Photoresistor

A photoresistor is a type of resistor whose resistance decreases when the intensity of light increases. In the absence of light, the resistance can be several megaohms, but in the presence of light, the resistance can drop to a few hundred ohms. This property is known as photoconductivity.

When light of a certain frequency is incident on a photoresistor, the absorbed photons give bound electrons enough energy to jump into the conduction band. The resulting free electrons conduct electricity, thereby lowering resistance.

Photoresistors are used in light-sensitive detector circuits and light-activated and dark-activated switching circuits. They can indicate the presence or absence of light and change the way a circuit behaves accordingly.

However, photoresistors are not suitable for applications requiring precise measurement of or sensitivity to light photons, as their photoresistivity may vary widely depending on ambient temperature. They also exhibit a certain degree of latency between exposure to light and the subsequent decrease in resistance.

It’s important to note that photoresistors are less light-sensitive than photodiodes or phototransistors. They do not have a PN junction. The resistance range and sensitivity of a photoresistor can substantially differ among dissimilar devices. Moreover, unique photoresistors may react substantially differently to photons within certain wavelength bands.