By Kim Lewis, updated March 24, 2022
Photocells, also known as photoresistors or light‑dependent resistors (LDRs), are semiconductor devices whose resistance drops dramatically when illuminated. They’re commonly made from cadmium sulfide or, for infrared detection, lead sulfide. To verify a photocell’s operation, a digital multimeter (DMM) is the tool of choice.
Set the DMM to the resistance (Ω) function. If your meter isn’t auto‑ranging, select a high range such as 2 MΩ to avoid saturating the display.
Place the red probe on one lead of the photocell and the black probe on the other. Probe polarity is irrelevant, but use alligator clips or a steady hand to keep contact stable.
Block all ambient light by covering the sensor with your hand or a dark cloth. In the dark, a typical LDR should show a resistance of several megohms.
Note the reading. If the display shows a value outside the meter’s range, adjust the range upward or downward until you obtain a clear figure.
Remove the shield and lower the DMM’s range to capture the lower resistance. Within a few seconds, the meter should read hundreds to a few thousand ohms, depending on light intensity.
Repeat the procedure with the photocell near various light sources—direct sunlight, moonlight, or a dim room. Record each resistance. Note that LDRs can take several seconds to minutes to stabilize after changing lighting conditions.
