A photodiode is the exact opposite of an LED. Instead of emitting light, the photodiode absorbs light and produces current. That current can be increased if voltage is applied to the photodiode, which is a process called biasing. Technically, the photodiode is reversed-biased, meaning that the voltage travels through it from the cathode to the anode -- the opposite direction of an LED. Likewise, the current that is produced is called reverse current and the amount produced depends upon the brightness of the light. The photodiode is very useful because, with a few other components, it can act as a light-sensitive switch which can turn portions of a circuit on or off.
Testing a Non-Biased Photodiode
Place the photodiode into the breadboard. Leg (pin) orientation is not important.
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Place the 100-ohm resistor into the breadboard and connect each leg of it to a leg of the photodiode.
Connect the probes of the digital multimeter to the legs of the resistor.
Turn the meter to the smallest voltage setting (millivolts) and you should see a very low voltage reading.
Cover and uncover the photodiode with your hand. Observe the change in the voltage reading. The reading should decrease when the photodiode is covered and increase when it is uncovered.
Reverse-Biasing a Photodiode
Place the photodiode into the breadboard and note the location of the longer leg (the anode).
Connect the positive (red) wire of the nine-volt battery clip to the cathode (shorter leg) of the photodiode.
Connect one leg of the 100-ohm resistor to the anode (longer leg) of the photodiode and connect the other resistor leg to the negative (black) wire of the battery clip.
Connect the multimeter probes to the legs of the resistor.
Cover and uncover the photodiode, observing the meter reading. You will get a higher voltage reading with a reverse-biased photodiode and you may have to place the meter on a higher voltage setting.