a security alarm sensor for mounting between two corner walls is described comprising a housing unit having a housing base for attachment to the corner walls and a selectively detachable housing cover ; a circuit board sized for engagement within the housing unit ; first and second openings on opposite sides of the housing base ; first and second mounting pins selectively movable within the first and second openings wherein said mounting pins have a length and a range of movement so as to have a first position wherein the mounting pins are totally retracted within the housing unit and a second position wherein said mounting pins extend outwardly from the housing unit sufficiently to pierce the corner walls and securely hold the security alarm sensor in place . the installation of the alarm sensor is usually accomplished by first removing the housing cover from the housing base and detaching the printed circuit board prior to mounting the housing base to the corner of the wall .