an electric key - operated switch adapted to be mounted on the exterior of a vehicle or structure to arm or disarm an alarm system . a key is to be rotated in a cylinder between two positions , each of which controls a separate circuit that is normally open when the contact rotor carried by the cylinder is not engaged with contacts connected to the respective circuit . the invention provides more positive electrical engagement and a novel structure for economically adjusting the limits of positions of the contact rotor so that the key may readily be inserted and removed .