Circuit Documentation

Summary

The circuit in question is designed to interface an ESP8266 NodeMCU microcontroller with a SIM900A GSM/GPRS module, allowing for cellular communication capabilities such as making calls, sending SMS, and receiving messages. The ESP8266 NodeMCU is powered by a 3.3V battery, while the SIM900A module is powered by a separate 5V battery. Communication between the ESP8266 and the SIM900A is established via serial communication with the appropriate TX and RX connections.

Component List

ESP8266 NodeMCU

• Description: A Wi-Fi enabled microcontroller module with an ESP8266 chip.
• Pins: D0, D1, D2, D3, D4, 3V3, GND, D5, D6, D7, D8, RX, TX, A0, RSV, SD3, SD2, SD1, CMD, SD0, CLK, EN, RST, VIN

SIM900A

• Description: A GSM/GPRS module for cellular communication.
• Pins: GND, DB9-3 (RXD), DB9-2 (TXD), 5V, 3VR, 5VR, 3VT, 5VT, VCC, Ring, RESTART, RESET, STATUS

5V Battery

• Description: A power source providing a 5V output.
• Pins: Positive, Negative

3.3V Battery

• Description: A power source providing a 3.3V output.
• Pins: +, -

Wiring Details

ESP8266 NodeMCU

• D2 (TX) connected to SIM900A DB9-2 (TXD)
• D3 (RX) connected to SIM900A DB9-3 (RXD)
• GND connected to SIM900A GND, 5V Battery Negative, and 3.3V Battery -
• VIN connected to 3.3V Battery +

SIM900A

• DB9-2 (TXD) connected to ESP8266 NodeMCU D2
• DB9-3 (RXD) connected to ESP8266 NodeMCU D3
• GND connected to ESP8266 NodeMCU GND and 5V Battery Negative
• 5V connected to 5V Battery Positive

5V Battery

• Positive connected to SIM900A 5V
• Negative connected to SIM900A GND and ESP8266 NodeMCU GND

3.3V Battery

• + connected to ESP8266 NodeMCU VIN
• - connected to ESP8266 NodeMCU GND

Documented Code

#include <SoftwareSerial.h>

// Define RX and TX pins for SoftwareSerial on ESP8266
SoftwareSerial SIM900A(D2, D3);  // RX, TX

void setup()
{
  // Start communication with SIM900A at 9600 baud rate
  SIM900A.begin(9600);
  // Start Serial Monitor communication at 115200 baud rate
  Serial.begin(115200);

  Serial.println("SIM900A Module Initialized");
  delay(100);
  Serial.println("Type 'c' for calling, 's' to send SMS, or 'r' to receive messages.");
}

void loop()
{
  if (Serial.available() > 0)
  {
    switch (Serial.read())
    {
      case 'c':
        MakeCall();
        break;
      case 's':
        SendMessage();
        break;
      case 'r':
        ReceiveMessage();
        break;
    }
  }

  if (SIM900A.available() > 0)
  {
    Serial.write(SIM900A.read()); // Display data from SIM900A
  }
}

// Function to make a call
void MakeCall()
{
  Serial.println("Making a call...");
  SIM900A.println("ATD+91XXXXXXXXXX;");  // Replace with actual number, don't forget to add ';' at the end of the number
  delay(10000);  // Wait for call to proceed
  SIM900A.println("ATH");  // Hang up the call
  Serial.println("Call ended.");
}

// Function to send an SMS
void SendMessage()
{
  Serial.println("Sending Message...");

  // Set SMS text mode
  SIM900A.println("AT+CMGF=1");  
  delay(1000);

  // Set the receiver's phone number (replace with actual number)
  SIM900A.println("AT+CMGS=\"+91XXXXXXXXXX\"");  
  delay(1000);

  // Message content
  SIM900A.println("Hello, this is a message from ESP8266 and SIM900A module.");
  delay(1000);

  // End the message with a CTRL+Z character (ASCII code 26)
  SIM900A.write(26);  
  delay(1000);

  Serial.println("Message sent.");
}

// Function to receive SMS
void ReceiveMessage()
{
  Serial.println("Receiving Message...");

  // Enable SMS reception
  SIM900A.println("AT+CNMI=2,2,0,0,0");  
  delay(1000);

  // The received SMS will be automatically displayed via Serial when available
  Serial.println("Waiting for messages...");
}

This code is designed to run on the ESP8266 NodeMCU and provides basic functionality for interfacing with the SIM900A module. It includes functions for making a call, sending an SMS, and setting up the module to receive SMS messages. The code uses the SoftwareSerial library to create a serial connection on pins D2 and D3 of the ESP8266, which are connected to the SIM900A's TXD and RXD pins, respectively.