Light-dependent resistor. Also known as photoresistors or photoconductors, light-dependent resistors are electrical devices that change their resistance with varying light levels. They are usually made from a cadmium sulphide based semiconductor. Light-dependent resistors can be used as light sensors and are typically 150 Ohms in bright light and 10 Megaohms in complete darkness. This light-dependent resistor is an ORP12, the most common type.