| 摘要 |
A photometer of a camera employs a MOS-type CPU having an open drain type transistor whose drain serving as an output terminal, a CMOS input terminal, and an input gate inverting an input with a value exceeding a predetermined potential. In the photometer, the transistor is set in a high-impedance state, thereby releasing a charge accumulated in a parasitic capacitor of a photodiode for generating a photoelectric current corresponding to an incident light amount. Thus, a voltage at the CPU terminal gradually increases, and, when it exceeds a threshold of a Schmitt inverter, the output of the Schmitt inverter is inverted. The time period from when the transistor is set in the high-impedance state to when the output of the Schmitt inverter is inverted is proportional to the photoelectric current. By measuring the time period, the photoelectric current value or luminance is detected.
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