When you walk up to a department store, the door opens for you automatically. Inside, you may use the restroom and the faucets and hand dryers seem to know you are there. These devices work flawlessly without the use of manual switches. This kind of intelligence comes from photoelectric sensors or photo eyes but have you ever wondered how these things really work? Let’s check them out and see.
Photoelectricity
In order to understand how a photo eye operates, you need to know something about photoelectricity, which is the creation of electric current from beams of light. In fact, this is what makes solar energy possible.
Long before photoelectric sensors, a physicist from Germany (Hertz) first discovered photoelectricity in the late 19th Century and by the turn of the 20th Century, Einstein was able to explain how it worked. So how does light become electric energy?
A basic law of physics states matter cannot be created nor destroyed but it can be changed form one form to another (matter to energy for example). Einstein proved that by shining a beam of light onto metal, it could produce electricity. Light is made up of tiny particles called photons and when they reach the metal, they displace some of the electrons (creating electric current). This was the forerunner of the famous Quantum Theory, resulting in Einstein winning a 1921 Noble prize.
Types of Photoelectric Sensors
There are three different types of sensors used today:
- Through beam – a transmitter shines light on a photo cell. When something passes by, it breaks the beam, triggering a switch. (Automatic doors)
- Reflective – transmitter and receiver are in the same area. (Luggage detector conveyor)
- Diffusive – only one unit is required because both transmitter and receiver are located in the same unit. The beam is bounced from transmitter, to the object, back to receiver. (Automatic water faucets)
