Link 2 is connected by turning pairs A and B to sliders 1 and 3 which move along fixed guides b-b and d-d. Point C of link 2 lies on a circle passing through point 0 of intersection of the axes of guides b-b and d-d, and through points A and B. When slider 1 moves along guides b-b point C describes straight line a-a passing through point 0.
$1540$SC,GI$