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Light-Dependent Resistors (LDRs)
A light-dependent resistor (LDR) (a.k.a. photoresistor, photocell, photoconductor) is a basic electronic component which can be used to measure light levels. It contains a resistor whose resistance changes when exposed to light. They are one of the cheapest and simplest light-detection components you can buy.
Construction
LDRs are made with semiconductor-based materials to allow them to be sensitive to light. A once popular semiconductor for LDRs was cadmium sulphide (CdS), however it’s use is becoming restricted due to the environmental problems that Cadmium can cause.
The semiconductor has a crystal lattice which, in the absence of light, greatly restricts the movement of electrons across it’s structure. This gives the component a large resistance in the dark. When light strikes the lattice, some of the photon energy is transferred to the electrons, which allows them to break free and conduct. This causes the resistance of component to decrease.
Even though an LDR contains semiconductor material, it is not considered an active device (such as a diode or transistor) as it does not contain a PN junction.
Resistance Changes
LDRs typically have a resistance in the hundreds of or in the dark, dropping to a few hundred under bright light1.
Further Reading
A phototransistor is a transistor-based component which is used to detect light levels. It is similar to a standard NPN BJT, but uses light to drive the base rather than an electrical signal. Most phototransistors are sensitive in the infrared region, whilst most LDRs are sensitive to visible light. One benefit of a LDR is that they are non-polarized, so they can work with voltages/currents in either direction.
Footnotes
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Nikhil Agnihotri (2021, Jul 12). What are the different types of light sensors?. Engineers Garage. Retrieved 2024-09-30, from https://www.engineersgarage.com/light-sensors-ldr-photodiode-phototransistor-solar-cell/. ↩