A resistor is an electronic component that reduces the amount of current flowing through a circuit. A variable resistor is one whose resistance varies depending on some condition. A photoresistor is a variable resistor that’s resistance changes depending on the amount of light it is exposed to. A photoresistor is made of a highly resistant semi-conductor material.
As a reminder, materials can be categorized in three ways, depending on their ability to conduct electricity. The electrons within an insulator cannot move freely within the structure, so electricity does not flow through them. In a conductor, electrons move freely allowing for free-flowing electricity. Semiconductors fall somewhere in between.
Let’s dive in a little to what happens within a semiconductor material to regulate the flow of electricity.
Within a semiconductor, the electrons have varying energy levels, and they arrange themselves so that similar energy levels are near each other. These levels are called energy bands.
The valence band is the level with the lowest energy, where electrons move the least freely. The conduction band has the highest energy and allows for the free movement of electricity. These bands are separated by an area called the energy gap. The resistance of the semiconductor depends on the amount of electrons available in the conduction band to carry electricity.
When light hits the photoresistor, the photons from the light excite the electrons in the valence band, increasing their energy levels and allowing them to cross the energy gap to the conduction band. Because more electrons are then available to conduct electricity the resistance of the photoresistor drops.
The changes in resistance can be measured and used to drive other electronic components. For instance, as a room grows darkers, a photoresistor can be used to turn a light on.