GB709809A - Improved method and apparatus for electrically producing copying stencils - Google Patents

Abstract

709,809. Copying-telegraphy ; local copying by scanning. RONEO, Ltd. Feb. 22, 1952 [March 8, 1951], No. 5608/51. Class 40 (3) In a method of producing a perforated stencil by scanning an original with a light spot in association with a photo-sensitive cell and passing the derived current in the form of sparks through a stencil sheet disposed between electrodes, the current applied to the electrodes is caused to vary in approximately linear relationship to the density of the part of the original being scanned. The light received by the cell varies inversely as the antilogarithm of the density of the original and a circuit arrangement is included to provide an approximate logarithmic reduction of the signal from the cell. In the arrangement shown, light from a source 23 is reflected from the original carried on a drum 12 to a photo-cell 25 whose output is fed to a valve 31. The anode load of the valve 31 includes a resistance 32 and a resistance 33 connected to the positive side of a battery 34 and by a potentiometer 43 to a tapping on the battery. Three diodes 35, 36, 37 are connected in parallel with the resistance 33 and have variable resistances 40, 41, 42 connected to points on the potentiometer 43 providing adjustable biasing voltages for the anodes of the diodes. As the anode of the diode 37 is the least negative, this diode will be the first to conduct in response to a signal and will reduce the gain of an output valve 38. For signals of greater amplitude conduction in diode 37 will be accompanied by conduction in one or both of the diodes 35, 36 with further reduction in the gain ot the valve 38, which is operated on the anode bend of its characteristic. The grid of the valve 38 is also fed via transformer 39 with a carrier frequency derived from an oscillator 44 so that the output of the valve 38 is an amplitude-modulated carrier which is further amplified by valve 45 and passed via an output stage 47 to the tungsten electrode 29 associated with the stencil sheet on the drum 13 which forms the second electrode of the spark gap. The stencil sheet may include metal powder, graphite or carbon black so that it is opaque to light except where perforations have taken place and the perforated stencil is equivalent to a film transparency. The transmitting and recording drums 12, 13 as shown are driven from the same shaft 14, but the recorder may be operated at a distance from the transmitter, and controlled by synchronizing currents. The shaft 14 is preferably driven from an intermediate shaft driven from the motor through a centrifugal clutch brought into operation when the motor is set into operation. By this arrangement the shaft 14 is free for manual adjustment when the motor is at rest. The scanning and recording units are advanced step-by-step by a threaded nut engaging a screw shaft which is given a slight rotation at each complete revolution of the shaft 14.