Door latch

A door-latch mechanism comprising:

• a. a unitary latchbolt constrained by a supporting structure to move linearly and only between a latched position and an unlatched position;
• b. a separate element that is separate from the latchbolt and is constrained by the supporting structure to move only between a first position and a second position, wherein the separate element is at the first position whenever the latchbolt is in the latched position;
• and
• c. a separate element movement means to move the separate element and therewith to move the latchbolt into the unlatched position;
• wherein movement of the latchbolt can move the separate element toward the first position but not away from the first position, and
• wherein the door-latch mechanism is mounted to an exterior surface of a door, and all substantial movements of the separate element are parallel to that door surface.

A spring-latch assembly for latching a door closed, comprising:

• a. a spring-return latchbolt movable by a first control means controlled from a first side of a door;
• b. a separate element movable by a second control means controlled from a second side of the door opposite the first side;
• c. a support structure mounted external to a door surface on the first side of the door and supporting the latchbolt and the separate element, with the separate element situated between the latchbolt and the first side of the door;
• wherein a first motion of the latchbolt in a first direction translates the latchbolt from a latched position to an unlatched position, and a second motion of the latchbolt in a second and opposite direction translates the latchbolt from the unlatched position to the latched position;
• wherein a third motion of the separate element in the first direction translates the separate element from a first position to a second position, and a fourth motion of the separate element in the second direction translates the separate element from the second position to the first position;
• wherein movement of the latchbolt to the latched position moves the separate element to the first position, but movement of the latchbolt using the first control means over a distance necessary to reach the unlatched position from the latched position does not necessarily move the separate element to the second position;
• and
• wherein the latchbolt resists movement of the separate element over that distance to the second position.