Patent GB-1467915-A (GB)
Title: Warning display device for an electric shutter of a camera
IPC: G03B7/083
Filed: 1975-01-15
Abstract: 1467915 Photographic cameras; electric exposure control and display SEIKO KOKI KK 15 Jan 1975 [16 Jan 1974] 1733/75 Heading G2A A warning display device for an electric shutter for a camera comprises an exposure control circuit A, and a warning display circuit having at least two stages L, N each of which includes a display circuit and a transistor 22, 20 for controlling the respective delay circuit, a reference time generating circuit M for producing an electrical signal representative of a predetermined time duration; the arrangement being such that, if the exposure time determined by the exposure control circuit A is less than the predetermined time duration, the display circuit of one stage L or N is energized, and if the exposure time determined by the exposure control circuit A is greater than the predetermined time duration the display circuit of the other stage, or one of the other stages, is energized. A switch 2 moves from position a to a position b when the shutter release is operated, whereupon power from a source 1 is applied to both the exposure control circuit A and the display circuit B. A switch 3 is separately operated to position b by a warning display button to apply power to both circuits A and B when it is desired to read the display without operating the shutter. A capacitor 4 is charged through a switch 5 and a resistance 7, when switch 5 is in position M', manual control, or a photo-conductive element 6 when the switch is in position A', automatic control. The voltage across the capacitor 4 is applied to a circuit 6 and when this voltage reaches a trigger voltage, the voltage at point P becomes positive. The trigger voltage is determined by a potentiometer comprising a resistor 12 and a resistor 10 or 11, according to whether a switch 9, ganged to switch 5, is in position M" or A". Resistor 10 is variable with stop number or film sensitivity. When the display only is to be used, warning display button is pressed to operate switch 3, which energizes circuits A and B without operating the shutter. The signal at point P is applied to three stages of circuit B. These stages L, M, N each have a transistor 20, 21, 22 driving a LED 14, 15, 16, and are connected by diodes 34, 35, 36 so that only one LED can be energized at one time. Stages M, L have timing circuits with resistors 26, 27, capacitors 30, 31 and discharge resistors 28, 29. A Zener diode 33 and a resistor 32 provide a stabilized voltage for stages M, N. If the subject to be photographed is so bright that there is risk of over-exposure, upon pressing the warning display button, capacitors 6, 30, 31 begin to charge. Transistor 20 begins to charge immediately, so energizing LED 14. In the case now considered, the voltage at point P becomes positive before capacitors 30, 31 have charged sufficiently to turn on transistors 21, 22: these transistors are prevented from conducting by transistors 23, 24, which conduct when the positive voltage at P is applied through resistors 39, 40 to their bases. Diodes 37, 38 prevent signals from the warning circuit B from reaching the exposure control circuit A, and maintain energization of LED's 15 or 16 when required. The result is that only LED 14 is energized. If the object is not so bright as to risk over-exposure, but not so dim as to require an exposure so long as to risk camera-shake, LED 14 will be energized when the warning button is depressed; but the voltage inversion at P will not take place until capacitor 30 has charged up sufficiently to turn transistor 21 on, so that LED 15 is energized and LED 14 de-energized. The positive voltage at P maintains energization of LED 15, and turns transistor 24 on so that transistor 22 does not conduct. Thus LED 15 only is energized to indicate correct conditions for taking a photograph with the camera held in the hand. If the object is so dim as to require an exposure longer than that which would avoid camera-shake, first LED 14 and then LED 15 will be energized as before. However, the positive voltage will not appear at P until condenser 31 has charged sufficiently to turn on transistor 22 to energize LED 16. Lamps or other display elements may be used instead of LED's. When, after release of the warning display button to return switch 3 to position a to discharge capacitor 4, the shutter is operated, switch 2 is operated, capacitor 4 begins to charge again and the shutter is opened by means of a solenoid 13. When the voltage across capacitor 4 reaches the trigger voltage, the voltage at point P becomes positive so that there is no voltage across solenoid 13 and the shutter closes. The display circuit B operates as before.