SIM800 GPRS MQTT Test: Bringing Cellular IoT Online Without Wi-Fi
A field-style note on SIM800 modem startup, GPRS attach, HTTP testing, MQTT connection attempts and display-based debugging.
Why this test matters
Wi-Fi is not available everywhere. For many real projects in Bangladesh, especially remote agriculture, security, energy meter or vehicle tracking projects, a cellular connection is more useful than Wi-Fi. That is why I built this SIM800 GPRS MQTT test bench.
The goal was not only to blink a modem. I wanted a visible debugging system where the display shows modem status, network registration, GPRS connection, IP address, HTTP test and MQTT progress. When a modem fails, the display should tell me where it failed.
Boot sequence
The firmware starts by checking the modem response. Then it waits for the network. After network registration, it attaches to GPRS and receives an IP address. In the captured test screen, the device shows network OK, GPRS OK, IP assignment, HTTP GET test and received bytes.
This type of visible log is very helpful. If I only depend on a serial monitor, the device is harder to show to other people. A TFT screen makes it portable and easier to explain in a lab or project exhibition.
MQTT over GPRS
MQTT over GPRS is useful for telemetry, but it is not as smooth as Wi-Fi. GPRS has latency, weak signal problems and sometimes unstable sessions. The firmware needs retry logic, keepalive handling and proper modem reset behavior.
In the MQTT log screen, the firmware shows modem init, GPRS connect attempts, MQTT host and port, handler binding and connection progress. These logs may look simple, but they are exactly what an embedded developer needs during bring-up.
Hardware lessons
SIM800 modules are sensitive to power. A weak USB supply is often not enough. The module may restart during transmit current peaks, and then it looks like the code is wrong. In reality, the supply and wiring are the first things to check.
Short wiring, common ground, enough current capacity and proper capacitors make a big difference. I learned to respect the power side before blaming the firmware.
Where this can be used
This test bench can become the base for an energy meter, security alarm, irrigation controller, GPS tracker or industrial sensor node. The same modem flow can publish sensor values to an MQTT broker or receive remote commands.
For future work, I would turn the modem code into a reusable driver with proper reconnect states, watchdog support, APN configuration and payload queueing. The project is small in appearance, but it teaches the real backbone of cellular IoT: power stability, state management and patient debugging.