LDRs and Thermistors

LDRs are light dependent resistors that are often used in circuits where it is necessary to detect the presence or the level of light. A light dependent resistor is a component that is sensitive to light; therefore when light falls upon it the resistance changes. The value of resistance of the LDR is subject to change over many orders of magnitude when the value of the resistance falls as the level of light increases.
It is fairly simple to understand the basics of how an LDR works, first of all it is necessary to understand that an electric current consists of the movement of electrons within a material. Good conductors have a large number of free electrons that can drift in a given direction under the action of a potential difference. However insulators with a high resistance have very few free electrons therefore it is hard to make them move and hence a current to flow. An LDR is made of any semiconductor material with a high resistance. Consequently it has a high resistance because there are very few electrons that are free and able to move; as the vast majority of the electrons are locked into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance. As light falls on the semiconductor, the light photons are absorbed by the lattice and some of the energy is transferred to the electrons. This gives some of them sufficient energy to break free from the crystal lattice so that they can then conduct electricity. Thus resulting in a lowering of the resistance of the semiconductor and hence the overall LDR resistance.
Light dependent resistors have become very useful to the world because without them lights would have to be on all the time or they would have to be manually adjusted. An LDR saves money and time what’s more another feature of the light dependent resistor is that it can be programmed to