Photocopier with servo motor and wheatstone bridge circuit - using wheatstone bridge instead of limit switches for direction reversal

摘要

Photographic copier comprises film, lens and master carriers (1-3), one fixed and the other two movable to vary the image-to-object ratio (n). One movable carrier is set by hand the other is adapted by a servo motor (5) connected to a Wheatstone bridge circuit (8). In the bridge circuit, two resistors (11, 12) in opposite branches have the value F (ohms), while those (7a, 9a) in the other two branches have the values n. F (ohms) and F/n (ohms), where F is in ohms and corresponds to the focal length in mm. Two opposite bridge terminals are connected to an amplifier (15) which emits a fault signal via a pole-changing switch (17) to the motor. The other two opposite bridge terminals (18, 19) are connected to a voltage divider comprising two resistors (21, 22) with the value F (ohms).