LI-FI is buzzing around the Internet since past few years; recently LI-FI is gained more popularity around the internet and developers. LI-FI stands for Light Fidelity which was coined by Harald Hass.

By: Girish Radhakrishnan

Circuit Objective

The objective of LI-FI is transfer data through visible light. Since the bandwidth of visible light is 10,000 times more than Radio waves, more data can be transferred through light at short period of time.

Visible light communication (VLC) eliminates the risk of some disease caused by the Radio waves due to long period exposure.

This protocol can be adapted where Radio waves are restricted, such as airplanes, hospitals, and in some research facilities. Researchers reached bit rate of 224 GB/s which is 100s of times faster than our average WI-FI connection at home or office.

This article explains about the basic idea how to make a very simple LI-FI circuit in which we will be able to transfer any audio source through light and receive it from the receiver which is placed few feet from the Transmitter.

Here it is explained about analogue communication through light, where as original LI-FI system uses digital communication, which is more complex and difficult to make one at hobby lab. But the concept is exactly the same.

analogue communication through light


Here is a simple block diagram explaining LI-FI:

block diagram explaining LI-FI:

The Design:

The circuit consists of two parts, which are receiver and transmitter. The transmitter consists of 3 transistors and few passive components paired with 1 watt LED. Which will changes its brightness with respect to audio signal.

But changes in brightness due to audio signal will not visible to human eye. We only see static illumination of white LED. The receiver consists of a photo detector (here I used solar cell) which is paired with an amplifier. The sound output is given by the speaker.

The transmitter is transistorized amplifier which consists of 3 amplifiers connected in parallel to drive the 1 watt white LED.

Each transistor base consists of voltage divider which gives necessary bias for the individual transistor. The input stage has capacitors at each transistor’s base for blocking DC signals which could degrade the quality of output.

LI-Fi Schematic

LI-FI circuit


Update: The above design can be also tried using a single transistor as shown below:


LI-FI circuit  with single transistor


You can use a current limiting resistor series with LED if you want operate the circuit at higher voltage (say 12V).You can also use standard 0.5mm white LED with current limiting resistor. For an audio source you can use mp3 player, mobile phone or a microphone with pre-amplifier etc.

The receiver consists of a 6 volt solar cell (3 volts above works fine) in series with 2.2uf capacitor which is paired with an amplifier. The amplifier need not to be the same illustrated here, but you can use any amplifier lying around your house. But make sure it as good sensitivity.


Amplifier Schematic


Li Fi receiver amplifier


Here is author’s prototype

practical Li-Fi circuit prototype


Video Clip:




You can use any amplifier with good sensitivity for receiver part. To test this circuit, go to a room where ambient light is dim and make sure no nearby electrical light source.

Place the 1 Watt LED parallel to solar cell. Turn ON the power supply for both transmitter and receiver, give audio input to transmitter, adjust the volume to transmitter. You can here clear audio sound on the receiving speaker.

The above explained Li-Fi circuit can also be tried using a photodiode as shown below, where the amplifier section is replaced with a LM386 amplifier circuit:

LiFi using photodiode


UPDATE:

Some Important Notes and Considerations Regarding the above Li-Fi Circuit


In this Li-Fi the LED does flicker, but it is in-significant for our eyes to detect.

If your eyes can detect those flickers, something wrong with the build.

The change in the brightness of LED due to the audio input is very small, but there is change in the brightness, where our eyes can’t detect.

If there is no audio input, the LED stays solid ON, the solar cell produce some voltage. The input capacitor at receiver blocks those DC signal giving almost zero voltage to amplifier.

When we apply audio signal at transmitter there will be change in the LED’s brightness (Very small). The solar cell replicates the small varying voltage, the capacitor will allow the small variation in voltage amplitude to amplifier and rejecting strong constant DC voltage.

The amplifier must have good sensitivity since the input is feeble. Probably that’s why many readers are commenting on loudness of the audio.

I have used old-school home theater’s amplifier which had very good sensitivity and the resulting output was LOUD and CLEAR.

- Girish Radhakrishnan

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