Lengths of yellow bars: a
Lengths of blue bars: b
Length of pink bar (input): c
Distance between centers of fixed pivots: d
Orange pen draws a portion of a circle of radius R (in orange).
Circle center is on the line that connects centers of the fixed pivots.
Distance between fixed pivot center of the pink bar and the circle center is L.
L = d.(b.b – a.a)/(d.d – c.c)
R = c.L/d = c.(b.b – a.a)/(d.d – c.c)
If (d - c) is chosen small, R can be very large.
This avoids the need to arrange a pivot at the circle center.
The mechanism has dead positions when all bars (except the pink one) are in line.
If (d – c) = 0 the circle becomes a straight line.