PHOTOELECTRIC SENSOR – PHOTORESISTANCE
What is photoresistance (LDR)?
The LDR (Light Dependent Resistor) or photoresistor is a resistance that varies its resistance depending on the light that falls on its surface. The greater the intensity of the light that strikes the LDR surface, the lower its resistance and the less light it hits, the greater its resistance.
How does an LDR work?
When the LDR is not exposed to light radiation, the electrons are firmly attached to the atoms that make it up, but when light radiation falls on it, this energy releases electrons, making the material more conductive, thus reducing its resistance. LDR resistors only reduce their resistance with light radiation located within a certain wavelength band. Those built with cadmium sulfide are sensitive to all visible light radiation, those built with lead sulfide are only sensitive to infrared radiation.
The most widely used photosensitive materials for the manufacture of LDR resistors are, thallium sulfide, cadmium sulfide, lead sulfide, and cadmium selenide.
If we measure between their extremes we will find that they can measure values in the dark close to MegaOhm (1MΩ) and, exposed to light, we will measure values around 100Ω.
They are used in lighting, turning off and on lighting (twilight switches), in alarms, in cameras, in light meters. Those of the infrared range in machine control and contagion processes and object detection.
Main characteristics of photo-resistors
Typical values range from 1 MΩ or more in the dark to 100 Ω in bright light.
Maximum dissipation, (50mW-1W).
Maximum voltage (600V).
The typical response time of an LDR is on the order of a tenth of a second.
Advantages of photo-resistors
High sensitivity (due to the large area it can cover).
Easy to use.
There is no bonding potential.
High light-dark resistance ratio.
Disadvantages of photoresistors
Narrow spectral response.
Low-temperature stability for faster materials.
The variation of the resistance value has a certain delay, different if it goes from dark to illuminated or from illuminated to dark. This limits not using LDRs in applications where the light signal varies rapidly.
Slow response on stable materials.