| 摘要 |
PURPOSE:To execute exactly an exposure control by a simple constitution of a circuit by converting a resistance value of a photoconductive element to a current value, and bringing a capacitor to a constant-current charging by its current. CONSTITUTION:An inversion input terminal of an operational amplifier 13 to whose non-inversion input terminal the reference volage 1 is impressed is grounded through a resistance 28. The emitter of an NPN transistor 24 whose base is connected to an output terminal of the amplifier 13 is grounded through a photoconductive element 37. In this way, when a negative feedback is applied to the operational amplifier, and the photoconductive element 37 is connected as a load of its operational amplifier 13, it is possible to bring a time constant capacitor 40 of an exposure controlling circuit to a constant-current charging by an optical current corresponding to a resistance value of the photoconductive element 37, and an exact exposure control can be executed by a simple circuit. |
