Two pairs of identical pulleys 3 and 3' rotate about symmetrically located axes B of board 1. Flexible link 4 runs over pulleys 3, and is attached at points E and D to the base. Flexible link 2 runs over pulleys 3', and is attached at points A and C to the base. Board 1 can be raised or lowered with translational motion along axis y-y.
$3549$CFL,ML$