Power Generation
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.
Light Detection
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 circuit diagram showing typical component values for the BPW41N detector.
Here is a circuit designed to pick up a stream of infra-red pulses.
- R1 and the BPW41N form a potential divider. The output depends on the level of infra-red light arriving at the detector.
- The 741 op amp is wired as a non inverting amplifier with a gain of 1 + R6 / R7 = 1001.
- The AC pulses from the photodiode are coupled via C1 to the non inverting input of the 741. The DC signal is blocked. C1 and the surrounding resistors form a high pass filter with a break point of f = 1 / (2 pi RC) Assuming the resistors surrounding C1 are about 1MΩ, the filter passes signals above 80 Hz.
- There is enough DC leakage to upset this circuit. This is compensated for as follows. R2, R3 and R4 form a potential divider. R3 is adjusted to set the DC potential of the non inverting input so the DC output of the op amp is zero.
- R5 couples the DC potential to the non inverting input without too much loss of the AC signal from the diode.
- The circuit provides a saturated +/-10 volts square wave output 0.5 metres away from the photodiode transmitter.
- This circuit can oscillate if the layout is poor. Use single point earthing and keep the output wiring away from the input wiring. Make sure the power supply is decoupled.