| 摘要 |
A display unit comprises a box housing two spools between which a strip of film frames is wound past a window first in one direction and then in the other direction. Reversal of the direction of film drive is initiated by two photo-electric cell arrangements adapted to detect the approach of the end of the strip in each direction. As each individual frame registers with the window, the strip is stopped to display the frame for a predetermined period. The registering of a frame with the window is detected by a photo-electric cell arrangement detecting marks suitably distributed along an edge of the strip to correspond to the individual frames. The three photocells are connected in electrical control circuitry, each photocell being connected to the input of a respective Schmitt trigger circuit. The outputs of the Schmitt triggers associated with the direction reversing photo-cells are fed to a first relay operative to effect reversal of a motor driving the spools. The output of the other Schmitt trigger is fed via a timing circuit to a second relay operative to initiate rear illumination of a frame registered with the window and to cause the application of a brake to the motor. To ensure that the film strip is tensioned during its movement in either direction, in one embodiment of the invention each spool is driven through a retarder providing resistance to rotation in the direction applying when the strip is being unwound from that spool.
|
