GB1099796A - Electronic timing devices - Google Patents

Abstract

1,099,796. Photo-electric control of photographic printing. ILFORD Ltd. Sept. 26, 1966 [Sept. 27, 1965], No.40950/65. Heading G1A. In a timer for a photographic printer of the type in which current from a photo-cell or other constant current source is integrated by a capacitor and the voltage thereacross fires an electronic relay, an initial potential is applied to the relay from a source of voltage k times the firing voltage of the relay by a potential divider comprising a fixed resistor and a variable resistor having a maximum value less than 1/(k-1) times the value of the fixed resistance. k lies between 1À0 and 2À0 and preferably between 1À05 and 1À30. With this arrangement, linear variations of the resistor to take account of film speed or subject matter cause corresponding logarithmic variations of the exposure interval. In the stand-by condition, relay A/3 is energized through R 5 and R 7 and the trigger of a cold cathode tube V 2 , Fig. 5, is connected to the junction of resistors R 1 and R 2 over a/1 and its potential is determined by the setting of the variable part of R 1 i. e. R 1B and R 1c . Exposure is initiated by opening S1. A/3 drops out to energize the exposure lamp L through a/3 and break a/1. The capacitor Cl is charged by the current through the photo-cell V 1 until the potential across it fires the tube V 2 . A/3 then pulls in to extinguish the lamp L and return the timer to its stand-by condition. In modifications, the capacitor may be in parallel with R 2 , Fig. 2, (not shown) or in series with R 1 and R 2 , Fig. 3, (not shown). Exposure may be terminated by a shutter instead of extinguishing the lamp.