Photoelectric sensor has basically 5 components, light source, light detector, lenses, logic circuit and output. However in this topic we are focusing on the light source and the light detector. Light source basically consists of a LED or a laser diode.
Light source
Due to the cheap price and availability, most of the devices use LED as the light source. LED (Light emitting Diode) is a semiconductor that produces light when current is passed through them.
Figure 1: LED construction
LED consists of a state and p state. When the current is passed through the LED the electrons from one side gets excited and tries to go to the next state lose photon and comes back to the original state. This photon is the light produced by the LED. The color produced by the LED depends on the wavelength of the lights produced.
The greatest advantage of using the LED is its tendency to be turned on and off rapidly. This allows creating different pulses of the source. The amount of light produced by the LED depends on the current flowing through it. The more the current, the range of amount of light produced is high. However, more current might also destroy the LED due to the excessive heat produced by it.
Figure 2: Laser System
A laser is an optical oscillator, which can be made out of a solid, liquid or gas. It contains two with mirrors at both ends: one is totally reflecting and the other only partially. To make it work, the material is excited or "pumped," with an energy source such as light or electricity.
When this energy source gives energy the electrons in the atoms, they become excited. But the interesting part is when a few of the electrons drop back to lower energy levels spontaneously, and then release a photon (or a quantum of light). The photons created stimulate other excited electrons and emit more and more photons. Then the light waves (photons), which passes through the laser medium, is reflected by the mirrors at both ends. It keeps the light back and forth creating a chain reaction. After that, the light beam goes out through the output.
Currently laser diodes are common in use due to its unique characteristics which include consistent wavelength of the emitted light, small beam diameter, and long range.
Light detector
The light detector is the component used to detect the light produced by the light source. They are commonly known as photodiode or phototransistor. Due to its capability of sensing the different pulses of light they are known as receiver. There best application is in the receiver end of the optical fiber data transmission. The transponder connected with it converts the light sensed by these detectors to electrical pulses.
Figure 2: Spectral Response of light detector
The spectral response of the light detector determines its sensitivity to different wavelengths in light spectrum. Thus, to increase the sensitivity, the spectral of source and the detector are matched.
The photo diode is connected in revearsed bias way. It operates by converting signals that hit the junction to a voltage or current. The junction uses and illumination window with an anti-reflect coating to absorb the light photons. The result of the absorption of photons is the creation of electron hole pairs.
When the light a above a cetain frequency strike on metal, electrons are escaped which are calld photoelectrons. The maximum kinetic energy depend on the frequency of light.The photoelectric effect can be explained by the particle behaviour of light. The photoelctric equation involves:
H = Plank constant
f = frequency of the light
Φ= the work function in joules(J) / minimum energy required to remove an electron fron teh surface of material.
Ek = the maximum kinetic enerygy of the emitted electrons in joules (J)
hf is the energy of a photon of light.
This equation can be rearranged as
Comparing with
By this second equation we can draw the graph as follows
Figure 3: Frequency graph
As seen by the above equation the energy of the photon of light depends on the frequency of the light.
Thus the working principle of photodiode can be explained in the opposite of LED working principle. The photodiodes are the semiconductors with conduction and valence energy bands. Electrons at the valence band cannot move hence no current flows through the material. However a small voltage can make the electrons move from the condution band. When a photon with energy E(as described above) strikes the photodiode, the photon is absorbed. The energy of the photon is absorbed by the electron, it gets excited and is able to move thus producing the electrical signal. This is how light power- a number of photons times a photon's energy per unit of time is converted to electrical signal or current which can be explained in Figure 4.
Figure 4: Principle of operation Photodiode
References
- Encyclopedia: Definition of Laser
http://www.pcmag.com/encyclopedia/term/45928/laser[Online], Accessed:27.3.2014
2. Photo electric sensor construction, Rockwell Automation
URL:http://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12041223/Photoelectric-Sensor-Construction.html [Online], Accessed 27.3.2014
3. Photodiodes, National Instruments
URL:http://zone.ni.com/devzone/cda/ph/p/id/168 [Online], Accessed 03.04.2014