1. Field of the Invention
The present invention relates to a photoelectric conversion device which converts light intensity (or illuminance) into an oscillation frequency in a digital signal form. The device according to the present invention is used to detect illuminance for discrimating night and day.
2. Description of the Related Art
It has been known that illuminance may be converted into an oscillation frequency by utilizing a photocurrent of a photodiode so as to derive a digital signal. However, in a prior art photo-electric conversion circuit using this known method, the oscillation frequency can vary widely depending on the ambient temperature of the photodiode. In general, in a photodiode a small dark current flows even when no light is radiated thereon. This is because a carrier present at a low energy level is excited by heat energy and is transferred to a conduction band. Therefore, the dark current varies in accordance with the ambient temperature of the photodiode.
In other words, in a conventional photoelectric conversion circuit, since not only a photocurrent but also a dark current, which varies in accordance with ambient temperature, flows into a capacitor for charging/discharging the photocurrent, the charging/discharging time of the capacitor also varies in accordance with the ambient temperature of the photodiode. Thus, the oscillation frequency of the photodiode varies widely.