A detector assembly for giving an infrared sensor a detecting range of about 180° includes a semi-cylindrical focusing lens and a pair of reflective mirror surfaces which is positioned behind the lens and oriented symmetrically with respect to each other with respect to a central symmetry plane and tilted with respect to both the axial direction of the semi-cylindrical lens and the symmetry plane such that infrared signals passing through the lens at azimuthal angles of incidence up to about 90° from the symmetry plane and impinging on either of the reflective surfaces will be received by the sensor. When the sensor thus detects the presence of an infrared-emitting source, a detection signal is outputted and power from a rechargeable battery is transmitted first at a higher voltage for a specified period of time and then at a lower voltage to a fluorescent tube through a contact piece being pressed against the tube. The contact piece is surrounded and supported by an electrical insulator which, in turn, is surrounded and supported by a socket member made of a thermally conductive material and provided with a planar protrusion which can be attached to a housing structure such that heat can be effectively conducted away from the contact piece.