Adafruit Industries, Unique & fun DIY electronics and kits Photo cell (CdS photoresistor) ID: 161 - CdS cells are little light sensors. As the squiggly face is exposed to more light, the resistance goes down. When its light, the resistance is about 5-10KΩ, when dark it goes up to 200KΩ.To use, connect one side of the photo cell (either one, its symmetric) to power (for example