Abstract: The invention relates to an optoelectronic keyboard comprising transmitting elements and receiving elements activated in multiplex operation, with the changes in the luminous flux caused by actuation of keys resulting in a signal which defines the actuated key. The invention resides in arranging the transmitting and receiving elements in a main body of the keyboard which comprises a separate cover acting as optical and electromagnetic shielding. The space serving to guide the radiation is preferably separated and shielded from the space accommodating the elements and the electronic evaluating system. Various kinds of multiplex operation can be used to operate the keyboard.

Abstract: An anti-entry device for surfaces in which the edge of the surface to be protected is provided with light-reflecting layers which conduct the light coming from a source of light to a receiver, said receiver being connected to an alarm which is activated when the light to said receiver is disturbed. In order to make the anti-entry device insensitive to adjustment, the light-reflecting layers are formed of elements which reflect an impinging ray of light independently of their angular position, in the plane extending between the layers and parallel to the direction of incidence thereof. These elements are developed as mirror strips which contain 90.degree. angle mirrors or 90.degree. prisms as reflecting elements. Several of such mirrors or prisms can be combined in a single mirror strip.The anti-entry device is used for the securing of surfaces, such as windows, as well as machines and open-field areas. The anti-entry device is adapted to be portable, particularly for the last-mentioned purpose of use.