Photoelectric Sensor Explained (with Practical Examples)

In this article, we will be talking about the three main types of Photoelectric Sensors: Through-Beam, Retroreflective, and Diffused.

A Photoelectric Sensor is a device that uses light to detect the presence or absence of an object.

Photoelectric sensor applications

Photoelectric Sensors can be used in many different ways and industries. For example, they can be used to detect objects or the orientation of an object on a production line, they can be used to count, and they can also be used to stop an automatic closing door.

Photoelectric Sensors are used in the automotive, food, transportation, and material handling industries to name a few.

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.

Photoelectric sensor working principle

The basic operation of a Photoelectric Sensor is, the sensor sends out a light beam from the part of the sensor called the emitter, and this light beam travels to the part of the sensor that collects the light called the receiver.

Types of photoelectric sensors

Depending on the type of sensor, the light can travel directly to the sensor’s receiver or can travel to a reflector or the object and then back to the receiver. We will go over this in more detail as we explain each of the photoelectric sensor types.

1) Through-beam photoelectric sensor

First, we will talk about the Through-Beam Photoelectric Sensor type. Through-Beam sensors have the emitter and the receiver in their own separate component.

For the Through-Beam sensor to work, the emitter and receiver have to be pointed at each other and be aligned.

When they are aligned and nothing is blocking the light, the output of the sensor will be on.

If you put something between the emitter and receiver to block the light, the output of the sensor will turn off.

The sensor’s output is the signal from the sensor to the PLC. Depending on the sensor, the output can be a positive signal or a negative signal.

The type of sensor output signal you will use depends on what type of PLC input card the sensor is connected to.

For example,

– If the sensor is PNP, meaning it has a positive output signal, the sensor’s output wire will have to be connected to a sinking input card.

– If the sensor is NPN the output signal is negative and the output wire will need to be connected to a sourcing input card.

If you would like more information on sinking, sourcing, PNP, and NPN check out our article titled Sinking and Sourcing PLC Inputs | What is the Difference?.

Light-on / dark-on mode

Some Photoelectric Sensors have a light-on, dark-on mode selector switch. With this switch, you can change when the sensor’s output turns on and off.

– In light-on mode, the sensor’s output will be on when the light can reach the receiver, and off when the light is blocked and is not reaching the receiver.

– In the dark-on mode, the sensor’s output will be on when the light is being blocked and not reaching the receiver, and off when the light is reaching the receiver.

Detection range

Through-Beam Photoelectric sensors have a longer detection range than Retroreflective and Diffused Photoelectric Sensors. This is because the light only has to travel in one direction to get from the emitter to the receiver.

Some disadvantages of using a Through-Beam sensor compared to using a Retroreflective or Diffused sensor are they cost a little more, they require more room to mount correctly, and they do not detect thin clear objects well.

Through-Beam sensors cost more because they have two components that require two cables and two mounts, this is also why they take up more space.

They do not detect thin clear objects because the light can travel straight through the object to the receiver.

2) Retroreflective photoelectric sensor

Retroreflective Photoelectric Sensors have the emitter and receiver together in the same component.

For the Retroreflective Sensor to work, the sensor’s emitter needs to be pointed at a reflector and aligned, so the light travels from the sensor’s emitter to the reflector and then bounces back to the sensor’s receiver.

The Retroreflective sensor output works the same as the Through-Beam sensor output. The output is on if the light is not blocked and the output is off if the light is blocked.

Retroreflective Sensors can also have a light-on, dark-on mode selector switch to change when the sensor’s output turns on.

Retroreflective sensors have a shorter detection range compared to Through-Beam sensors. This is because the light has to travel to a reflector and then back to the sensor instead of just traveling straight to the receiver.

Some disadvantages of using a Retroreflective sensor are you have to install the sensor with a reflector, if the object is shiny it might turn on the sensor’s output instead of the reflector, and the light beam is not as focused as a Through-Beam sensor’s light beam.

If the object is shiny, you can try adjusting the angle of the sensor and reflector compared to the object.

3) Diffused photoelectric sensor

Diffused Photoelectric Sensors have the emitter and receiver together in the same component.

For the Diffused sensor to work, the sensor’s emitter needs to be pointed at an object so the light travels from the sensor’s emitter to the object and then bounces back to the sensor’s receiver.

The Diffused sensor output works the same as the Through-Beam and Retroreflective sensor outputs.

Diffused Sensors can also have a light-on, dark-on mode selector switch to change when the sensor’s output turns on.

The main disadvantage of using a Diffused Sensor is it has the shortest detection range of the three sensors. Because depending on the object’s shape, size, and color it might not reflect light very well back to the sensor’s receiver.

Summary

In review, by reading this article you have learned about the three basic types of Photoelectric Sensors:

– Through-Beam,

– Retroreflective,

– Diffused.

You learned that all three sensors use light to detect objects and all three sensors have output signals that trigger a PLC input.

You also learned about the different sensing ranges and some of the disadvantages of each sensor.

As mentioned, we recommend checking the following related article, if you haven’t already, to have a better understanding of Photoelectric Sensors output signal:

Sinking and Sourcing PLC Inputs | What is the Difference?

Please let us know if you have any questions about the three basic types of Photoelectric Sensors in the comments below and we will get back to you in less than 24 hours.

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