A position-coding pattern which extends across a surface and codes a plurality of positions on the surface is described. The CMOS optical sensor and at least one of the analog processing, analogdigital conversion, logic control and data processing units are integrated in a single chip. A locking element of the apparatus is configured for automatically holding the cover member in the second position, the locking element being automatically operable for releasing the cover member when a downward force is applied thereagainst by the cotton receiver for moving the cover member to the first position. The first stop has a first position in relation to the base wherein the first groove is exposed and has a second position in relation to the base wherein the first groove is covered by the first stop.