A mounting assembly is provided for mounting a sensing component of an alarm system in a predetermined orientation adjacent to the inner edge of a door or window frame having an outer surface. The assembly includes two angle brackets adapted to fit over the inner edge of the frame, each having a support flange and a positioning flange. A flat planar support surface on each support flange is arranged in a predetermined orientation for receiving thereagainst a flat planar mounting surface of the sensing component mounting member. Locating pins on the support surfaces are receivable in locating holes in the mounting member to facilitate alignment of the screw apertures in the mounting member and bracket for fastening them together and to the door frame. Steps on the positioning flange facilitate cutting same to a desired length.