Light Dependent Resistors (LDRs) are little light sensors, and this is a BIG pack of 5 of them! As the squiggly face is exposed to more light, the resistance goes down. When it\’s light, the resistance is about 5-10K ohms, when dark it goes up to 200K ohms. These are the same KLDRs we use in our popular Edukit #2 Sensors. To use, connect one side of the photocell (either one, it\’s symmetric) to power (for example 5V) and the other side to your microcontroller\’s analogue input pin. Then connect a 10K pull-down resistor from that analogue pin to ground. The voltage on the pin will be 2.5V or higher when it\’s light out, and near ground when it\’s dark. Specification 5mm Diameter Resistance Decreases as the Light on the Device Increases Dark resistance: 1M ohm Resistance @ 10 Lux: 10-20k ohm Resistance @ 100 Lux: 2-4k ohm Max. voltage: 150V peak AC or DC Power dissipation: 100mW Lead pitch: 3mm Operating temperature: 30C to +70C