This project is a fire detection system that continuously monitors ambient temperature using a DHT11 sensor connected to an ESP8266 microcontroller. When the sensor reading exceeds a set temperature (50 °C for example), a buzzer is activated and an SMS alert is sent via a SIM800L GSM module to a predefined phone number. This device provides immediate local warning and remote notification, helping to prevent fire-related incidents in homes or workspaces.

The system consists of both hardware and software modules working together:

• ESP8266 Microcontroller: Reads temperature data, processes the threshold logic, controls the buzzer, and interfaces with the GSM module.
• DHT11 Temperature Sensor: Provides real-time temperature readings to the microcontroller over a digital signal.
• Buzzer: Emits an audible alarm when the temperature crosses a set value.
• SIM800L GSM Module: Sends SMS alerts over UART to notify remote users.
• Push button: Stops the buzzer alarm.

Schematic made in Fusion 360. The ESP8266 was made manually because there seems to be no schematic for it on the internet.

- Temperature Sensor DHT11

• Data (OUT) → pin D2 (GPIO4) of the ESP8266 board
• VCC → 3.3V
• GND → GND

- Button (Reset Alarm)

• First pin → pin D1 (GPIO5) of the ESP8266
• Second pin → 3.3V
• Third pin → GND
• Software configuration: internal pull-up resistor is enabled with pinMode(D1, INPUT_PULLUP);

- Passive Buzzer

• Signal pin → pin D0 (GPIO16)
• GND → GND

- SIM800L GSM Module

• TX (SIM800L output) → pin D7 (ESP8266 RX)
• RX (SIM800L input) → pin D8 (ESP8266 TX)
• VCC → separate 3.7V power source (Li-ion battery) or stabilized step-down converter
• GND → Common GND with ESP8266

- ESP8266 (NodeMCU)

• USB → PC (for power and serial communication)

The firmware is developed in the Arduino IDE targeting the ESP8266 platform.

- DHT11 Temperature Sensor

• Reads the ambient temperature through digital pin D2 (GPIO4).
• If the temperature exceeds 26.1°C, an alarm state is triggered.
• The reading is done using the `Adafruit_Sensor`, `DHT`, and `DHT_U` libraries.
• Temperature is read periodically (every ~0.5 seconds) and logged to the serial monitor.

- Alarm System (Passive Buzzer and Button)

• The buzzer is connected to pin D0 (GPIO16) and activated using the `tone()` function when an alarm is triggered.
• A push button connected to D1 (GPIO5) is configured with an internal pull-up resistor (`INPUT_PULLUP`).
• When the button is pressed, the alarm is silenced, and the system enters a cooldown period of 1 minute to prevent retriggering.
• `alarmActive` and `smsSent` boolean flags are used to manage system state.

- SIM800L GSM Module

• Communicates with the ESP8266 via `SoftwareSerial` on pins D7 (RX) and D8 (TX).
• During setup, AT commands are used to initialize the module and verify signal/SIM status.
• Sends an SMS alert once per alarm event using the `sendSMS()` function.
• The SMS content is: *”ALERTA: Temperatura depaseste 26.1C!”*

• Upon startup:
  1. Serial communication is initialized.
  2. The DHT11 and SIM800L modules are set up.
  3. AT commands check SIM status, signal quality, and configure SMS mode.
• In the main loop:
  1. The system reads temperature from the DHT11 sensor.
  2. If temperature > 26.1°C and no alarm is active:
     1. The buzzer is activated.
     2. An SMS is sent (only once per event).
  3. If the user presses the button:
     1. The alarm stops.
     2. A 60-second cooldown prevents immediate retriggering.
• Messages and debug information are printed to the serial monitor for monitoring and troubleshooting.

Libraries Used

- `Adafruit_Sensor.h`, `DHT.h`, `DHT_U.h`: For interfacing with the DHT11 temperature sensor. - `SoftwareSerial.h`: Enables serial communication between the ESP8266 and the SIM800L GSM module.

Working on this project was both challenging and fun. At first, I wasn’t sure if everything would work together, especially the SIM800L module, which needed a lot of trial and error to set up properly. I also had to be careful with wiring and power, since I didn’t want to damage any components.

I learned a lot about how to use interrupts, how UART communication works, and how to read data from the DHT11 sensor. I also got some practice debugging both hardware and software issues, which sometimes took a while but felt great once they were solved.

In the end, I managed to build a system that detects high temperatures, triggers an alarm, and sends an SMS alert. It even has a button to silence the alarm, which works using interrupts.

Even though it’s a simple version, I think it’s a good starting point, and I’m happy with how it turned out. With more time, I’d like to improve the design and maybe add more features. Overall, it was a great learning experience.

ESP8266 with DHT11 – Newbiely guide – A detailed tutorial on how to connect and read temperature from a DHT11 sensor using the ESP8266.

SIM800L GSM Module with Arduino – Last Minute Engineers – A complete guide for using the SIM800L module to send SMS messages via AT commands.

ESP8266 Interrupts – Official Documentation – Official Arduino documentation on using interrupts.

LM2596 datasheet and guide – Technical overview of the LM2596 buck converter module