Photoresistors
(41)Photoresistors are in fact semiconductor elements which change their electrical properties depending on the environmental factors. This means that their resistance depends on the intensity of light falling on their surface.
Photoresistors’ design
A photoresistor features two leads, which are connected to comb electrodes. There is a monocrystalline or polycrystalline light-sensitive material between them. That thin structure is placed on a substrate, which is made of an insulator (e.g. ceramic or glass), while the whole device is treated with a resistant and transparent material (often with epoxy resin). Various chemical compounds, e.g. silicon, lead sulphide, cadmium sulphide, can be used in photoresistors, and their properties influence the characteristics of the device, in particular its spectral range, i.e. the frequency band of electromagnetic radiation in which the photoresistor is the most sensitive. Considering the area of application of photoresistors, this frequency usually falls within the range of visible light frequency.
Application of photoresistors
One of the most essential advantages of photoresistors is their simple implementation. They work like standard resistors, so they can be used in many traditional analogue and digital circuits, acting as light detectorsor even sensors. What’s more, thanks to this property, they can be replaced with resistors or potentiometers in the process of prototyping, to facilitate precise tests that simulate certain environmental conditions.
Another important advantage is that photoresistors are cheap, which results in a significant reduction of costs in mass production settings.
Due to all of the above mentioned reasons, photoresistors are commonly used in a multitude of devices. On the one hand, these are very simple circuits, e.g.an automatic brightness controller of a LED display or a twilight sensor (connecting the lighting circuit after dark). On the other hand, photoresistors are also applied in more complex and precise applications such as lux meters, optical conditions measuring/monitoring devices or in industrial sensor modules. In such applications, the photoresistor is usually integrated into a calibration circuit to reach high measurement precision.
Key parameters of photoresistors
Most of the photoresistors’ parameters are the same as those of passive components. The elements have the defined maximum power (which in the TME offer varies from 90 mW to 0.5 W), as well as the maximum operating voltage (which may amount to even hundreds of V). Naturally, manufacturers also define components’ thermal tolerance as well as their mounting method (it is almost always THT, i.e. through-hole technology).
Typical parameters of photoresistors include the turn-on and turn-off time (several dozen ms). While choosing a component for a specific application, take into account its sensitivity, which is shown by two values: wavelength of peak sensitivity and resistance at a given light intensity. The first one defines the light colour to which the element is adjusted, whereas the other value helps predict the element’s behaviour in given conditions. The technical documentation of every product includes more detailed information about it. It is presented in the form of a chart or an equation defining the exact dependence between the light intensity (in lux) and the components’s resistance. However, it’s worth noting that a photoresistor is usually connected to the circuit as a part of a voltage divider or through an operational amplifier. This means that the photoresistor can be adjusted for a specific application by choosing external resistors with appropriate values.
Warehouse:
