Introduction to Industrial Automation

in two possible states.

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Hardware Components for Automation and Process Control 

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57

Optical level switch: The optical level switch can detect the level of transparent liquids, while its 

operation principle is based on the refraction of infrared light through the liquid. Due to its very small 

dimensions, it is suitable for mounting in small devices for general use or containing small vessels of water 

or any other transparent liquid. Typical examples include switches used in drink vending machines, 

medical machines and devices, in motor vehicle technology, etc. As shown in Figure 2.45a, the optical 

level switch consists of plastic housing, a transparent hollow hemisphere, an infrared diode, and a light-

sensitive semiconductor. The infrared diode acts as a transmitter of a light beam and the semiconductor 

as a receiver of the light beam. When the switch is not covered by liquid, the infrared light beam is fully 

reflected on the surface of the hemisphere and is guided to the receiver. When the switch is covered 

with a transparent liquid, the infrared light is mostly refracted into the liquid and less light reaches the 

receiver, as shown in Figure 2.45b. The incidence or irregularity of the infrared beam on the receiver is 

then converted to a switching output that can be used suitably in an automation system.