A display apparatus for displaying a clip-on sunglass, sunglass or other product and a method of assembling the display apparatus. The linear actuator furthermore comprises a sheath stretching across the intermediary member, which sheath is inrotatably, but longitudinally slidably arranged in relation to the first and second members. Air introduction means for forcing air into the cylinder portion is also provided. The demodulator circuit extracts the serial data bit-stream and a constant-frequency clock from an FSK carrier signal in the 1\u02dc20 MHZ range, which can power the wireless system as well.