Link 2 connects lever 1 to link 3. To set the switch, lever 1 is shifted from support B to support A. At this, link 2 turns link 3 about fixed axis 0 and tie-rods 4 and 5 operate the switch. Weight 6 locks lever 1, holding the switch in either position. Lever 1, link 2 and the base have been conditionally turned through 90° in the drawing.
$901$LW,FD$