Patent GB-1510173-A (GB)
Title: Optical shutters
IPC: G03B7/097
Filed: 1975-04-11
Abstract: 1510173 Optical shutters WEST ELECTRIC CO Ltd 11 April 1975 [18 April 1974] 14898/75 Heading G2A An optical shutter comprises shutter blades 4, 5 which are movable between an adjustable open position and a closed position by a rotatable stepping motor 3 under the control of circuitry which provides shutter opening pulse signals, the number of which depends upon the brightness of the subject, to the motor 3 to open the blades to provide an appropriate aperture, and shutter closing pulse signals to the motor 3 after the blades have been open for a time interval depending upon the brightness of the subject, to close the blades. When a switch 66 which is interlocked with a shutter release button is closed a capacitor 65 is discharged to turn on a thyristor 64, which remains conductive, and so to turn on a transistor 67 to provide current to a set signal generator 95 and a power source connection unit 68. The generator 95 applies set signals to ring counters 20 and 39, flipflops 37, 52, 53 and 54 and the motor control circuit 57 for the motor 3, and the unit 68 supplies power to the circuitry, see also Fig. 4 (not shown), for details of the generator 95 and unit 68. A pulse oscillator 14 feeds pulses to the ring counter 20 whereby transistors 21 to 24 are turned on to provide a first voltage at the input terminal 56 of a differential amplifier 19. A photo-conductive element, e.g. a CdS element, 16 measures the brightness of a subject to provide a second voltage to a second terminal 55 of the amplifier 19. The two voltages are compared by the amplifier 19 which provides an output in proportion to the brightness of the subject. The output is applied to a differentiating circuit 31, 32 which provides negative pulses which are converted by a diode 33 and an inverter 35 into positive pulses for application to the motor 3 to rotate the latter so as to open the blades 4, 5. The exposure time is deter-  mined by the ring counter 39, a time constant circuit 58 and an integration level control circuit 69. One input of counter 39 is connected to the motor control circuit 57 of the motor 3 to provide pulses at output terminals of the counter 39 leading to flip-flops 52 to 54 and transistors 49 to 51 for short-circuiting resistors 47, 47' and 48 to determine the voltage across a resistor 45 and so the operating level of the time constant circuit 58 so that the exposure time can be selected. The exposure time is determined by a photo-conductive element, e.g. a CdS element, 15, the circuit 58 including an integrating capacitor 42 and switching transistors 40 and 41 and 43 and 44. In response to a voltage across a resistor 94 the transistor 40 is rendered conductive and the transistor 41 is cutout whereby the capacitor 42 starts to integrate the current from the element 15. When the voltage across the capacitor 42 reaches the operating level of the circuit 58 the transistor 43 is made conductive and the transistor 44 is cut-out whereby clock pulses from the oscillator 14 are inverted by an inverter 60 to provide negative pulses which are amplified by an amplifier 59 and then fed to control circuit 57 of the motor 3 to rotate the motor to close the blades 4, 5. In an alternative embodiment the stepping motor 3 is reversed by switching the polarity of excitation in each phase of twophase field coils of the motor, Fig. 8 (not shown), via gate circuits controlling flip-flops and transistor bridge circuits connected to the field coils. A reset signal generator 95', see also Fig. 10 (not shown), resets the circuit elements and disconnects the power after switch contacts 104, 105 are closed by closing of the blades 4, 5. Instead of the circuitry being activated by actuating a shutter release button it may be activated remotely, e.g. by a wireless signal, by applying an electric signal to terminals 66', and the shutter may be operated repeatedly at desired time intervals in response to pulsing of the electric signal.