The resistance values of an LDR may change over several orders of magnitude. The resistance value will be dropped when the light level increases. The resistance values of LDR in darkness are several megaohms whereas in bright light it will be dropped to hundred ohms.
So due to this change in resistance, these resistors are extremely used in different applications. The designing of LDRs can be done by using semiconductor materials to allow their light-sensitive properties.
The famous material used in this resistor is CdS cadmium sulfide , even though the utilization of this material is currently restricted in European countries due to some environmental issues while using this material.
Likewise, CdSe cadmium selenide is also restricted and additional materials that can be employed mainly include PbS lead sulfide , InS indium antimonide. Even though for these resistors, a semiconductor material is used, because they are simply passive devices and they do not have a PN-junction. In electronic circuits, the LDR symbol is used that mainly depends on the resistor symbol; however, it illustrates the light rays in the arrows form. The LDR circuit symbols are shown below.
The construction of an LDR includes a light-sensitive material that is placed on an insulating substrate like ceramic. The material is placed in a zigzag shape in order to get the required power rating and resistance. The area of zigzag separates the metal-placed areas into two regions. Where the Ohmic contacts are made either on the sides of the area. The resistances of the contacts must be as less as possible to make sure that the resistance, mainly varies due to the light effect only.
The working principle of an LDR is photoconductivity, which is nothing but an optical phenomenon. When the light is absorbed by the material then the conductivity of the material enhances. When the light falls on the LDR, then the electrons in the valence band of the material are eager to the conduction band.
But, the photons in the incident light must have energy superior to the bandgap of the material to make the electrons jump from one band to another band valance to conduction. Hence, when light having ample energy, more electrons are excited to the conduction band which grades in a large number of charge carriers.
As explained above, the main component for the construction of LDR is cadmium sulphide CdS , which is used as the photoconductor and contains no or very few electrons when not illuminated. In the absence of light it is designed to have a high resistance inthe range of megaohms. As soon as light falls on the sensor, the electrons are liberated and the conductivity of the material increases.
When the light intensity exceeds a certain frequency, the photons absorbed by the semiconductor give band electrons the energy required to jump into the conduction band. The equation to show the relation between resistance and illumination can be written as. Values usually range betwee 0. Practical LDRs are available in a variety of sizes and package styles, the most popular size having a face diameter of roughly 10 mm.
The active semiconductor region is normally deposited onto a semi-insulating substrate and the active region is normally lightly doped. In many discrete photoresistor devices, an interdigital pattern is used to increase the area of the photoresistor that is exposed to light. The pattern is cut in the metallisation on the surface of the active area and this lets the light through. The two metallise areas act as the two contacts for the resistor. This area has to be made relatively large because the resistance of the contact to the active area needs to be minimised.
This type of structure is widely used for many small photoresistors or light dependent resistors that are seen. The interdigital pattern is quite recognisable. Each material gives different properties in terms of the wavelength of sensitivity, etc. In view of the environmental concerns of using Cadmium, this material is not used for product in Europe.
Regardless of the type of light dependent resistor or photoresistor, both types exhibit an increase in conductivity or fall in resistance with increasing levels of incident light. The sensitivity of photoresistors is shown to vary with the wavelength of the light that is impacting the sensitive area of the device.
The effect is very marked and it is found that if the wavelength is outside a given range then there is no noticeable effect. Devices made from different materials respond differently to light of different wavelengths, and this means that the different electronics components can be used for different applications. It is also found that extrinsic photoresists tend to be more sensitive to longer wavelength light and can be used for infrared.
However when working with infrared, care must be taken to avoid heat build-up caused but he elating effect of the radiation. One important aspect associated with photoresistors or light dependent resistors is that of the latency, or the time taken for the electronic component to respond to any changes. This aspect can be particularly important for a circuit design.
However when the light changes take place over a period of time they are more than adequate. The rate at which the resistance changes is called the resistance recovery rate. It is for this reason that one of the specifications normally quoted in the electronic component datasheets for photo-resistors is the dark resistance after a given time, typically in seconds.
Often two values are quoted, one for one second and another for five seconds. These given an indication of the latency of the resistor. Photoresistors are found in many different applications and can be seen in many different electronic circuit designs. It flows from the positive battery terminal, through R1, the LED, and the transistor down to the negative battery terminal. The resistor R1 controls the amount of current going through the LED.
I have written an article on how to calculate the resistor value for an LED. What if you want to power the circuit with something other than a 9V battery?
Then you need to change the resistor value to get the right amount of current flowing through the LED. The variable resistor R2 is used to change the trigger point for the LED.
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