Light dependent resistors (LDRs) are variable resistors that are controlled by light. They are also known as photoresistors or photo-conductive cells and are usually used in light or dark-activated switching circuits, or light sensitive detector circuits. They are used in circuits which are required to sense light. For example, you can program your circuit so that when the LDR does not detect light, it will trigger a light to turn on.
How do LDRs work?
When the LDR sensor is exposed to light, the resistance of the LDR decreases as the intensity of light increases. The wavelength of the light affects the resistance of the resistor. LDR light dependent resistors are made of a high-resistance semiconductor, which absorbs photons until bound electrons are able to jump the conduction band. The free electrons conduct electricity and therefore lower resistance. Depending on the light, the resistance of the LDR will reach up to 1MΩ. Ω is the symbol for ohms.
Intrinsic LDR vs. Extrinsic LDR
In intrinsic LDRs, photons must jump the entire bandgap, so they need to gain more energy before the resistance will decrease. Extrinsic LDRs have impurities, which means the electrons don't need to jump as far, making them more sensitive to lower energy photons.
Intrinsic LDR resistors contain silicon or germanium and are pure semiconductor devices. Extrinsic LDRs have impurities.
When intrinsic LDRs receive light their electrons from the valence band to the conduction band get excited and the number of charge carriers changes. For extrinsic LDRs, when the light hits the surface, the impurities create new energy bands over the valence band. There is a decrease in the band gap. Extrinsic photo resistors are ideal for sensing light with longer wavelengths.
What are light dependent resistors used for?
LDR resistors have many practical applications in situations in which the resistance needs to change depending on the amount of light available. For example:
Outdoor solar-powered lamps