1 Preface
Visible light communication technology , referred to as VLC , is the latest technology in the field of wireless communication, and it can develop rapidly. In the next period of time, no matter which field it is in, this technology will definitely develop greatly, and it will form a strong competition with the existing wireless communication technology, which will have a huge impact on the progress of human civilization. If we want to build an indoor visible light communication network, we need to carry out the following two steps: first, add data transmission functions to public lighting facilities; second, organically combine communication and indoor light sources. In this way, the purpose of building VLC can be achieved , so that information data, etc. can be wirelessly transmitted from the server to the client.
The communication industry must be a major theme in various electronic information technologies. People only look at one aspect of the communication industry, such as mobile communication, the number of various mobile communication users in the world will increase and grow stronger. Despite the explosive increase in demand for wireless communications, this hasn't changed much for the wireless industry as a whole. On the contrary, the fatal flaw of lack of wireless spectrum is exposed in the case of people's continuous increase in its absorption. Therefore, the electromagnetic wave frequency band that has not been exploited by scientists, that is, visible light, can be used as a new carrier frequency band by us and properly developed.
2 Visible light communication system analysis
2.1 Visible light communication
2.1.1 Overview of LED or Laser Visible Light Communication
As we all know, the modulation method of LED or laser light-emitting devices is simple and convenient, and the visible light communication technology is based on the characteristics of convenient modulation of LED or laser devices. The high-speed light wave signal that cannot be identified and resolved, and then the light containing the transmission signal is received by a sensitive element at the receiving end, and the original signal is obtained after a certain method of processing.
The system model of optical communication is certain. According to the definition of communication, the transmitting module and receiving module are two important components of the system model. The main function of the transmitter is to transmit the signal to be transmitted, it can process the signal to be transmitted, the processed signal is suitable for LED or laser, and the processed signal is given to the LED or laser, so the LED at the transmitter Or the laser has the content of the signal we want to transmit. Moreover, due to the change of the signal at the transmitting end, the signal loaded on the LED or laser will also undergo corresponding changes of the same type, for example, the amplitude becomes larger and smaller, and the frequency becomes higher and lower. The main task of the receiving end is to extract the signal in the light, and this conversion method is to convert the optical signal into an electrical signal. Because the received signals are very small, and due to external environmental factors, certain noise will be generated, so the filter part and amplification are required at the front end of the receiving module. In addition, if digital signals are required, the obtained signals can be Give it to signal processing equipment or chips to get the signal people want.
2.1.2 Analysis of the system
Based on visible light communication technology, this system modulates the audio signal to LED or laser, and detects the audio signal by the photodetector at the transmitting end, and finally plays the audio signal through the speaker after being amplified by the power of amplification and filtering.
2.2 Modulation and demodulation principle
What is modulation? Modulation is to take a signal and use it as a basic signal, let this basic signal be the carrier of the signal, and load the data we want to transmit or other data we want to spread onto this carrier , so that the two signals can be combined, and the carrier signal can be changed with the change of the input signal, making it a dynamic process. There are many methods for modulation, which can be divided into many kinds according to various methods.
Optical modulation is essentially a way of superimposing information signals, and the modulator of the device completes this process in the form of a carrier. The modulator can make the signal to be transmitted change some parameters based on the light wave signal of the light source. Usually, when changing these parameters, the amplitude of the signal, the frequency of vibration, the phase transformation of the signal, and the wavelength are generally changed. The application range of optical modulation is very wide, and it is widely used in distance measurement and communication.
What is demodulation? It can be seen from the literal meaning that the main function of this step is to extract the signal, because the signal received at the receiving end is the combined signal of the carrier signal and the transmission signal. The combination of the two signals is sometimes necessary and sometimes is No, the main task of demodulation is to separate the two signals, and the separation should be good to ensure that the signal distortion rate is low and the information can be perfectly separated.
2.3 Working principle and characteristics of photodetector
- TEMT 6000 is a photosensitive sensor, which is composed of a high-sensitivity visible light photosensitive (NPN type) triode, which can change and amplify the captured tiny light by about 100 times.
- Can be used in occasions that are sensitive to changes in visible light, such as lighting control, screen backlight control, etc.
- Working voltage: 3.3~5V
2.4 speakers
Loudspeakers, commonly known as speakers, can also be called speakers. Electrical signals can be converted into sound signals. Speakers are especially important for the quality of the sound signal, but are more susceptible to damage. Speakers come in a variety of shapes and types, and their prices vary depending on the composition or materials used, and sometimes the prices can vary greatly.When the AC electrical signal is given to the coil, the coil will drive the paper cone to vibrate due to the force, and as the frequency and amplitude of the input signal change, the vibration amplitude and frequency of the paper cone will also change. The vibration of the paper cone drives the air to vibrate, so it can make sound.
3 program design
The process of adding the signal to be transmitted to the light wave is called "modulation". Optical modulation is the process of making the carrier carry information by changing the amplitude, intensity, frequency, etc. of the carrier (light wave).
The system consists of three components: transmitting module, power supply module and receiving module. The block diagram of the system is shown in Figure 3.1 . In order to provide power for the system, the power module converts the 220V mains power into ±5 V and ±9 V DC required by the system (the transmitting circuit can use either 5V or 9V voltage, but the receiving module can only Use soil 5V power supply for power supply, and this system needs two sets of power supplies, the transmitting and receiving ends cannot use the same set of power supply. If the same set of power supply is used, it will cause greater interference); the transmitting end outputs the mobile phone, computer, MP3, etc. through the white LED light The audio signal is converted into an optical signal, and then transmitted wirelessly in the form of visible light; the receiving module restores the optical signal to an electrical signal by the TEMT6000 module, and then completes the wireless transmission of the signal. The system uses the output signal of the PC terminal as the input signal of the system, and the frequency is about 20HZ-150KHZ. Design a relatively common amplifier and triode drive circuit to drive the brightness of the LED to change, and convert the analog audio signal into a light signal. The TEMT6000 module at the receiving end restores the optical signal to an electrical signal, and then processes the signal through a band-pass circuit composed of a multi-stage amplification, high-pass filter circuit and low-pass filter circuit, and then drives the speaker to produce sound through the drive circuit based on the TDA2030 chip .
Figure 3.1 Scheme system block diagram
In addition, the circuit board used in the experiment has high requirements on the size of the circuit design, and the more refined the layout and welding, the better, and because this scheme can already meet the requirements of basic design and improved design, and its principle is simpler and easier. Understand, intuitive and clear, fewer components, more convenient connection, more reliable welding, and easier to implement, so this solution was finally selected.
4 unit modular design
4.1 Function introduction and circuit design of each unit module
4.1.1 Transmitter circuit
Figure 4.1 Schematic diagram of LED transmitter circuit
Figure 4.2 Schematic diagram of infrared laser emission circuit
Working principle: Load the weak audio signal to the LED light or infrared laser to play the function of sending the signal. As shown in Figures 4.1 and 4.2, the main components of the entire transmitter module are two operational amplifiers, an NPN transistor, and a white LED lamp or infrared laser. The voltage follower formed by the operational amplifier ensures the input of a stable 2.5V DC voltage, and the non-inverting adder formed by the operational amplifier loads the audio signal to the input 2.5V DC voltage, amplifies it through the triode together, and drives the LED to emit light. The earphone jack is the audio signal input end, and the other input end is the DC 2.5V input end, which is used for the LED light to glow even when there is no audio input. Q1 is an NPN transistor used to drive the LED light. C1 and R5 form an emitter bias circuit to provide a stable operating point for the triode, which is a commonly used circuit in analog circuit design. R6 and R7 are used for voltage division to provide 2.5V superimposed DC voltage. After the signal is superimposed on the DC power supply, the LED or infrared laser keeps emitting light. The frequency of the light changes with the change of the input signal, and the voltage of the sensor also changes with the change of the light, so as to realize the conversion from electrical signal to optical signal.
4.1.2 Receiving and filtering circuit
Working principle: the receiving part is firstly restored to the electrical signal by the TEMT6000 optical sensor, and then the signal is passed through the high-pass filter circuit and the low-pass filter circuit, so that the passed signal is between 20Hz and 20KHz, and then processed by the amplifier circuit, and then input to the speaker. The function of the filter is to filter out unwanted signals. It can separate useful signals from noise, improve signal anti-interference and signal-to-noise ratio, and filter out unwanted frequency components. The function of the amplifier is to amplify the signal, but it does not distinguish whether it is a useful signal or a useless signal, and all signals passing through the amplifier can be amplified. This module first filters and then amplifies .
Figure 4.3 Schematic diagram of filter circuit
As shown in Figure 4.3, the filter circuit uses a Fliege high-pass filter and a Fliege low-pass filter to form a 20Hz to 20kHz active bandpass filter. The main components are OP07, resistors, and capacitors. The Fliege filter uses a dual-operation Compared with the filter with single op amp structure, it is a filter with higher cost, but it has a very strong control ability on the corner frequency or Q value, which can be adjusted very conveniently, and the filtering effect is better.
4.1.3 Amplifying circuit
Figure 4.4 Schematic diagram of the amplification circuit
Working principle: As shown in Figure 4.4, this part of the circuit is mainly composed of OP07 chip, which constitutes a negative feedback circuit to achieve the effect of amplification. The output voltage of the circuit is proportional to the magnitude of the input voltage, the phase difference is 0 degrees, and the amplification factor is v 0 = 1+ R 3 R 2 v i , in the actual circuit, R3 is changed to an adjustable resistor, so that the magnification can be adjusted, mainly to amplify the signal, so that the subsequent power amplifier circuit can work normally .
4.1.4 Power amplifier circuit
Working principle: As shown in Figure 4.5, the main body of this module is TDA2030. This module mainly includes negative feedback circuit, input and output DC blocking circuit and vibration elimination circuit. The 200k resistor, 10k resistor and 10uF capacitor to ground form a negative feedback loop.
After the signal enters the speaker drive circuit, it is connected to the amplifying circuit behind by AC coupling through a 1uF DC blocking capacitor, then passes through the TDA2030, and then passes through the output DC blocking circuit. The DC is filtered out by the capacitor C3, and only the AC signal is output. The anti-vibration circuit consists of a 1 ohm resistor and a 220nF capacitor connected in series between the output and the ground to form a anti-vibration circuit to prevent the circuit from self-oscillating. The 100uf and 0.1uf capacitors at the +VCC terminal and -VEE terminal are called decoupling capacitors. Remove the interference caused by the power supply and stabilize the power supply.
Figure 4.5 Schematic diagram of power amplifier circuit
4.2 Introduction of Special Devices
4.2.1 OP07
The OP07 chip is a low-noise bipolar operational amplifier integrated circuit; it has the characteristics of very low input offset voltage, low input bias current and high open-loop gain. The characteristics of this low offset and high open-loop gain make OP07 especially suitable for High-gain measuring equipment and amplifying weak signals of sensors, etc.
Pin function description: 1 and 8 are bias balance (zero adjustment terminal), 2 is reverse input terminal, 3 is positive input terminal,
4 is grounded, 5 is empty, 6 is output, and 7 is connected to power.
Figure 4.6 OP07 pin diagram
4.2.2 TDA2030
Figure 4.7 74LS148 pin diagram
Main features: large output current, low harmonic distortion, output-to-ground short-circuit protection function, automatic limit power consumption, protection of output transistors working in a safe working area, and built-in overheating protection circuit. Pin 1 is signal input, pin 2 is negative feedback, pin 3 is negative power/ground, pin 4 is output, pin 5 is positive power.
5 Debugging and Simulation
5.1 Debugging environment
Proteus simulation software, debugging in kind.
5.2 Simulation debugging
5.2.1 Transmitter Circuit Simulation Diagram
By loading the sinusoidal signal to simulate the audio signal, the audio signal and the DC signal can be superimposed and then output to the LED or laser light.
Figure 5.1 Simulation diagram of transmitting circuit
5.2.2 Enlarging the simulation graph
Successfully amplify the signal in positive phase, change the resistance value of R3, and the magnification factor will change accordingly.
Figure 5.2 Simulation diagram of the amplification circuit
5.2.3 Filter simulation diagram
As shown in Figure 5.3, the filtering effect is a 20-20k bandpass filter.
Figure 5.3 Simulation diagram of filter circuit
5.3 Physical Test
Physical connection and testing, the entire system connection test, each module is working normally, the input audio signal can be successfully loaded to the LED or laser light, and loaded to the speaker input after being processed by the post-stage circuit, the audio quality is good without obvious distortion.
Figure 5.4 Physical overall test diagram
Work link: https://live.csdn.net/v/243484