Ratchet wheel 1 rotates about fixed axis A. Pawl 2 is actuated by spring 3 and can slide in fixed guides b-b. Tooth a of pawl 2 engages teeth d of wheel 1. Pin 4 slides in a vertical slot of the base and indexes the position of pawl 2. As shown, pawl 2 prevents counterclockwise rotation of wheel 1. Pawl 2 can be lifted and turned through 180° in which case it prevents clockwise rotation instead.
$2718$RG,ML$