Abstract: A security device, such as a passive infra-red motion detector (1) is provided with a plurality of fasteners (15, 25). One fastener (25) removably fastens a lens (4) to a housing of the passive infra-red detector, such that the lens (4) can be removed from the outside of the housing, whilst the other fastener (15) fastens a front section (2) of the housing to the rear section (3). Both fasteners (15, 25) are transparent/translucent and act as light guides.