Control circuit for automatically controlling a diaphragm of a camera

Abstract

The control circuit for automatically controlling a diaphragm of a camera for the proper exposure has a servomotor coupled with the diaphragm and provided with a pair of coils each for driving a servomotor coupled with the diaphragm in the opposite direction upon energization thereof so as to open or close the diaphragm and a circuit including a photoelectric element adapted to receive scene light through the diaphragm so as to generate a first and a second input for the pair of coils varying in relation to each other in accordance with the scene light quantity passed through the diaphragm and received by the photoelectric element. The control circuit comprises two sets of circuits each consisting of a first transistor and a second transistor with their emitter grounded while the collector of the first transistor is connected to the base thereof to effect diode connection and the base of the second transistor is connected to the base of the first transistor, the collector of the second transistor in each circuit being connected through a resistor to the first and the second input, respectively, while the collector of the first transistor in each circuit is connected through a resistor to the other of the inputs than that connected to the collector of the second transistor belonging to the same circuit of the two sets, and a third transistor with its emitter grounded while its base is connected to the collector of the second transistor in each circuit so as to form emitter grounded amplifier, so that amplified output is obtained at the collector of the third transistor in each circuit proportional to the variation in the first and the second input. The collector outputs of the third transistors of the two sets are connected to the pair of coils, respectively, while they are fed back to the inputs of the two sets of circuits in negative feedback connection. Thus, the current flowing through the coils is made to the minimum when the first and the second input are balanced each other at the time the diaphragm is set at the proper exposure condition.