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Photodiodes produce a voltage when light shines on them. This is how solar cells work. Solar cells are arrays of large numbers of photodiodes. They have a large surface area and coatings to maximise light absorption. This is the best arrangement for generating power.
If a reverse leakage current is flowing in the photodiode, this current is greatly increased by incident light. This is the best arrangement for detecting light.
A typical photodiode is the BPW41N. Here is a diagram showing the pinout and typical component values. Note that the diode is reverse biased and its operation relies on leakage current caused by incident light.
The current through the diode is about 0.5 µA in the dark rising to 100 µA when brightly lit by infra red light.
Here is a layout and circuit diagram showing typical component values for the BPW41N detector.
On the right is an Op Amp, current to voltage converter. It converts the diode leakage current into a voltage.
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