Photoelectric Sensor

Last Updated : 02 May, 2024

A photoelectric sensor is a type of sensor used to detect the presence or absence of objects, as well as to measure distance, based on the principle of light reflection or interruption. It typically consists of a light transmitter and a receiver, which are positioned opposite each other with a gap between them.

Here in this article, we will discuss the definition of photoelectric sensors, the construction and working, various types, and the characteristics briefly. There are many advantages and applications of photoelectric sensors in day-to-day life. Let us discuss all those in this article.

Table of Content

Photoelectric Sensor
Components and Construction
Types
Characteristics
Advantages
Disadvantages
Applications

What is a Photoelectric Sensor?

A photoelectric sensor is a device used to detect the presence or absence of an object and detects various objects present at large distances by using a light transmitter and a receiver. Simply, we can say that, it detects the change in light intensity. These sensors are commonly known as photo eyes. It uses visible and infrared light as a light source for transmitting and receiving. The output signal of the photoelectric sensor can be in analog or digital mode. It mainly consists of an emitter for emitting light and a receiver for receiving light.

These are widely used in industrial automation, robotics and security systems, medical, food and beverage, detecting vehicles in racing events, and many others. These sensors are usually made up of phototransistors (i.e. used as a receiver) , a light source, a signal converter, and an amplifier (i.e. for amplifying the signals). This sensor responds in microseconds. It uses light to detect an object without physical contact. These sensors have ability to detect very small objects. These are influenced by their color while detecting the object. These are also well known for their compact size and their ability to detect even invisible markings on the products.

A photoelectric sensor converts optical signals to electrical signals based on the photoelectric effect, where light of a specific frequency causes electrons to be emitted from a material’s surface . This absorption of photo energy induces an electrical effect .

Components and Construction of Photoelectric Sensor

It is a device that detects various objects over long ranges and very small objects like invisible markings in microseconds. It detects presence or absence of an objects by using light transmitter and receiver without any physical contact.

Components

Photoelectric sensor consists of :

Emitter
Receiver
Processing Circuit

Emitter : It is the source of photoelectric sensor that emits light . Light emitting diodes and lasers are mainly used as source of light in photoelectric sensors. The selection of source depends on various factors like wavelength , intensity of light etc.
Receiver : It detects the change in the intensity of light that is reflected by an object . Receiver can be phototransistor or photodiodes as they are having very fast response to light . So , basically it receives reflected light by an object .
Processing circuit : This processing circuit receives signals from the receiver and processes it by using amplifiers , filters , comparators and other processing components. It gives an output whether an object is present or not.

Circuit of Photoelectric Sensor

Working Principle

The working principle of photoelectric sensor is based on photoelectric effect . i.e., whenever an object is exposed to light or whenever a light incident on a material then it emits some electrons . These electrons excites the phototransistor or receiver which generates a voltage or current signal proportionally to the intensity of light . If an object interrupts the light , then the electrons received by the phototransistor will be reduced . Which further changes the output voltage and current signal . Then the sensor senses the change and reacts accordingly . This change in output signal indicates presence or absence of an object .

Here , emitter is the source of light , mostly light emitting diodes are used . Receiver that detects the intensity of light and mainly we use phototransistors as receivers. For observing output we can use interfaces like analog outputs or digital outputs .Analog outputs gives voltage or current signals in waveforms , whereas digital outputs gives voltage and current values in digits. These are commonly used for object detection , counting and positioning .

Types of Photoelectric Sensor

There are mainly three types of photoelectric sensors that are classified based on various applications . They are :

Through beam sensors
Reflective sensors
Diffuse reflective sensors

Through Beam Sensors

In through beam sensor , the emitter and receiver are housed in different cases that are separated from each other .Here receiver is placed within the line-of-sight of the emitter. The emitter transmits light to the receiver only in one direction. If an object interrupts the light , then intensity of the light decreases . So that output voltage or current level changes . Sensor senses this change in signal and acts accordingly . These sensors offer long sensing distances and are highly reliable as they directly detects the interruption of the light . These are suitable for applications where precise object detection is required . Through beam photoelectric sensors have a longer detection range than retroreflective and diffused photoelectric sensors . Because the light only has to travel in one direction to get from the emitter to the receiver .

Through Beam Sensor

Limitation : These are costly as we need to buy both emitter and transmitter that require two cables . These sensors requires more space as they require two cables that should be mounted in opposite directions . It must install at two points on system i.e., emitter and receiver . It cannot detect transparent objects because light can travel straight through the object to the receiver .

Retro-Reflective Sensors

In reflective sensor , the emitter and receiver are placed in a single housing . The emitted light from the emitter is reflected back from a reflector to the receiver . If an object interrupts the reflected light then it fails to reach the receiver. It indicates the presence of an object. The receiver detects the change and acts accordingly . These are used in applications where the object can reflect light such as door and gate control systems. These are used where space is limited . These are very reliable and cost is less when compared to through beam sensor. These sensors can also have a light on and dark on mode selector switch to change when the sensor output is on .

Retro-Reflective sensor

Limitation : Sensing range is less when compared with through beam sensor. This is because the light has to travel to reflector and then to the sensor instead of just travelling straight to the receiver. It must install at two points on system i.e., emitter and reflector .

Diffuse Reflective Sensors

The structure of diffuse reflective sensor is same as in reflective sensor , the emitter and receiver are placed in a single housing . But , it does not have reflector .The emitter emits a beam of light , which is directly reflected by a beam of object .The light incident on the object at any angle and it is diffused in all directions and some light is reflected back. When the object is detected within the reflective surface then the sensor acts accordingly . The presence of the object is determined by the intensity of the reflected light reaching the receiver . These are suitable applications where precise alignment of separate emitter and receiver units is not possible. The cost of diffuse reflective sensor is less than through beam sensor and reflective sensors. It must install at only one point .

Diffuse reflective sensor

Limitation : Suitable for detection of objects within limited range .Because depending on the object’s shape , size and color it might not reflect light very well back to the sensor’s receiver. This is less accurate than through beam and reflective sensors .

Characteristics of Photoelectric Sensor

Sensing range : The sensing range of photoelectric sensors vary from millimeters to several meters . As we discussed in types, through beam sensor have longest sensing range whereas diffuse reflective sensor has shorter sensing range .
Accuracy : These are well known for its high accuracy . They can detect invisible markings on the products .
Quick response : They offer fast response time ,in the range of milliseconds .
Versatility : There are various types of photoelectric sensors . These are classified based on different applications. So, this allows for their adoption to diverse detection requirements .
Non contact detection : They have ability to detect objects without any physical contact .This non contact operation prevents the object damage or sensor itself.
Maintenance : They require less maintenance . But periodic cleaning of lens ensures optimal performance overtime .
Reliability : They are well known for their reliability and long operational life , requiring minimal maintenance once installed properly.
Cost : The cost of diffuse reflective sensor is less than reflective and through beam sensor . Reflective sensor cost is less than through beam sensor .

Advantages of Photoelectric Sensor

They can detect objects in long distances and even in objects present in millimeters also .
They are easy to install and requires minimal setup time which reduces installation costs and downtime .
They have longer life span .
Diffuse photoelectric sensors detects very small objects like color markings and invisible markings on the products .
They provide fast response times and high accuracy in object detection ,making them suitable for applications where precise detection is required .
They detect objects without any physical contact , that reduces wear and tear on both the sensor and the objects being detected .
These sensors require less maintenance which reduces maintenance costs . And these sensors size is compact .
They detect all types of objects . These sensor require less equipment as it only requires reflector and sensor itself .

Disadvantages of Photoelectric Sensor

It offers various sensing ranges but they may not be suitable for applications that requires extremely long distance detection .
Interference of other sensors operating at same frequencies can disrupt the the operation of photoelectric sensors.
Proper alignment of photoelectric sensors is necessary for optimal performance. In through beam sensor proper alignment between emitter and receiver is necessary , which can be time consuming .
Reflective and diffuse reflective sensors can be sensitive to the surface properties of objects that can be detected .
They require periodic maintenance for optimal performance .

Applications of Photoelectric Sensor

These sensors are used in many industries like automotive , food and beverage , doors and gates , machine engineering , material handling , pharmaceutical industries etc.
These photoelectric sensors are used in the logistics industry and also in elevators as well as automatic doors .
These sensors are used to detect device size and spot errors , installation bag seals , object height etc.
It detects automobiles within parking areas and also detects whether crystal clear bottles are filled or not .
These are also used for checking different objects on conveyors , production lines , counting objects , color detecting ,distance measuring and bigger areas monitoring for objects through light grids .
They are commonly used in automatic door monitoring systems to detect the presence of individuals approaching the door .
These are used in racing events to detect which vehicle that first arrived the destination .

Conclusion

Thus photoelectric sensors are used to detect presence or absence of objects with help of beam of light . There are various types of photoelectric sensors .They can be used to detect most materials like metal , plastic and wood . They can even detect clear items like glass , plastic and liquids depending on what type of sensor it is .

Here in this article , we discussed about through beam sensors , reflective sensors and diffuse reflective sensors . Through beam photoelectric sensors have a longer detection range than retroreflective and diffused photoelectric sensors . These are helpful in determining the distance and absence or presence of object.

In industrial automation , they enable precise control of machinery, ensuring efficient sorting , packaging and assembly. In robotics, they facilitate obstacle avoidance and precise positioning, enhancing automation capabilities. In everyday applications like automatic doors and parking systems, they provide convenience and efficiency by detecting movement. They play vital roles in improving efficiency, safety and accuracy in various applications i.e. from controlling conveyor belts to security premises. These sensors ensure smooth operation in industrial settings , enhancing productivity and reducing errors .