Patent GB-1359105-A (GB)
Title: Information holding apparatus
IPC: G03B7/08
Filed: 1972-07-06
Abstract: 1359105 Electric shutter control circuits OLYMPUS OPTICAL CO Ltd 6 July 1972 [6 July 1971; 25 Dec 1971; 10 Jan 1972; 21 Jan 1972 (2); 22 April 1972] 31686/72 Heading G2A [Also in Division H3] An analogue store comprises a device which passes a current of value dependent upon information being received, a field effect transistor connected in series with the device, a capacitor connected between source and gate of the transistor such that the capacitor charges to a value which makes the transistor current equal to the device current and holds the current at this value. Fig.3 shows a camera shutter timer circuit including a solar cell 12 or a transistor Fig. 7, (not shown), controlled by the light input. When the switches of the store are in the position shown the cell is reverse biased and the capacitor 18 charges to such a value that the current  through the transistor 6 is equal to the cell  current. When the switches are reversed to  initiate an exposure, the transistor current which is held by the capacitor at the same value, charges a capacitor 100 until a transistor 36 conducts and so terminates the exposure.  Variations of the store are described with reference to Fig. 1 and to Figs. 4-12 (not shown). In Fig. 1, switches 10, 10a and 3 are replaced by a changeover switch which connects the output current either to the cell or to the load. Fig. 5 is similar but of complementary conductivity. In Fig. 4 the switch 10a, which by-passes the output, is omitted. Fig. 6 uses an enhancement type M.O.S. transistor, switch 18 being connected between drain and gate and Fig. 7 is a similar circuit replacing the cell with a transistor having its base current controlled  by the light. Figs. 8 and 9 use P and N channel junction F.E.Ts respectively and Fig.10 provides a diode in series with the cell to prevent it being discharged during read-out and uses a changeover switch instead of 11a and 18. Fig.11 is similar to Fig. 10 but has multi-vibrator control of switching and a similar circuit, Fig.12, adds a transistor in series with switch 18 to allow the gate and source potential to rise by the voltage across the cell. The circuit of Fig. 3 is adapted for use in a through-the-lens camera for controlling the exposure, the light level being measured before operation of the shutter release and stored during exposure, lamps being provided to indicate excessive or insufficient light. The light level is indicated by the current through transistor 6 when the switches in the light unit 20 are as shown. In the intermediate position of the release button the switches 3, 11a and 10a are changed over and contact 26 is as shown. The current from transistor 6 charges a capacitor 100 which is periodically discharged by a transistor 30 controlled by a multi-vibrator 76. The resulting pulses are applied via transistors 36 to 40 and 42 which supply two inputs of a bi-stable circuit 90. Other inputs receive the output from transistor 60 of the multi-vibrator and fixed length pulses from 81. The resulting effect is that if the light is within the usable range one of the lamps 92, 94 is lit. If the light is too large the other is lit and if too low the lights alternate. If the shutter release control is moved further the reflector of the camera is moved away, contact 18 is opened to store the light level because the cell is not now receiving light and contact 26 is changed over. A focal plane shutter is opened by solenoid 58 and when capacitors 10 have charged sufficiently to render a transistor 36 conducting the solenoid is de-energized releasing the rear membrane of the shutter to effect  closure. The tappings on resistor 46 adjust for film speed and on capacitor 100 for aperture. Fig. 11 illustrates an alternative arrangement of the multi-vibrator and light measuring circuit which omits the voltage across the "load" (14) from the capacitor circuit and Fig. 12 includes an additional transistor in series with switch 18 which is switched off when 30 is off so as to allow the gate and source potentials to rise by the voltage across the cell.