Disclosed is apparatus for use with: an RFID proximity card; a frame including a pair of jambs; a door hingedly connected to one of the jambs, the other being hollow and having a rabbet defined by a pair of surfaces, one surface presenting to the closed door and defining part of a stop against which the door is positioned when closed and the other surface flanking and presenting towards the door edge when closed. The other surface width is in the direction which defines the one jamb depth. The one jamb thickness is in the direction which defines the doorway width. The apparatus comprises a sensor which: produces a signal when the card is presented thereto; and has thickness T, width W and height H. T