WHAT IS A PHOTORESISTOR OR LDR?
HOW DOES A LDR OR PHOTORESISTANCE WORK?
When the LDR (photoresistor) is not exposed to light, the electrons are firmly united in the atoms that make it up, but when light radiates on it, this energy releases electrons with which the material becomes more conductive, and from this way decreases its resistance. The LDR resistors only reduce their resistance with a luminous radiation located within a certain band of wavelengths. The photoresistor built with cadmium sulfide are sensitive to all visible light radiations and those built with lead sulfide are only sensitive to infrared radiation.
TYPES OF LDR
The types of photoresistors can be classified according to photosensitive materials or classified according to their linearity. The most commonly used photosensitive semiconductor glass materials for the manufacture of LDR resistors are thallium sulphide, cadmium sulphide, lead sulphide, and cadmium selenide.
- Cadmium Sulfate: The photoresistors produced with this chemical are extremely sensitive to all types of light radiation that are visible in the human being's spectrum.
- Lead sulfate: The photoresistors made with this chemical are especially sensitive to infrared radiation.
The most common classification is through linear and nonlinear:
- Linear LDR: They are better known as photodiodes but in some applications it is possible to use as photoresistors due to the linear behavior they present and their operation. (It polarizes in reverse)
- Non-linear LDR: They are the most common and are those whose behavior does not depend on the polarity with which it connects.
OHMIC VALUE OF THE LDR
When we measure between its extremes we will find that they can reach values in the dark near megaohm (1MΩ) and exposed to light we will measure values around 100Ω .
There are two basic ways to connect our LDR, they can be used depending on the desired purpose. If we have a controller it is possible to modify the results programmatically.
- Higher light, higher voltage: When connecting the photoresistance to the positive node of our voltage source we will have that, when a greater amount of light is affected, it will cause a lower voltage drop or potential differential between the source and the reference pin (Vout), therefore there will be a major reading
- Higher light, lower voltage: In short, the ldr is connected to the GND node and will cause a behavior opposite to point 1.
The resistor can be replaced by a potentiometer if we are going to change from one state to another, therefore the lighting will vary, with this we avoid modifying the programming code.
TECHNICAL CHARACTERISTICS OF THE LDR
- Typical values vary between 1 MΩ or more in the dark and 100MΩ in bright light.
- Maximum dissipation (50mW – 1W).
- Maximum voltage (600V).
- Spectral response.
- The typical response time of an LDR is in the order of one tenth of a second.
Note: It is advisable to verify the data sheet.
LDR ADVANTAGES AND DISADVANTAGES
- High sensitivity (due to the large area it can cover).
- Easy employment.
- Low cost.
- There is no union potential.
- High light-dark resistance ratio.
- Narrow spectral response.
- Hysteresis effect.
- Low temperature stability for the fastest materials.
- The variation of the value of the resistance has a certain delay, different if it goes from dark to illuminated or from illuminated to dark. This limits the use of LDRs in applications where the light signal varies rapidly.
- Slow response in stable materials.