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.
Place the photodiode into the breadboard. Leg (pin) orientation is not important.
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.
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.
You can also measure the current produced with the meter. Connect the meter in series, between the anode of the photodiode and the leg of the resistor. Set the meter on the current ("I" or "amps") setting to get a reading. Try using a smaller or larger value resistor if you are having difficulty getting a reading.
If the battery, photodiode or resistor becomes hot or if you notice a burning smell, disconnect the battery immediately. Let everything cool down and double-check your connections. If it happens again, it may be a faulty photodiode or the resistor needs to be larger in value, to limit current in the circuit.