A light dependent resistor, or LDR, varies resistance in a circuit based on how much light shines on it. When fully illuminated, the LDR has no resistance and current flows freely, but with darkness the resistance increases and current flow stops. In most circuit designs, the LDR acts like an on/off switch based on how much light shines on it.
There are two types of photo conductors used in an LDR. The intrinsic type uses material that has free electrons available to carry a charge and represents the most common form of LDR. The extrinsic type uses materials that require an impurity to carry a charge. The LDR has two contacts mounted on either side of the photo conductor. When light shines, the photo conductor has free electrons. Current flows from one contact to the other and closes the circuit. Without light, the semiconductor will resist current flow and keep the circuit open.
Based on the design and material used, an LDR can utilize a wide variety of light, from visible to infrared. Some LDR designs use coherent laser light or visible light with a specific set of frequencies. Light meters on cameras, streetlights and smoke detectors use LDRs as trigger mechanisms. The elaborate laser alarms seen in some movies also use the LDR.