Patent GB-488499-A (GB)
Title: Improvements in timing devices for camera shutters
IPC: G03B9/58
Filed: 1937-01-07
Abstract: 488,499. Valve relays. TRUE COLOUR FILM, Ltd., and WHALEN, F. C. Jan. 7, 1937, No. 501. [Class 40 (v)] [Also in Group XX] The timing of a camera shutter is controlled by the rate of charging of an electric condenser by the plate current of a valve. The essential circuit is shown in Fig. 1, in which a relay B controlling the shutter is in the cathode lead of a valve V. A condenser c is connected to the grid and its other terminal can either be short circuited to the grid or connected to cathode, by a switch S. The condenser is charged by the anode current of a second valve V<1> at a rate controlled by a potentiometer P, R for adjusting the grid bias. On moving switch from short circuiting position to charging position the current through valve V and hence relay B first increases to operate relay and open shutter and then when condenser has attained a certain potential decreases to release the relay and close the shutter. In the application to a " repeating back " camera for the production of colour separation negatives, Fig. 2, a carrier 4 bearing a focusing screen 10 and a series of colour filters 5, 6, 7, 8 behind which the sensitive plates are mounted, is adapted to be moved in guides 3 across the camera in steps by an electric motor 13 driving a disc 12 bearing a pin 11 engaging in slots 19. The carrier is retained in each position by a spring catch, with hand release 28, engaging inclined stop members 29. With the carrier in the right hand position the focusing screen is opposite the camera and the normally closed contacts 25 are opened by cam 24 to effect the opening of the shutter. The carrier is then slid one step to the left. The contacts 25 close, closing the shutter and cam 24 presses contacts 20 to close them. The exposure timing circuit, Fig. 3, is then set in operation by a press button 31 and the shutter opens to expose the plate behind the filter 5, for a time pre-set in the first of the potentiometer circuits controlling grid bias to valve V<1> and closed by contacts 20 ... 23. At the end of the exposure the shutter closes and the motor 13 operates whereby the pin 11 moves the carrier 4 one step to the left. Rotation of disc 12 closes contacts 16 to maintain motor in operation for one revolution of the disc. After the carrier has stopped, but at the end of the movement of the disc 12, a projection 18 closes contacts 17 momentarily to initiate a fresh exposure on the plate behind filter 6, the time being determined in this case by the closing of contacts 21 by the cam 24 on the carrier, and the sequence is repeated for each plate on the carrier, the contacts 20 ... 23 being closed in turn. In the timing circuit, Fig. 3, the time of charging a condenser c by a valve V<1> is controlled by grid bias obtained through one of a set of potentiometers P<1> ... P<4> and the second valve V operates a relay B in accordance, in turn controlling the shutter. On depressing knob 31, relay A starts the charging of the condenser c while relay B operated at the beginning of the charging of the condenser forms a holding circuit for relay A and also energizes a shutter relay Sh to open the shutter, the release of the relay B at the end of the timing permitting, owing to the slow-release of relay A, the operation of a further relay C which in turn operates relay M to start the motor 13, continuation of the operation being ensured by contacts 16 and the initiation of the new exposure being performed by contacts 17, after the carrier has come to rest, reenergizing the relay A. To ensure that the relays C, M are held for long enough for the motor to rotate disc 12 until contacts 16 are engaged, a delaying condenser 36 is used or the polarizing current of an electrolytic condenser 37 is made use of.