Adafruit Photo cell (CdS photoresistor)

CdS cells serve as compact light sensors with their resistance varying based on light exposure. In bright conditions, the resistance is approximately 1KΩ, whereas in darkness, it increases to around 10KΩ.

To employ a CdS photocell, attach one side to a power source, such as 5V, and the opposite side to an analog input on a microcontroller. Additionally, connect a 10K pull-down resistor from the analog pin to the ground. When illuminated, the pin voltage exceeds 2.5V; when dark, it approaches zero.

These photocells are symmetric, making setup straightforward, though it's worth noting they are not RoHS compliant. If compliance is necessary, consider a phototransistor as an alternative.