Types of Photosensitive Sensors
A photosensitive sensor converts light energy, also known as photons into an electrical signal. This can then be used to trigger events or output a digital signal.
There are several different types of photoelectric sensors. Each has a distinct mode of detection. These include diffused, thru-beam and retroreflective. Let’s look at the details of each.
Light Dependent Resistor (LDR)
LDRs are light-sensitive resistors that vary their resistance based on the intensity of light that strikes them. They are also known as photoresistors and can be categorized according to their linearity & photosensitive materials. The photosensitive materials used for making LDRs include cadmium sulfide, thallium sulfide, cadmium selenide, Microwave sensor and lead sulfate. Linear type LDRs have a high light-to-dark resistance ratio while non-linear ones have a low light-to-dark resistance ratio.
The operating theory behind an LDR is that when light falls on its photoconductive semiconductor material, it absorbs the energy of the photons. This energy causes electrons in the valence band to move to the conduction band, which increases the device’s conductivity. Since the electrons in the valence band have more energy than the bandgap of the material, the device’s resistance decreases.
To test the resistance of an LDR, you can use a multimeter in Ohms mode. You can also test it in the dark by covering the zig-zag shape with your hand. The result should be a high reading. If you want to measure the sensitivity of an LDR, you can try using a light-to-dark meter.
While LDRs are an essential part of most electronic circuits, they can have some drawbacks. One is that they are less responsive than other light sensors like photodiodes and phototransistors. Another is that they can be difficult to work with in certain environments.
Cadmium Sulphide (CdS)
Cadmium sulfide is the yellowish solid inorganic compound with the formula CdS. It is soluble in acid and is used as a pigment. The compound is highly toxic and carcinogenic. It is found in nature as the rare minerals greenockite and hawleyite, as well as in the more common zinc ores sphalerite and wurtzite. It is also an important component of some types of solar cells.
Due to its wide range of applications, CdS is often mixed with other semiconductors. These heterojunctions are designed to suppress carrier recombination and photocorrosion. This allows for more efficient and longer-lasting performance than unitary CdS.
In order to create CdS nanoparticles (CdS NPs), researchers have employed chemical, physical, and biological methods. The physicochemical properties of these particles vary depending on their size, shape, and coating components. These NPs can be characterized by UV-Vis spectroscopy, Fourier transform infrared spectroscopy, photoluminescence, powder X-ray diffraction spectroscopy, scanning electron microscopy, and transmission electron microscopy.
In one study, a hydrothermal method was used to create CdS NPs with a diameter of 50.8 nm. The synthesis was photosensitive sensor performed in three steps. The first step involved adding a stoichiometric amount of the cadmium precursor CdCl2 to dH2O. Then, a solution of sodium sulfate and cetyltrimethylammonium bromide was added to the mixture. Finally, the resulting solution was subjected to high-intensity ultrasound irradiation.
Transmissive
This photosensitive sensor converts radiant energy in the visible and infra-red light spectrum into an electrical signal output. This module can be connected to the Arduino or any other development board and will produce an analog value that will vary according to light intensity. This is a great little module for light level detection, night/day switches and sun tracking applications.
A photoconductive light sensor, commonly called a photoresistor, is a semiconductor material that becomes more electrically conductive when exposed to electromagnetic radiation such as visible light or infrared. The most common photoconductive cell is made from cadmium sulfide, which has a peak sensitivity in the visible range and a high resistance when unlit (dark).
When light shines on the sensor the crystalline structure of the material causes electrons to move across it. This motion induces a current in the semiconductor which can be detected by the circuitry attached to it. This current can be used to trigger a switch to turn on and off, for example a relay or a LED.
Other photo-detectors include Photodiodes, which are very versatile light sensors that can be turned “ON” and “OFF” in nanoseconds. These are found in CD and DVD-ROM drives, TV remote controls, scanners, fax machines, burglar alarms and many other devices. Photodiodes are also available in a variety of packages including surface mount, SMD and leaded versions.
Electronic Background Suppression
For applications requiring the sensor to sense objects in close proximity to a background, a photoelectric sensor with built in background suppression is recommended. Background suppression sensors are able to ignore background surfaces that have similar reflectivity as the target by modifying the light returned from the LED to the sensor. This is important because if the LED light spot from the sensor reflects off the background surface it will appear to be the same as the target and the sensor will activate a signal.
To avoid this issue, the light from the LED is transmitted through a lens to two different receiver elements (E1 and E2 on the ifm photoelectric sensor shown below). If the target obstructs the beam the reflected light will shine on E1 and not on E2. This allows the sensor to differentiate between the incoming signals and activate an output only if there is a target present.
Background suppression sensors are available in both mechanical and electronic types. Mechanical types have a screw that can be adjusted to set the point of interest, while electronic versions use software processing to configure and determine the object detection zone. The advantage of using an electronic sensor is that they are smaller and require less power than their mechanical counterparts. This means that they can fit into space-critical conveyors or machines without compromising performance or accuracy.