555 film showing automatic protection closing down circuit

This kind of circuit is composed of the reflective type infrared sensor head, the 555 time-base trigger circuit and the relay control circuit.etc, as the figure 14-42 shows. When the film is projecting, the infrared light which is reflected by the substrate conducts the photosensitive tube VD1 to make the pin-2 of IC1(555) to have the high electric potential, the output port pin-3 has the low electric potential. Then the film is over or interrupt, the photosensitive tube is in the cut-off state because of there is no reflection infrared ray, the 555's pin-2 has the low electric potential, the 555 sets, pin-3's high electrical level makes the K to close, the normally closed contact point cuts off, the screening motor has no power to work. The normally closed contact point of relay K connects with the screening motor's control circuit.