4G LTE Module Mastery: Sending Messages & Making Calls with

Welcome to another exciting IoT communication project — 4G LTE Module Mastery using SIM A7670C!
In this comprehensive guide, we’ll explore how to use your A7670C 4G LTE module to send SMS messages and make phone calls directly through AT Commands.

This tutorial is designed to help you understand, configure, and master the working of cellular communication with microcontrollers such as Arduino, ESP32, or Raspberry Pi. Whether you’re an IoT beginner or an advanced embedded developer, this guide will give you the complete foundation you need to control GSM/LTE networks through serial communication.

📡 What is the SIM A7670C 4G LTE Module?

The SIM A7670C is a 4G LTE CAT1 module developed by SIMCom, designed for low-power IoT applications requiring high-speed data transmission. It supports LTE, GSM, and GPRS networks, making it perfect for projects that need SMS, calling, or cloud connectivity.

Unlike older GSM modules like SIM800L or SIM900, the A7670C supports modern LTE bands, ensuring stable connectivity even as 2G networks phase out globally.

🔹 Key Features of the A7670C Module

Feature	Description
Network Support	LTE CAT1 / GSM / GPRS
Communication Interface	UART (AT Command based)
Power Supply	3.4V – 4.2V (typically 4V)
Data Rate	Up to 10 Mbps (DL) / 5 Mbps (UL)
SMS Support	Text & PDU Mode
Voice Calling	Full voice functionality
SIM Slot	Micro SIM (Standard 2G/4G)
Antenna Support	LTE Main + GPS Antenna
USB Interface	For firmware update or debugging

This module is commonly used in:

Smart IoT gateways
Vehicle tracking systems
Remote sensors
Industrial communication devices
Smart metering solutions

⚙️ Why 4G LTE for IoT?

IoT systems often need reliable, long-distance connectivity. Wi-Fi or Bluetooth have limited range — but LTE modules overcome this by using cellular networks that cover large geographical areas.

Using 4G LTE modules like the A7670C, you can:

Send data to cloud servers via HTTP/MQTT
Send/receive SMS alerts
Initiate voice calls for security systems
Transmit real-time sensor data without Wi-Fi

This makes it ideal for remote IoT applications such as:

Smart farming systems
Home security devices
GPS tracking
Industrial monitoring

🔋 What You’ll Learn in This Project

In this project, you’ll master:

Understanding AT Commands used for LTE communication
Setting up your A7670C module with Arduino or ESP32
Sending and receiving SMS messages
Making and answering voice calls
Debugging and testing serial communication

By the end of this guide, you’ll have complete hands-on knowledge of:

“How to integrate a 4G LTE module into your IoT projects and control it using simple AT commands.”

🧩 Required Components

Component	Description
SIM A7670C LTE Module	Core communication module
Arduino UNO / Mega / ESP32	Main controller
4G SIM Card (Data + SMS plan)	For network access
Antenna (LTE + GPS)	Signal reception
USB to TTL Converter (optional)	For direct PC communication
Jumper Wires	Connections
Power Supply (4V, 2A recommended)	Stable power for the module

💡 Understanding AT Commands

AT Commands (Attention Commands) are a set of textual instructions that control GSM/LTE modules. You send them through the Serial interface, and the module responds accordingly.

For example:

Command	Function	Response
`AT`	Test communication	`OK`
`AT+CSQ`	Signal quality	`+CSQ: 20,99`
`AT+CMGF=1`	Set SMS mode (Text)	`OK`
`AT+CMGS="+91xxxxxxxxxx"`	Send SMS	`>` (enter message)
`ATD+91xxxxxxxxxx;`	Make a call	`OK` (dialing)

These commands can be sent using the Arduino Serial Monitor or directly from your microcontroller sketch.

🧠 Project Flow

Connect the A7670C module to Arduino / ESP32 using UART.
Power the module with a stable 4V supply (important!).
Send AT commands via the Serial interface to check connectivity.
Send SMS and make calls using predefined command sequences.
Add automation logic to trigger messages or calls based on sensor data.

🧾 Project Goals

✅ Successfully connect the A7670C 4G LTE module to Arduino
✅ Send and receive SMS messages
✅ Make and receive voice calls
✅ Understand and experiment with AT commands
✅ Build a foundation for future IoT projects (like GPS tracking, HTTP requests, etc.

⚙️ 1. Hardware Setup Overview

Before we start sending AT commands or coding, you need to connect the A7670C 4G LTE module properly to your microcontroller (Arduino / ESP32) and ensure stable power.
Incorrect wiring or insufficient current can prevent the module from booting or responding to commands.

🔋 2. Power Supply Requirements

The A7670C module is power-sensitive and requires:

Voltage: 3.4V to 4.2V (recommended 4.0V)
Current: Peak 2A during LTE transmission

⚠️ Important Note:
Do not power the module directly from the Arduino 5V pin — it can’t supply enough current.
Instead, use:

A 4V 2A DC adapter, or
A separate Li-ion battery (3.7V), or
A buck converter (e.g., LM2596) set to 4.0V output

Make sure GND of both power supply and Arduino are common.

🧩 3. Pin Configuration (SIM A7670C)

Pin	Function	Description
VCC	Power Input	3.4V–4.2V
GND	Ground	Common ground with MCU
TXD	UART Transmit	Connects to MCU RX
RXD	UART Receive	Connects to MCU TX
PWRKEY	Power On Pin	Needs a LOW pulse to start module
NETLIGHT	Network LED	Blinks to show status
DTR / RTS	Optional	Sleep control (not required for basic setup)

⚡ 4. Connection with Arduino UNO

A7670C Pin	Arduino UNO Pin	Description
VCC	External 4V Power	Use stable supply
GND	GND	Common ground
TXD	D2	SoftwareSerial RX
RXD	D3	SoftwareSerial TX
PWRKEY	D7	To power ON module
NETLIGHT	—	Optional (Status LED)

You can use SoftwareSerial(2,3) for communication between Arduino and A7670C.

⚡ Connection with ESP32

A7670C Pin	ESP32 Pin	Description
VCC	4V External Power	External source only
GND	GND	Common ground
TXD	GPIO16 (RX2)	Receive pin
RXD	GPIO17 (TX2)	Transmit pin
PWRKEY	GPIO5	Power ON control

Use Serial2.begin(115200, SERIAL_8N1, 16, 17); in code for hardware UART.

🔌 5. Powering ON the Module

To start the module manually:

Hold the PWRKEY pin LOW for 2 seconds, then release.
The NETLIGHT LED will start blinking.

Blink Pattern	Meaning
1 blink every second	Searching for network
1 blink every 3 seconds	Connected to LTE network
No blink	Module off

If you want Arduino/ESP32 to do this automatically, you can control the PWRKEY pin using a digital pin.

🧰 6. Serial Communication Setup

Once connected, open the Serial Monitor at 115200 baud and send the command:

AT

You should receive:

OK

If you don’t see any response:

Check baud rate (try 9600 or 115200)
Ensure RX/TX connections are not swapped
Confirm power supply stability (min 4V)
Make sure the module is powered ON (NETLIGHT blinking)

💡 7. Verifying Network Registration

Use these commands to check module status:

Command	Function	Expected Response
`AT`	Test command	`OK`
`AT+CPIN?`	Check SIM ready	`+CPIN: READY`
`AT+CSQ`	Signal strength	`+CSQ: 20,99` (higher = better)
`AT+CREG?`	Network registration	`+CREG: 0,1` (registered)
`AT+COPS?`	Operator name	`+COPS: 0,0,"Airtel"`

Once these responses are OK, your module is fully ready for sending SMS or making calls.

📈 8. Tips for Stable Communication

✅ Always ensure common GND
✅ Avoid using long jumper wires
✅ Use external power (4V, 2A)
✅ Wait 10–15 seconds after power-up before sending AT commands
✅ Use logic level shifter if connecting to 5V Arduino (optional but recommended)

🧩 9. Optional: Testing via USB to TTL

If you want to test the A7670C directly from your computer:

Connect the module to USB to TTL converter (TX–RX cross).
Open PuTTY / TeraTerm / Serial Monitor at 115200 baud.
Send AT commands manually and observe responses.

This method helps to confirm that your module and SIM card are working before coding.

🎯 End of Part 2 Summary

By now you have:

Correctly wired your A7670C LTE module
Verified serial communication
Checked SIM card and network registration

Your setup is now ready for sending SMS messages and making phone calls using AT Commands from Arduino or ESP32.

📡 1. Introduction

Now that our SIM A7670C module is fully powered and communicating properly with Arduino or ESP32, it’s time to perform real-world LTE operations —
✅ Sending SMS messages, and
✅ Making voice calls

These two actions will help you understand how AT commands work in real-time and how the module communicates through the cellular network.
This part will cover both manual AT command testing and automated code examples using Arduino.

🔹 2. Understanding AT Command Basics

AT (Attention) Commands are text-based instructions used to control GSM/LTE modems.
Each command starts with AT and ends with a carriage return (Enter).

Common AT Commands for communication:

Command	Description	Example Response
`AT`	Check communication	`OK`
`AT+CSQ`	Check signal strength	`+CSQ: 20,99`
`AT+CMGF=1`	Set SMS to Text Mode	`OK`
`AT+CMGS="number"`	Send SMS	`>` (then type message)
`ATD+91xxxxxxxxxx;`	Dial a number	`OK`
`ATH`	Hang up a call	`OK`
`ATA`	Answer an incoming call	`OK`

📲 3. Sending SMS Using AT Commands

🧠 Step-by-Step Logic

To send a message manually:

Set SMS mode to text → AT+CMGF=1
Enter the phone number → AT+CMGS="+91XXXXXXXXXX"
When > appears, type your message.
End the message with Ctrl + Z (ASCII 26)

Example sequence:

AT
OK
AT+CMGF=1
OK
AT+CMGS="+919876543210"
> Hello from A7670C 4G LTE Module!
+CMGS: 45
OK

You’ll receive this message on your phone instantly if the module is connected to the LTE network.

🔧 4. Arduino Code for Sending SMS

Here’s the full Arduino sketch for sending a text message automatically:

/***************************************************
  4G LTE Module (A7670C) - Send SMS Example
  by YaranaIoT Guru
****************************************************/

#include <SoftwareSerial.h>

SoftwareSerial A7670C(2, 3); // RX, TX pins for Arduino

void setup() {
  Serial.begin(9600);
  A7670C.begin(9600);
  delay(1000);
  
  Serial.println("Initializing 4G LTE Module...");
  A7670C.println("AT");
  updateSerial();
  
  A7670C.println("AT+CMGF=1"); // Set SMS mode
  updateSerial();
  
  A7670C.println("AT+CMGS=\"+919876543210\""); // Enter your number
  updateSerial();
  
  A7670C.print("Hello! Message from YaranaIoT Guru 4G LTE Project."); // Your message
  A7670C.write(26); // Ctrl+Z to send
}

void loop() {
  // nothing here
}

void updateSerial() {
  delay(500);
  while (Serial.available()) {
    A7670C.write(Serial.read());
  }
  while (A7670C.available()) {
    Serial.write(A7670C.read());
  }
}

✅ Explanation:

A7670C.begin(9600) initializes communication.
AT+CMGF=1 sets SMS mode to text.
AT+CMGS specifies the recipient.
A7670C.write(26) sends Ctrl+Z to complete message sending.

You’ll see the module’s response (OK / +CMGS:) on the Serial Monitor.

☎️ 5. Making a Voice Call Using AT Commands

Manual Test in Serial Monitor:

AT
OK
ATD+91XXXXXXXXXX;
OK

You’ll hear the recipient’s phone ringing.
To hang up:

ATH
OK

To answer an incoming call:

ATA
OK

⚙️ 6. Arduino Code for Calling

/***************************************************
  4G LTE Module (A7670C) - Make Call Example
  by YaranaIoT Guru
****************************************************/
#include <SoftwareSerial.h>

SoftwareSerial A7670C(2, 3); // RX, TX

void setup() {
  Serial.begin(9600);
  A7670C.begin(9600);
  delay(1000);
  
  Serial.println("Dialing Number...");
  A7670C.println("AT");
  updateSerial();
  
  A7670C.println("ATD+919876543210;"); // Replace with your number
  updateSerial();
  
  delay(15000); // Let the call ring for 15 seconds
  A7670C.println("ATH"); // Hang up
  updateSerial();
}

void loop() {}

void updateSerial() {
  delay(500);
  while (Serial.available()) {
    A7670C.write(Serial.read());
  }
  while (A7670C.available()) {
    Serial.write(A7670C.read());
  }
}

✅ Explanation:

ATD+91XXXXXXXXXX; dials a number.
The semicolon ; indicates a voice call, not a data call.
ATH ends the call.

You can also modify the code to:

Trigger calls automatically when a sensor threshold is reached.
Receive and process incoming call notifications.

🔍 7. Checking SMS Inbox

You can view stored messages with:

AT+CMGL="ALL"

Delete all SMS with:

AT+CMGD=1,4

📶 8. Troubleshooting

Problem	Cause	Solution
No response from module	RX/TX swapped or module off	Check wiring & power
“+CMS ERROR: 500”	SIM not ready	Check SIM card
“NO CARRIER”	Network issue	Check signal strength (`AT+CSQ`)
Cannot send SMS	Wrong mode	Use `AT+CMGF=1`
Call drops quickly	Low voltage	Use stable 4V 2A supply

🧠 9. Optional Enhancements

You can expand this project to:

Send sensor data via SMS (e.g., temperature alerts)
Trigger calls automatically on motion detection
Integrate GPS tracking using A7670C’s GNSS support
Log events to cloud using HTTP/MQTT

🎯 End of Part 3 Summary

By now you’ve:

Sent SMS messages using AT commands
Made and ended voice calls
Learned the structure of LTE communication
Written Arduino code for both features

Your 4G LTE module is now fully functional for SMS and calling automation!

🚀 1. Overview

In this final part of the 4G LTE Module Mastery series, we’ll take your basic SMS and calling knowledge to the next level by introducing:

Advanced AT commands,
Automated SMS and call triggers,
IoT-based control logic (using sensors), and
Integration with cloud platforms (HTTP/MQTT).

By the end, you’ll be able to build real-world LTE-powered IoT systems that don’t need WiFi at all — perfect for remote monitoring, alert systems, or industrial IoT automation.

🧠 2. Advanced AT Commands Reference

Here are some additional AT commands that can make your system smarter:

Command	Description	Example
`AT+CSQ`	Check network signal	`+CSQ: 20,99`
`AT+CCID`	Read SIM card number	`+CCID: 8991101234567890`
`AT+COPS?`	Get current operator	`+COPS: 0,0,"Jio 4G"`
`AT+CREG?`	Check network registration	`+CREG: 0,1` means registered
`AT+CMGR=1`	Read SMS from SIM slot 1	Displays message content
`AT+CMGD=1,4`	Delete all stored SMS	Clean inbox
`AT+CLIP=1`	Enable caller ID display	Shows incoming number
`AT+CLCC`	Check call status	Displays active/incoming calls
`AT+CNMI=2,2,0,0,0`	Show new SMS instantly on serial	Real-time notification

With these commands, you can build interactive IoT systems that respond to incoming SMS, network status, or external events.

🔧 3. Automation Example – SMS Alert System with Sensor

Let’s build a real-time alert system where the A7670C module sends an SMS when a sensor (like a temperature or motion sensor) crosses a limit.

🧰 Hardware Required

Arduino Uno or ESP32
A7670C 4G LTE Module
DHT11 (for temperature) or PIR (for motion) sensor
Power Supply (5V 2A or 4.1V 2A for module)
Jumper wires

⚙️ Circuit Overview

DHT11 → Arduino pin 4
A7670C RX → Arduino pin 3
A7670C TX → Arduino pin 2
Common GND
Module powered from external 4V 2A

💻 Arduino Code – Automated SMS Alert

/***************************************************
  4G LTE Module (A7670C) - SMS Alert System
  by YaranaIoT Guru
****************************************************/
#include <SoftwareSerial.h>
#include <DHT.h>

#define DHTPIN 4
#define DHTTYPE DHT11
DHT dht(DHTPIN, DHTTYPE);

SoftwareSerial A7670C(2, 3); // RX, TX

void setup() {
  Serial.begin(9600);
  A7670C.begin(9600);
  dht.begin();
  delay(2000);
  
  Serial.println("Initializing 4G LTE Module...");
  A7670C.println("AT");
  updateSerial();
  A7670C.println("AT+CMGF=1");
  updateSerial();
}

void loop() {
  float temp = dht.readTemperature();
  Serial.print("Temperature: ");
  Serial.println(temp);
  
  if (temp > 35.0) { // threshold
    sendAlert(temp);
    delay(30000); // wait before next alert
  }
  delay(2000);
}

void sendAlert(float temp) {
  String message = "Alert! High Temperature Detected: " + String(temp) + "°C";
  Serial.println("Sending SMS: " + message);
  
  A7670C.println("AT+CMGS=\"+919876543210\""); // Your number
  delay(1000);
  A7670C.print(message);
  A7670C.write(26); // Ctrl+Z
  updateSerial();
}

void updateSerial() {
  delay(500);
  while (Serial.available()) {
    A7670C.write(Serial.read());
  }
  while (A7670C.available()) {
    Serial.write(A7670C.read());
  }
}

✅ How It Works:

Reads temperature using DHT11.
If temperature > 35°C → sends alert SMS automatically.
Can be adapted for motion sensors, gas leakage, or fire detection.

☁️ 4. IoT Integration with HTTP & MQTT

Once SMS alerts work fine, you can push data to cloud servers too using HTTP POST or MQTT.

Example: HTTP POST (uploading sensor data)

AT+HTTPINIT
AT+HTTPPARA="CID",1
AT+HTTPPARA="URL","https://api.thingspeak.com/update?api_key=YOURKEY&field1=35.6"
AT+HTTPACTION=0
AT+HTTPTERM

These commands let your 4G module send sensor data directly to IoT platforms like:

ThingSpeak
Blynk Cloud (via API)
IFTTT
Google Sheets (via Apps Script endpoint)

✅ Result: You can build fully standalone IoT systems that upload data over 4G — no WiFi, no router!

⚙️ 5. Power & Stability Tips

To ensure stable operation:

Use 4V 2A regulated power for the A7670C.
Always connect common ground (GND) with Arduino/ESP32.
Keep delay (1000–2000 ms) between commands.
For ESP32, use HardwareSerial(1) instead of SoftwareSerial for better stability.

🔍 6. Real-World Use Cases

You can now create:

🚨 SMS-based fire or temperature alert systems
🏡 Home security modules with call alerts
🏭 Industrial IoT systems with 4G backup
🚗 Vehicle tracking and remote diagnostics (with GPS)
🌾 Smart agriculture projects in remote areas

🧩 7. Troubleshooting & Debugging

Issue	Possible Cause	Fix
“+CMS ERROR: 500”	SIM not ready	Check SIM and network
Module resets	Power drop	Use external 4V 2A
SMS not sent	Incorrect mode	Use `AT+CMGF=1`
“NO CARRIER” on call	Weak signal	Move antenna outside
AT commands lagging	Serial overflow	Add delay between commands

🏁 8. Final Summary

You have now mastered the A7670C 4G LTE Module.
Here’s what you’ve accomplished in all 4 parts:

Part	Focus	Skills Learned
Part 1	Introduction & Setup	Wiring, Power, and Basics
Part 2	Communication Test	Serial setup & network test
Part 3	SMS & Call	Real-world communication
Part 4	Automation & IoT	Sensor alerts & Cloud upload

🎓 Conclusion

With this guide, you’ve built a complete IoT communication backbone that works anywhere with mobile network coverage.
No WiFi, no router — just pure 4G connectivity.

You can now confidently move forward and integrate data logging, remote control, and even GPS tracking for advanced projects!

🌐 Next Steps

👉 Coming soon on YaranaIoT Guru:
“LTE IoT Tracker with GPS and Cloud Logging using A7670C + ESP32”
Stay tuned for that complete project walkthrough!

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