Patent GB-1054127-A (GB)
IPC: H03K17/28
Abstract: 1,054,127. Shutters and timing means therefor. INTERNATIONAL POLAROID CORPORATION. June 1, 1964 [May 31, 1963], No.22599/64. Headings G2A and G3T. A shutter control mechanism has a member movable from an unblocking position in which exposure is effected to a blocking position in which exposure is terminated, said movement being operated by electromagnetic means controlled by a first timing means to operate the shutter member after an interval dependent on the level of scene brightness, and in case this first timing means does not come into operation, by a second timing means to operate the shutter member after a pre-selected time independent of the level of scene brightness. Fig. 1 shows a suitable type of shutter to which the invention is applicable. The two shutter blades 19, 20 are shown in full lines in the tensioned position prior to exposure. Blade 20 is retained in such a position that its opening 21 is aligned with the optical axis A-A, and blade 19 in such a position that its opening 21 is out of said alignment, so that exposure is blocked. Blade 19 is retained, against spring 23, by a latch 24 which engages with a pin 26 on the blade. Blade 20 is retained, against spring 29, both by the magnetic action of a solenoid 37, 39 and also by a bar 28 fixed to blade 19. An exposure is made by depressing the lever end 30 of latch 24, disengaging it from pin 26, and so allowing blade 19 to be pulled across by spring 23 into the unblocking position in which its opening 21 is on the optical axis. Thus an exposure is made through lens 11, diaphragm 13, and the openings in both shutter blades, on a film 12. Depressing lever 30 also closes a switch S 1  in a circuit which energizes the solenoid so as to retain blade 20 for a certain time, and then release it so that it is pulled across by spring 29 to block the optical axis, so terminating the exposure. The shutter is re-set by a lever system 33, 34, which engages against bar 28 and so pushes both blades back to the "set" position. The timing and control of the shutter is effected by the circuit of Fig. 2, which is a three-transistor trigger circuit responsive to the output of a timing circuit 44 which includes a photo-conductive element 45 whose resistance varies according to the level of scene brightness. When switch S 1  is closed initially, the solenoid 38 is instantaneously energized, so that a magnetic induction is set up in pole-piece 39 and keeper 40 (which is mounted on blade 20), this induction being sufficiently strong to retain blade 20 in the unblocked position even though the restraining action of bar 28 has been removed. The initial movement of the opening blade 19 opens a switch S 2  which activates the timing network 44, thus generating a time variable voltage dependent on the level of scene brightness. This voltage is applied to the base of transistor Q 1 , and when it builds up sufficiently to trigger this into conduction, transistor Q 3  is caused to cease conducting, the solenoid is deenergized, and blade 20 is released, so terminating the exposure. At lower levels of scene brightness, the resistance of element 45 increases, and Q 1  may not have sufficient gain to hold Q 2  and Q 3  cut off for the blade release time, in which case re-energization of the solenoid causes the retaining action to be exerted on blade 20 once more before it has had time to begin its closing movement. Thus the closing blade 20 fails to be released. An auxiliary circuit is therefore provided, which operates independently of the timing circuit, to ensure that the solenoid is de-energized, and so preventing the possibility of the closing blade failing to be released. An A. C. coupling is provided between transistors Q 1  and Q 2  , as shown in Fig. 2, and by suitable choice of resistors 52 and 57 and capacitor 56, Q 2  and Q 3  can be arranged to cut off at a predetermined time which is independent of the level of scene brightness, so de-energizing the solenoid to release the closing blade 20 if the operation of Q 1  has not already accomplished this. An alternative somewhat modified circuit is given in Fig. 5 (not shown). Although the principal application is to the control of a camera shutter, the invention is also claimed in more general terms for the control of any element actuated by an electromechanical means which can have one of two conditions of operation.