Patent GB-1354811-A (GB)
Title: Electric shutter control system for a camera
IPC: G03B9/58
Filed: 1972-05-11
Abstract: 1354811 Electric shutter control systems ASAHI KOGAKU KOGYO K K 11 May 1972 [11 May 1971] 22193/72 Heading G2A An electric shutter control system, e.g. for a single lens reflex camera having interchangeable lenses, comprises a photo-sensitive element 3 for detecting light from an object to be photographed, a converting circuit 5 for converting the output of the photo-sensitive element into a corresponding signal, and a switching circuit 12 controlled in dependence upon the signal and operative to control energization of an electromagnet 2 which when energized holds the shutter from being closed, wherein a D.C.-D.C. converter providing power to the converting and switching circuits also applies a D. C. output plus an A. C. component to energize the electromagnet. The A.C. component reduces the time lag between the de-energization of the electromagnet and the closing movement of the shutter due to magnetic hysteresis of the electromagnet. The photo-sensitive element 3 is connected in series with a logarithmic compression diode 4, and the voltage at the junction therebetween is fed to the converting circuit 5. Converting circuit 5 has three potentiometers 19, 20, 21 connected in parallel, slidable contact of potentiometer 20 being coupled to a ring for pre-setting the diaphragm aperture and slidable contact of potentiometer 21 being coupled to another ring for setting the film speed. An interchangeable lens having an aperture pre-setting ring is preferably used and has a pawl on the pre-setting ring which contacts the sliding contact of potentiometer 20 and also opens the normally closed contacts 20a. When a lens not having such a pawl is mounted, contacts 20a remain closed and adjustment of the lens diaphragm has no effect on circuit 5, light measuring being conducted through the already adjusted diaphragm. Additionally, a switch 18 is closed to contact 18a only when a diaphragm pre-setting ring is provided on the mounted lens; when light measurement is carried out using an aperture already stopped down switch 18 is closed to contact 18b to disable potentiometer 20 and in its place connecting potentiometer 19 which is pre-set during manufacture. The output of the converting circuit is sorted via mirror movement switch 11 on storage capacitor 9. The shutter speed that will result from the voltage stored upon capacitor 9 is indicated by an ammeter present in an indicator circuit 13. To this end a signal is fed via resistor 9a to the base of high input impedance field effect transistor 26, the output of which is connected to the indicating circuit. A switching circuit 12 has a switching element 25 which when switched to a state of conduction results in shutter closure. The trigger electrode of element 25 is connected to the sliding contact 24 of potentiometer 23. Electronic control of exposure times longer than 1/500 sec. can be achieved with precision, but shorter times tend to be unstable, and a mechanical timing mechanism may be employed instead. In that event, the electrical locking of the shutter in the opening position is prevented by coupling the change-over from electric to mechanical control with movement of slider 24 so that element 25 is always conducting. The switching action of element 25 is coupled to the base of transistor 14 via a Zener diode 27 so that counter electro-motive forces generated when the electromagnet is deenergized produces no delay in the switching action of transistor 14. The electromagnet 2 is of high power consumption and normally a time delay between closure of energizing switch 15, coupled to shutter release operation, and full activation of the electromagnet is allowed. To reduce this time delay to a minimum, a storage capacitor 40 of high value is provided to provide a larger current to the electromagnet initially than that which the battery 17 can provide alone.  The exposive time can be manually set by connecting switch 11 to a second closed terminal 11b which is connected to a variable voltage source 28. The position of potentiometer slider 32 is selected according to a scale of exposure times. The potential of battery 17 may be checked by closure of switch 33, with switch 16 open, so that indicating circuit 13 is energized. The battery testing circuit is connected to the indicating circuit via a diode 36 so that the voltage testing circuit does not influence the indicating circuit when indicating exposure times. Power to the measuring circuit, converting circuit 5 and switching circuit 12 is delivered from battery 17 via a D. C. -D.C. converter 1. Voltage regulating means 6, 7 are inserted in each load circuit which is susceptible to a thermal influence.