Simple Li-Fi Transmitter Receiver Circuit

Just imagine using light instead of radio waves for internet.

Li-Fi is a technology that does exactly that.

This post shows you how to build a basic Li-Fi system to send and receive information using light.

The transmitter uses a common transistor and a bright LED to send the signal.

The receiver uses a special light sensor like a solar cell to pick up the signal and an amplifier to make the sound clear enough to hear from a speaker.

This is a simple project to understand how Li-Fi works, but it wont replace your Wi-Fi.

What is a Li-Fi Transmitter Receiver Circuit:

Li-Fi or Light Fidelity is a wireless communication technology that uses visible light or infrared light to transmit data.

In a Li-Fi system data is modulated onto light waves and transmitted by a light source typically an LED.

A Li-Fi receiver detects the modulated light signals and decodes them to retrieve the transmitted data.

Transmitter Working:

Parts List:

The 2N2222 transistor is configured as a common emitter amplifier.

The voltage divider at the base VBias is established using resistors R1 and R2​.

Audio input to the transmitter causes variations in the transistors output.

These variations modulate the brightness of the 1 watt LED in response to the audio signal.

Capacitors at each transistors base prevent the passage of DC signals preserving the quality of the audio signal.

If operating at higher voltages e.g.12V a current limiting resistor in series with the LED ensures proper functionality.

Formulas and calculation for transmitter working:

The formula below calculates the base voltage VBias in a voltage divider biasing circuit for a transistor:

V_Bias = R₂ / (R₁ + R₂) × V_Supply

here,

• V_Bias: This represents the voltage at the base of the transistor, which is the voltage we are trying to calculate.
• R₁ & R₂: These are the resistances of the two resistors used in the voltage divider circuit.
• V_Supply: This is the supply voltage applied to the entire circuit.

Using the voltage divider concept, the formula effectively determines the voltage across resistor R2, which is linked to the transistors base.

The closer V_Bias is to VS_upply, the larger the ratio of R2 to (R1 + R2).

Steps to Calculate VBias:

Substitute the given values:

• R1 = 4.7kΩ (kilo-ohms)
• R2 = 1kΩ (kilo-ohms)
• VSupply = Let us assume VSupply = 12V (typical supply voltage)

Calculate the total resistance:

Rtotal = R1 + R2 Rtotal = 4.7kΩ + 1 kΩ Rtotal = 5.7kΩ

Calculate the voltage divider ratio:

Voltage divider ratio = R2 / Rtotal

Voltage divider ratio = 1kΩ / 5.7kΩ

Voltage divider ratio = 0.175 (rounded to three decimal places)

Calculate VBias:

VBias = Voltage divider ratio × VSupply

VBias = 0.175 × 12V

VBias = 2.1 V (rounded to one decimal place)

Therefore, in this example, the base voltage (VBias) is approximately 2.1V

Note:

Ideal resistors perfect conductors with no voltage loss between them are assumed in this formula.

Resistor tolerances may cause small differences in real world situations.

A simple and reliable technique for adjusting a transistors base voltage is the voltage divider biasing approach.

The transistors operating point in the circuit will be determined by the resistance values used.

Transmitter Construction:

To construct the transmitter employ a 2N2222 transistor configured as a common emitter amplifier.

This alteration in LED brightness corresponds to the audio signal received.

Include capacitors at each transistor base to block DC signals and preserve audio signal quality.

For operation at higher voltages incorporate a current limiting resistor in series with the LED.

Compatible audio sources include mp3 players, mobile phones, or microphones with pre amplifiers.

Receiver Working:

Parts List:

The receiver incorporates a solar cell in series with a 2.2μF capacitor.

The solar cells voltage output is sensitive to variations in ambient light.

The varying voltage from the solar cell is fed into an amplifier LM386 or similar.

The amplifiers sensitivity ensures it can pick up the relatively weak audio signal.

The amplified audio signal is then sent to a speaker producing clear sound.

Receiver Construction:

The receiver involves a 6V solar cell (3V will also do) in series with a 2.2μF capacitor combined with an amplifier.

While the amplifier can vary opt for one with excellent sensitivity.

Test the circuit in a dimly lit room without nearby electrical light sources.

Formulas:

An audio amplifier using a single transistor and a transformer, below mentioned are the relevant formulas:

1. Voltage Gain Emitter Follower:

Voltage Gain (Av) = 1

2. Transformer Impedance Ratio:

The transformers impedance ratio is the square of the turns ratio (Np / Ns):

3. Impedance Ratio = (N_p / N_s)²

here,

• N_p: Number of turns in the primary winding
• N_s: Number of turns in the secondary winding

Note:

A high quality audio amplifier needs to be carefully designed and assembled using the right parts.

Use multi stage amplifier circuits with appropriate biasing and filtering methods for optimal performance.

Overall Circuit Operation:

The circuit is designed to operate in a dimly lit environment with minimal interference from electrical light sources.

The LED may exhibit slight flickering which is insignificant for human perception.

With audio input to the transmitter the LEDs brightness undergoes subtle changes though these changes are not visually detectable.

The solar cell at the receiver replicates these small voltage changes allowing the amplifier to produce clear and audible sound.

To test the circuit, ensure both transmitter and receiver are powered provide audio input to the transmitter and adjust the volume.

Clear audio output should be heard from the receivers speaker.

By understanding and implementing these principles you can successfully build and operate a simple Li-Fi circuit for wireless audio transmission.

Procedure:

Assemble the transmitter and receiver circuits separately.

Connect the 1 watt LED in parallel with the solar cell.

Power both transmitter and receiver and adjust the transmitters volume.

Provide audio input to the transmitter.

In a low light environment listen for clear audio from the receivers speaker.

Experiment with a photodiode in the Li-Fi circuit replacing the amplifier section with an LM386 amplifier circuit.

The circuit remains effective providing flexibility in component selection.

Important Notes and Considerations:

The LED may flicker imperceptibly visible flickering indicates potential issues.

Minute changes in LED brightness undetectable, to the human eye occur with audio input.

In the absence of audio input the LED remains solid ON with the input capacitor blocking DC signals.

The solar cell replicates small voltage variations allowing the capacitor to transmit the audio signal while rejecting constant DC voltage.

Select an amplifier with good sensitivity to enhance the circuits performance.

Conclusion:

By following these process you can construct a reliable Li-Fi circuit for wireless audio transmission offering an alternative communication solution in low light environments.

Experiment with different components to optimize performance based on your specific requirements.

References

System architecture of Li-Fi transmitter and receiver.

Li-Fi