Lengths of short bars: a
Lengths of long bars: a + a
Distance between centers of the two fixed pivots is b.
Orange pen draws a portion of a circle of radius a (in orange)
If the pink crank rotates regularly, the pen moves regularly on the circle.
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 2b.
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.