4G (GSM) quick start

Use the ThingsLinkerGSM class when a device must work without WiFi, for example in the field or in a vehicle. Every widget function is the same as on WiFi.

Hardware

• An ESP32 (any variant).
• A Simcom A7672SA, A7672E, A7672S or SIM7672 4G LTE modem.
• A SIM card with an active data plan.
• A power supply for the modem that can deliver at least 500 mA; the ESP32's 3.3 V pin is not enough.

Typical wiring:

The 4G variant needs only the ArduinoJson library.

The sketch

#include <ThingsLinkerGSM.h>

ThingsLinkerGSM iot(Serial2, "YOUR_AUTH_TOKEN", "YOUR_BLUEPRINT_KEY");

#define MODEM_PWR_PIN 4   // GPIO wired to PWRKEY, or TL_NO_PWRKEY

void setup() {
  Serial.begin(115200);
  Serial2.begin(115200, SERIAL_8N1, 16, 17);   // RX = 16, TX = 17

  iot.begin("airtelgprs.com", MODEM_PWR_PIN);  // your SIM's APN
}

void loop() {
  iot.run();

  static unsigned long last = 0;
  if (millis() - last >= 10000) {
    last = millis();
    iot.chart("V1", 24.5);   // replace with your sensor
  }
}

The library powers the modem on, registers on the network, opens the data connection, connects securely to ThingsLinker and reconnects after any drop.

APN

The APN comes from your mobile operator. Some common ones:

Operator	APN
Airtel India	airtelgprs.com
Jio India	jionet
Vodafone India	www
AT&T US	phone
T-Mobile US	fast.t-mobile.com

If your operator needs an APN username and password, pass them after the PWRKEY pin: iot.begin(APN, MODEM_PWR_PIN, "user", "pass").

PWRKEY

Your board	Use
PWRKEY wired to an ESP32 GPIO, for example 4	#define MODEM_PWR_PIN 4
PWRKEY not connected, or tied so the modem starts when powered	#define MODEM_PWR_PIN TL_NO_PWRKEY

If the ESP32 pin drives PWRKEY directly, with no transistor, the key is pressed by pulling the pin low. Tell the library before begin():

iot.setPowerKeyActiveLow(true);
iot.begin(APN, MODEM_PWR_PIN);

The key is only pressed when the modem is not answering, so restarting the ESP32 never switches off a modem that is already running.

Optional: MQTT login

As on WiFi, no MQTT login is needed for the ThingsLinker broker. For a broker of your own that asks for one, put the username and password first:

iot.begin("MQTT_USERNAME", "MQTT_PASSWORD", "airtelgprs.com", MODEM_PWR_PIN);

Useful functions

Function	Returns
iot.networkConnected()	true while the mobile data connection is up
iot.mqttConnected()	true while connected to ThingsLinker
iot.signalQuality()	Signal strength, 0 to 31 (99 means no signal)
iot.getIP()	The modem's IP address

Every reading sent over 4G uses mobile data. Send only as often as you need.