Cellular modems

Six modems that give a board its own connection to the world — no site network, no IT department, no cable.

This is how a setup in a field, a pump station or somebody else's building sends an alarm at three in the morning.

Choosing

You need	Reach for
SMS only, cheapest, 2G	`SIM800L`
SMS and data, 2G, with GPS	`SIM808`
LTE-M / NB-IoT — the modern low-power networks	`SIM7080G`, `ADRASTEA`
LTE-M / NB-IoT and GPS	`SIM7000G`
full LTE where bandwidth matters	`EC25`

Check what your country still runs. 2G is switched off in a growing number of places, and a SIM800L on a dead network is a dead board. LTE-M and NB-IoT are the replacements built for this kind of device — low power, good building penetration, small data.

What a modem gives the program

Three blocks — see Service blocks:

SMS_SEND sends a message from the project's table
GSM_INFO reports whether the modem is registered and how strong the signal is
the modules with GPS also feed position and time into variables

Message texts and phone numbers live in the project, not the logic — so changing a number when somebody changes job is an edit and an upload, not a program change.

The traps

A modem needs current the moment it transmits

Transmission bursts pull far more than the average — hundreds of milliamps, briefly. A supply sized for the average browns out on the first burst, and the symptom is a board that resets when it tries to send. Size the supply for the peak and put a capacitor next to the modem.

Signal strength is measured where the antenna is

Not where the phone in your pocket has bars. An enclosure is a Faraday cage; an antenna inside a steel cabinet may see nothing at all. Show GSM_INFO's signal on a screen during installation so the person mounting the board can find a spot that works — instead of discovering the problem months later when the first real alarm fails to arrive.

Rate-limit what you send

A fault that chatters can send hundreds of messages and a bill to match. One fault, one message: a delay in front of the trigger and a latch behind it.

SMS is a notification, never a control path

Messages get delayed, dropped and duplicated by networks you do not control. Nothing on the equipment may depend on one arriving.

The chips

Connectivity

Chip	Bus	What it is
`ADRASTEA`	serial	Würth Adrastea-I cellular module (Sony ALT1250): LTE-M + NB-IoT + GNSS in one module, 3.0–3.6 V, AT over UART. Used by the gsm: service (SMS alarms), not a pin chip; its GNSS replaces a separate gps: receiver. PPP data dials but the module FW 06.006 wedges its IPCP on a standard ConfRej (bench 2026-08-14) — until a module FW fixes it, gsm data: needs an EC25/SIM7xxx instead. European supply chain and operator certificates — the product-line pick.
`EC25`	serial	Quectel EC25 cellular module: LTE Cat 4 with 2G/3G fallback, GNSS variants, AT over UART. Used by the gsm: service, not a pin chip. Higher data rates and legacy fallback where LTE-M coverage is thin.
`SIM7000G`	serial	SIMCom SIM7000G cellular module: LTE-M + NB-IoT + 2G fallback + GNSS, AT over UART. Used by the gsm: service, not a pin chip. The budget alternative to the Adrastea-I.
`SIM7080G`	serial	SIMCom SIM7080G cellular module: LTE-M + NB-IoT + GNSS (no 2G), AT over UART. Used by the gsm: service, not a pin chip.
`SIM800L`	serial	SIMCom SIM800L cellular module: 2G GSM/GPRS only, AT over UART. Used by the gsm: service, not a pin chip. CAUTION: 2 A supply bursts and the EU 2G phase-out — prototype use, not for the product.
`SIM808`	serial	SIMCom SIM808 cellular module: 2G GSM/GPRS + GNSS in one module (AT+CGNSPWR/CGNSINF — the SIM7000 dialect), AT over UART, separate GPS antenna connector. Used by the gsm: service, not a pin chip. Same 2 A supply bursts and 2G phase-out caution as the SIM800L.

Every entry above is in the editor: pick the chip, give it an address or a chip-select pin, and its channels become variables. See Adding a peripheral for the click-through.