Automatic door openers are usually triggered by a sensor which senses the presence of a person near the door. There are basically two types of sensors which either sense pressure (or weight) or the interruption of a beam of visible or infrared light. In a beam sensor type door opener, the source of the beam and the detector are placed opposite each other in the passageway in front of the door in such a way that the beam crosses the passageway. When the beam is interrupted by a moving person a relay sets the door opening mechanism into operation. If the sensor is of the pressure type it is placed below a movable platform in front of the door. When a person steps on the platform his weight pushes it down slightly activating the pressure sensor and the door opening mechanism.
