweets (or similar) in the room. When he needs to enter the room, he can tell a member of the staff “I would like to offer you some candy, but I cannot access my room.Then when the staff member unlocks the door, he will be doing it for his own benefit. Second, he quotes Horav Elyashiv zt”l who suggests telling the manager upon check-in that he can only rent the room if the room is accessible over Shabbos, and that he cannot enter the room with a card. If the manager then agrees to provide some other arrangement to allow the guest to enter the room, such as providing a staff member to unlock the door, the manager is doing so for his own benefit and not for the benefit of the Charedi client. He also maintains the the key-card becomes muktze because of “chisaron kis” or fear of monetary loss, and cannot be moved at all.
Rivevos Efraim (7: 104) cites from Teshuvos Hashavit (Rav S. T, Stern zt”l) the implementation of an added magnetic card inserter. This device was supposed to introduce indirectly by magnetic attraction the card into the lock, thus making the opening operation only a g’romo or a causative work. However the device did not seem to be usable or available.
Horav Shlomo Miller’s Shlit’a opinion is that since activating the lock mechanism could be a Biblical prohibition or may only be an issur d’rabonon, entering the room constitutes a sh’vus d’shvus or double rabbinic injunction that is permitted for a mitzva, in need or because of oneg Shabbos.
Therefore, he suggest as an alternative option to the preferred one mentioned above, namely, that when in need, the guest may leave his card at the front desk before Shabbos, and then on Shabbos ask a non-Jewish staff member to open his door. One should not ask a Gentile to open the door for minor reasons, only when the need is imperative. (See similar ruling at; The Traveler’s Halachic Guide to Hotels from Star K on-line).

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