Works LDR 1

LDR - photo resistor

A photoresistor is a semiconductor whose resistance value is light-dependent. It is also called LDR (Light Dependent Resistor).
All semiconductor materials are sensitive to light and would therefore work well for a photoresistor. Since this effect does not appear to the same extent in every semiconductor, there are special semiconductor mixtures in which this effect occurs particularly strongly.
In addition to cadmium sulfide (CdS), there are also lead sulfide (PbS), lead selenide (PbSe), indium arsenide (InAs), germanium (Ge) or silicon (Si) for photoresistors. These semiconductor mixtures have a particularly strong internal photoelectric effect. There are many other semiconductor mixtures, depending on the electrical properties and manufacturer.

An LDR consists of two copper combs that are attached to an insulated base (white). In between lies the semiconductor material in the form of a twisted ribbon (red). If the light (photons) falls on the light-sensitive semiconductor material, the electrons are released from their crystals (pair formation). The LDR becomes more conductive, that is, its resistance value becomes smaller. The more light falls on the component, the smaller the resistance and the greater the electrical current. However, this process is very slow. The delay lasts several milliseconds.


Not all light is the same. Each light not only has a different intensity, but also a different color. Photoresistors react very differently to the light wavelength (color). There are photoresistors which have their maximum sensitivity for a certain colored light.
There are also special photoresistors that have their maximum sensitivity in the ultraviolet or infrared range.

Ultraviolet0.38 λ / µm
violet0.4 λ / µm
blue0.435 λ / µm
Cyan0.489 λ / µm
green0.546 λ / µm
yellow0.576 λ / µm
orange0.615 λ / µm
red0.7 λ / µm
Infrared0.77 λ / µm

Circuit symbols


The photoresistor is used in DC and AC circuits.
If inertia does not play a role, a photo resistor is used as a light intensity meter, flame monitor, twilight switch and as a sensor in photoelectric barriers.

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