Photocells are detectors that are light dependent. When they are not near light, they have a high resistance. When placed near light, their resistance falls. When placed inside circuits, they allow current to flow based on the amount of light that illuminates them, and so are called photoresistors. They are also called light dependent resistors or LDRs.
Photocells are made from semiconductors, most commonly cadmium sulfide. Those made from lead sulfide are used to detect infrared. To check a photocell, use a digital multimeter.
Turn the multimeter on, and place it on the setting for resistance. Resistance is usually indicated by the Greek letter omega. If the multimeter is not auto-ranging, change the knob to a very high level, such as megaohms.
Place the red probe of the multimeter on one leg of the photocell, and the black probe on the other. The direction does not matter. You may need to use alligator clips to make sure that the probes do not slip from the photocell’s leads.
Shield the photocell so that no light falls on it. Do this by placing your hand over it or by covering it, for example.
Record the resistance. It should be very high. You may need to adjust the resistance setting up or down a notch to get a reading.
Unshield the photocell. Adjust the knob on the multimeter by lowering its resistance setting. After a few seconds, the resistance should read hundreds of ohms.
Repeat the experiment by placing the photocell near various light sources, such as sunlight, moonlight, or a partially darkened room. Each time, record the resistance. Photocells may take from a few seconds to a few minutes to readjust when they are removed from a light source then placed into darkness. As before, you may need to change the resistance settings to get proper readings.