We've already used the SIM800L GSM module to let SMS messages control a lamp remotely — this project flips that direction, using the same module to send temperature data out to your phone on request rather than receiving control commands.
How it works
A DHT22 sensor continuously monitors ambient temperature. When a text message containing a specific keyword arrives, the SIM800L module passes that message to Arduino over serial, the sketch recognizes the trigger keyword, reads the current temperature, formats it as a short text string, and sends an SMS reply back to whichever phone number sent the request.
Components needed
An Arduino Mega 2560, a DHT22 temperature and humidity sensor, a SIM800L GSM/GPRS module, an LM2596 buck converter to step 12V down to the module's required voltage, and supporting wiring.
Wiring it up
The DHT22's data pin connects to a digital pin on the Mega, while the SIM800L communicates over serial (TX/RX), needing its own regulated power from the LM2596 converter since it draws current spikes that a direct 5V pin usually can't supply reliably.
How this compares to our earlier SMS lamp project
Our earlier SMS-controlled lamp project used incoming text messages as commands to switch a relay — data flowing from your phone to the Arduino. This project reverses that: it's the Arduino sending data back out, replying to a request with a live sensor reading rather than acting on an instruction. Both share the same SIM800L wiring basics, just used for opposite directions of communication.
Where this is used
Remote temperature reporting like this fits security and facility monitoring where checking in physically isn't practical, greenhouse or storage monitoring, and any setup where getting a quick temperature check by text is more convenient than a live dashboard.
You can find SIM800L GSM modules and DHT22 sensors at Juthour Tech.
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