Abstract: A time switch device for X-ray diagnostic apparatus, in particular for the preparation of a tooth or dental exposure characterized by the device including a housing, a knob mounted on the housing for rotation to set the exposure period, a time scale of exposure periods concentrically arranged to the knob, a symbol carrier concentrically arranged to the knob. The symbol carrier has a symbol of each object being X-rayed arranged thereon one relative to the other at such a spacing relative to each other so that upon setting one of the symbols to the exposure period that is associated therewith, the other symbols will be located opposite their respective associated exposure periods on the time scale. The knob is movable in an axial direction and the symbol carrier and knob have coacting coupling means which will become engaged when the knob assumes a predetermined axial position to couple the symbol carrier for rotational movement with the knob.