How to send Text Message with Arduino in a BPSK system

Binary Phase Shift Keying (BPSK) is a simple and efficient digital modulation technique widely used in communication systems to transmit binary data over a carrier signal. By shifting the phase of the carrier wave, BPSK effectively encodes the binary data (0s and 1s) for transmission. In this article, we will demonstrate how to send a text message, such as the word "Hello," with an Arduino in a BPSK system using a Colpitts oscillator as the carrier signal generator and the MC1496 as the modulator.

Overview of BPSK Modulation with Arduino

In a BPSK modulation system, two main signals are required:

Data Signal: The binary data signal representing the text message to be sent.
Carrier Signal: A high-frequency signal (often in the RF range) that carries the binary data signal. In this setup, a Colpitts oscillator is used to generate the carrier signal.

The MC1496 Balanced Modulator is a popular choice for BPSK modulation. It modulates the binary data signal onto the carrier signal, shifting the carrier's phase by 180 degrees depending on whether the data is a binary "1" or "0".

Components Required

Arduino: To generate and send the binary data signal for the text message.
Colpitts Oscillator: Generates a high-frequency carrier signal.
MC1496 Modulator: Performs BPSK modulation by combining the binary data signal with the carrier signal.
Microphone (Optional): For audio input if needed.
Other Electronic Components: Resistors, capacitors, breadboard, jumper wires, etc.

Circuit Design

The circuit implementation involves three parts:

Generating the Carrier Signal: The Colpitts oscillator is used to generate a stable carrier signal. In previous article it was illustrated how you can build a practical Colpitts oscillator. For calculating the values of the components needed for the oscillator, you can use the oscillator calculators available online.
Connecting the Arduino to the MC1496: The Arduino sends the binary data signal (representing the text message) to one of the input pins of the MC1496 modulator. This signal controls the phase shift of the carrier signal.
Modulation with MC1496: The MC1496 receives both the data signal from the Arduino and the carrier signal from the Colpitts oscillator. It performs the BPSK modulation by phase-shifting the carrier signal according to the data bits. A detailed circuit implementation guide for a BPSK system with the MC1496 can be found in BPSK system circuit with Arduino.

Arduino Code to Send "Hello" in Binary Format

The following Arduino code converts the text "Hello" into its binary ASCII representation and sends the binary bits to the MC1496 modulator via a digital output pin. The word "Hello" is repeatedly sent, allowing for continuous transmission.

Arduino Code

const int dataOutPin = 9;  // Digital output pin for binary data
const int bitDelay = 100;  // Delay in milliseconds between each bit (adjust as needed)

// ASCII binary representations of "Hello"
const byte message[] = {
  0b01001000,  // 'H'
  0b01100101,  // 'e'
  0b01101100,  // 'l'
  0b01101100,  // 'l'
  0b01101111   // 'o'
};
const int messageLength = sizeof(message) / sizeof(message[0]);

void setup() {
  pinMode(dataOutPin, OUTPUT);  // Set the data output pin as OUTPUT
}

void loop() {
  sendBinaryMessage();  // Send the binary message "Hello"
}

void sendBinaryMessage() {
  for (int i = 0; i < messageLength; i++) {
    sendByteAsBits(message[i]);  // Send each character as a sequence of bits
  }
}

void sendByteAsBits(byte character) {
  for (int i = 7; i >= 0; i--) {  // Loop through each bit in the byte from MSB to LSB
    bool bitValue = bitRead(character, i);  // Read each bit of the byte (MSB first)
    digitalWrite(dataOutPin, bitValue);  // Output the bit value to the pin
    delay(bitDelay);  // Delay to maintain bit period
  }
}

Explanation of the Code

dataOutPin: This pin is used to send the binary data to the MC1496 modulator.
bitDelay: The delay between each bit transmission. Adjust this delay to control the data rate.
message[] Array: Contains the ASCII binary representation of each character in the word "Hello."
sendBinaryMessage() Function: Loops through the message[] array and sends each byte to the modulator.
sendByteAsBits() Function: Converts each byte into a sequence of bits and sends them one by one to the output pin.

Setting Up the Carrier Signal with a Colpitts Oscillator

A Colpitts oscillator is ideal for generating the carrier signal needed for BPSK modulation. It is known for its stability and simplicity. For practical guidance refer to the building of Colpitts oscillator on a breadboard. The carrier signal frequency should be chosen based on the application requirements and can be calculated using various online tools, such as the oscillator calculators.

Connecting and Modulating with the MC1496

Carrier Input: Connect the output of the Colpitts oscillator to the carrier input of the MC1496.
Data Input: Connect the dataOutPin of the Arduino to the modulation input of the MC1496.
Output: The modulated signal (BPSK) is available at the output of the MC1496.

Testing and Receiving the Modulated Signal

To verify the transmitted BPSK signal, you can use an AM radio receiver configured to decode the BPSK signal. An example of how to set up an AM radio receiver using Arduino was explained previously. By tuning the receiver to the correct frequency, you can demodulate the BPSK signal and recover the original "Hello" message.

Conclusion

Sending text messages using Arduino in a BPSK system is a fascinating way to explore digital communication techniques. By combining the power of Arduino with a Colpitts oscillator and an MC1496 modulator, you can build a simple yet effective BPSK communication system. This project is a great way to understand the basics of modulation, digital signal processing, and the practical applications of microcontrollers in wireless communication.

With the code provided and the circuit setup described, you're well on your way to implementing a functional BPSK system capable of transmitting text messages over a carrier signal.