| name | gsm-lte-modules |
| description | GSM, LTE, and NB-IoT module interfacing: SIM800/SIM7000 AT commands, logic shifting, and power constraints. |
gsm-lte-modules
Goal: Interface cellular modems (SIM800L, SIM7000G, A7670) with microcontrollers to send text messages, perform HTTP(s) requests, or establish PPP TCP/IP bridges.
1. Power Requirements (Crucial)
Cellular modems emit bursts of RF transmission that pull up to 2A peaks.
- Connecting the VCC of a SIM800L directly to a 3.3V or 5V pin of an Arduino/Raspberry Pi will cause a brownout and infinite reboot loops.
- Rule: Use an external 3.7V - 4.2V LiPo battery or a beefy Buck Converter capable of 3A sustained, wired in parallel to the MCU.
2. UART and Logic Levels
- Modems usually operate at 2.8V logic.
- If using a 5V Arduino, you must use a logic level shifter or a resistor voltage divider on the
RX pin of the modem string.
3. The AT Command Sequence
Modems are driven via serial AT commands. Terminate all commands with \r\n.
AT // Check communication. Expect: OK
AT+CPIN? // Check if SIM is unlocked. Expect: +CPIN: READY
AT+CSQ // Check Signal Quality. (0-31, 99 is no signal).
AT+CGATT? // Check GPRS attachment. Expect: +CGATT: 1
Making an HTTP GET Request (SIM800)
AT+SAPBR=3,1,"Contype","GPRS"
AT+SAPBR=3,1,"APN","internet" // Set APN of your carrier
AT+SAPBR=1,1
AT+HTTPINIT
AT+HTTPPARA="URL","http://example.com/api"
AT+HTTPACTION=0 // 0 = GET
AT+HTTPREAD // Read the response payload
AT+HTTPTERM
4. OPSEC & Triangulation
- Connecting to a cell tower registers the SIM and the module's IMEI on the Mobile Network Operator (MNO) logs.
- The location is known to the MNO via triangulation.
