Introduction to Industrial Automation

Figure 2.34  Through-beam photoelectric switch.

Emitter

Reflector

Detecting 

object

Sensing distance

Receiver

Emitter

Transmitted beam

Reflected beam

Figure 2.35  Retro-reflective photoelectric switch.

---

Hardware Components for Automation and Process Control 

◾

 

49

surface, it is possible to play the role of an artificial false reflector which is normally an unwanted 

faulty situation. In such a case, polarizing filters can be placed in the light beam in order to prevent 

the sensor from false triggering due to non-polarized light signals.

Diffuse-reflective photoelectric switches, which base their operation on the reflection of the 

light beam directly on the surface of the detected object, are also called “direct reflection” 

photoelectric switches for this reason. Obviously, in this case the emitter and the receiver are 

embedded together in one housing unit, as in the retro-reflective type. The emitter emits a beam 

of light that is not returned by reflection to the receiver. When the target object is inserted in 

the light beam trajectory, the beam is diffused in many directions, one of which is reflected back 

to the receiver, as shown in Figure 2.36. Since the reflection of the beam is performed on the 

detected object, the color and the type of its surface affects the operation of the sensor. Light 

colors usually have a better behavior, offering the maximum of sensing distance, while shiny 

opaque objects affect the reflection of the beam by type and quality of the surface rather than 

by color.

Photoelectric switches come in a variety of designs, sizes, and technical characteristics, each 

type being suitable for a specific application. The terminology that has been presented for prox-

imity switches is also used in the case of photoelectric switches in a similar way. The detection 

distance for the through-beam and retro-reflective types of photoelectric switches is defined as 

the maximum distance between the emitter and receiver, or between the emitter and reflector 

correspondingly. In the case of diffuse reflection types, it is the maximum distance between the 

photoelectric switch and the detected object. The detection distance varies with the type, size, 

and model of the photoelectric switch and ranges usually from less than 10 cm up to 1500 cm. 

Photoelectric switches have a residual current, which is necessary to power the sensor, while their 

frequency or ON-OFF output cycles per second depend on the AC or DC voltage of operation 

and may range from a few Hz to 700 Hz or more. A new term, used only in photoelectric switches, 

refers to their “Dark On” or “Light On” operations. The “Dark On” operation means that the 

photoelectric switch is energized when the beam of light is interrupted, or simply when the switch 

is “in the dark”. Instead, the “Light On” operation means that the switch is energized when the 

beam of light reaches the receiver or simply when the switch is “in the light”. All cases for the three 

basic types of photoelectric switches are presented in Table 2.4.

The photoelectric switches can be connected to an AC or DC power supply source through a 

two-wire or three-wire connection, similarly to the proximity ones. The output of photoelectric 

switches is usually an SPDT contact that changes state when the switch is energized. The NO or 

NC contact change is transformed into a digital signal that can be sampled by an external control 

unit (e.g., a motion controller, PLC or an automation circuit) in order to trigger a variation in 

the operation of the overall controlled system. In addition to the above-described photoelectric 

switches, various other types, such as fiber optics types or laser types, may be used for specific 

applications.

Emitter

Detecting

object 

Receiver

Transmitted beam

Reflected beam