Patent GB-1474038-A (GB)
Title: Photographic exposure control mechanism
IPC: G03B9/10
Filed: 1974-05-14
Abstract: 1474038 Shutter control circuits POLAROID CORP 14 May 1974 [22 May 1973] 21290/74 Heading G2A A shutter diaphragm is opened and closed by a stepper motor driven by forward and reverse pulse trains, the forward drive pulse train being terminated if a light-sensitive circuit signals the passage of a predetermined quantity of light through the diaphragm as it opens or if a maximum number of permitted steps is counted, whichever comes first, the signal from the light-sensitive circuit calling for the generation of a reverse drive pulse train, both forward and reverse pulse trains being produced by dividing down by the same factor the output of a common pulse generator and the reverse pulse train being initiated in response to the first pulse from the generator following the appearance of the signal from the light-sensitive source so that the reverse train is not necessarily in phase with the forward train. As seen in Fig. 4 (right centre), following a reset function which puts timing into synchronism with forward drive pulses, bi-stables ka, kb, divide the output of pulse generator 92 by a factor of four, the so divided output of ka being gated over 158 (see bottom left) to become forward drive pulses KZ from gate 202. Decade counters M and N switch a bi-stable R after twenty pulses and by closing 158 end forward drive at maximum aperture. The pulses step motor 60 conditioned for forward drive from 65 as instructed by the state of R. When R switches, the motor is conditioned to drive in reverse to any further pulses KZ. The light-sensitive circuit 134 (see top right) measures the light admitted by the diaphragm and when it reaches a threshold set by 142 a signal STO appears to switch R, on completion of the next or existing forward drive pulse, if this has not happened already in response to counting. Signal STO also sets a bi-stable STX (see bottom right) so that it switches to the next pulse from generator 92 and starts bi-stables K1, K2, dividing down the generator output to provide forward drive pulses to gate 270 (see bottom left). It takes two pulses from the generator before the first pulse is available at the output of K2 and, if STO occurs before the final forward pulse is delivered, this first pulse from K2 will overlap or be too close to the final forward pulse. To avoid this, bi-stable K3 is included so that the first pulse from K2 is suppressed since it is needed to switch K3 which then opens gate 270. Bi-stables G1, G2, G3 absorb contact bounce from start circuits and start timing in synchronism with forward drive pulses. Bi-stables H1, H2, H3, monitor contacts S1 which open as soon as the diaphragm starts opening and close when the diaphragm is fully closed. H1 and H2 isolate H3 from effects of contact bounce, H3 permitting an exposure cycle once S1 is open and ending the reverse arrive pulse train when S1 closes by switching G3 and closing gate 270. The shutter has two sliding plates (26, 28), Figs. 1 and 2 (not shown), with exposure apertures (30, 32) and apertures (46, 48) which work with the photo-sensitive circuit and may have an anticipitation feature. The sequence of operations required automatically to control all the operations of an SLR camera is set out.