Introduction to Industrial Automation

Table 2.4  The Dark-on and Light-on Operation of a Photoelectric Switch

Photoelectric Switch 

Type

Light ON

Dark ON

Through beam

Target

Retro-reflective

Reflector

Diffuse reflective

Target

Return

spring

F

Operation

plunger

NO

NC

Movable

contacts

Fixed contacts

Switch

housing

Actuator head

Switch body

(a)

(b)

Figure 2.37  Typical form of a limit switch with (a) an interior view of switch body, and (b) a 

switch body equipped with an actuator head.

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Hardware Components for Automation and Process Control 

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specific motion of the actuating object to be acceptable with safety from the limit switch. A wide 

range of limit switches, with various actuator heads, exist in the market, thus being able to detect 

the motion of any kind of moving objects. In all these types, the switch body is the same, with 

the only difference being the type of sensor head. Some basic types of limit switches are shown 

indicatively in Figure 2.38.

Limit switches are cheaper in comparison to proximity or photoelectric switches, and thus may 

be preferred in any application where noncontact actuation is not required. There is also a wide 

range of limit switches regarding their size, from subminiature size for use inside small machines 

to heavy-duty ones for large mechanical systems. Furthermore, limit switches are available in a variety 

of rotary arm styles, operating forces, reset forces, over-travel distances, operation requirements, and 

environmental factors (including moisture, contamination, temperature, shock, and vibration). 

Limit switches have contacts with electrical ratings that are usually around 250 V AC and between 

10 and 15 A. Some manufacturers of plug-in style limit switches offer switch bodies that have 

a LED or neon lamp status indicator. As with all other sensors, the actuation of a limit switch 

Switch

body

Operating

head

Actuator

Object motion with overtravel of the actuator

Object motion without overtravel of the actuator

Adjustable lever length

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 2.38  Various types of actuator head for transmitting the external force to the change-

over mechanism suitably and thereby engaging the movable contact’s (a) push plunger, (b) roller 

plunger, (c) roller lever plunger (motion from the front or side), (d) roller lever plunger (motion 

from the back or side), (e) fork type, (f) adjustable roller arm, (g) adjustable rod, and (h) spring 

flexible rod.

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  Introduction to Industrial Automation

provides an electrical digital signal (change of contact state) that causes an appropriate control 

system response.