Light-sensing circuits are used in outdoor lighting, camera sensors and other applications where it is necessary to detect whether the environment is illuminated or dark. Light-dependent resistors, or photoresistors, are one method of changing the behavior of a circuit in response to light. Light-dependent resistors conduct electricity when exposed to light, but in the dark, their resistance increases so that current will not flow.
How Light-Dependent Resistors Work
Light-sensitive, semiconducting compounds, such as cadmium sulfide and lead selenide, normally act as low-conducting resistors. However, photons from light within a specific range of wavelengths interact with the electrons in these materials. Photons excite enough electrons in the materials that they no longer act as resistors but instead become conductive. To create a light-dependent resistor, light-sensitive semiconductors are deposited onto an insulator and covered in clear glass or plastic. Each end of the deposited semiconductor is connected to metal contacts so it can be placed in a circuit.
Types of Light-Dependent Resistors
Light-dependent resistors are either intrinsic or extrinsic. Intrinsic refers to the intrinsic light sensitivity of the material. No additional materials are added to intrinsic light-dependent resistors to modify their behavior or conductivity. Extrinsic refers to properties added to the resistor by adding impurities to the semiconducting material. Adding impurities can shift the wavelength of photons to which the light-dependent resistor is sensitive.
Resistor Wavelength Sensitivity
The wavelength of light that activates a light-dependent resistor varies with the semiconductor used to make the resistor. Light-dependent resistors made with cadmium sulfide, or CdS, are sensitive to the visible light spectrum. When the only light illuminating a CdS light-dependent resistor is outside of the visible spectrum, the CdS device retains its resistor behavior. It only becomes conductive in the presence of visible light. Many lead-based photoresistors, such as lead selenide, only become conductive in response to infrared wavelengths.
Building a Light-Sensitive Circuit
Simple light-activated circuits can be made by placing a light-dependent resistor in series with a load and a power source. When the resistor is illuminated, the resistor becomes a conductor, and the circuit will function. Dark-activated circuits are made by placing the electrical load in parallel with the light-dependent resistor. When illuminated, the light-dependent resistor will have less resistance than the load, so the electricity flows only through the resistor. When light is removed, the light-dependent resistor will have a resistance higher than the load, so electricity will now flow through the load instead.