Li-Fi - Wireless communication using light

The term Li-Fi(Light Fidelitly) was first introuduced by Harald Hass in 2011. It is a way of wireless communication that uses light pulses as a medium for transmitting data. Transition speed is much faster than that of regular Wi-Fi, and can theoretically reach up to 100Gb/s.

 

 

Solar Panels

how a solar panel works

Solar panels are essential to Li-Fi technology because it converts light into electricity. When light enters a solar cell, an electron and an electron hole is created due to photoelectric effect. Due to the electromagnetic field located near the p-n junction, the electron moves towards the metal grid and the electron hole in the other direction. The electron flows through an electric circuit connected to the solar panel, and merges with an electron hole at the end of the circuit. The total charge or the number of electrons in the solar panel does not change during this cycle, so the process can continue for a long time.

 

Photoelectric Effect

https://www.khanacademy.org/science/ap-chemistry/electronic-structure-of-atoms-ap/bohr-model-hydrogen-ap/a/photoelectric-effect

Photoelectric effect is when electrons are emitted from a metal which light with an energy level higher than the metal's work function is absorbed. The kinetic energy of the emitted electron is equal to the energy of absorbed light minus the metal's work funtion.

Below are the work function for several metal elements.

Semiconductors and P-N Junction

how a p-n junction is formed

Semiconductors mostly consist of silicon. Silicon atoms have four electrons in their valance shell, which are used to connect with other silicon atoms in a semiconductor. A p-type semiconductor is doped with a type of atom of a similar size to a silicon atom, but with one less valance electron(Barium, for example). On the other hand, a n-type semiconductor is doped with a type of atom with five valance electrons(for instance, Phosphorus). When a p-type semiconductor and a n-type semiconductor are put close, free electrons in the n-type semiconductor which aren't used for connecting atoms flow to the 'empty spaces'(where there are no electrons to connect atoms) and fill them up. The area that this event happens is called the p-n junction. In the p-n junction, an electomagnetic field in the direction of the p-type semiconductor is formed, pushing the electron towards the n-type semiconductor and the electron hole towards the opposite.

 

An Example of Li-Fi Technology

A Li-Fi speaker is a common example of Li-Fi technology, so I decided to make one myself. My Li-Fi speaker is composed of two circuits- the transmitter and the reciever. The transmitter circuit uses a transformer to add electric signals from the sound source to another part of the circuit consisting of a power source and LED. The reciever circuit recieves light signals from the transmitter via a solar panel, and sends electrical currents made by the solar panel to a speaker. 

reciever circuit

transmitter circuit

The LED is always on, but the brightness of the emitted light slightly changes by the different voltages carried by the transformer, which is connected to the sound source. 

 

In the presence of ordinary light(room light), the speaker gave a low humming sound. Upon covering the solar panel, the sound stoped. After connecting the AUX cable to a sound source, I could hear the song coming out from the speaker - faint but for sure. After a day of researching and tinkering, my Li-Fi speaker finally worked!