Communication and location

SIM800L GSM with ESP32

Almost every SIM800L that answers nothing has a power problem. The module wants about 4 V, within 3.4 to 4.4 V, so a 3.3 V pin is too low and 5 V is over its range. Transmit bursts reach 2 A for a few milliseconds, and below 3.4 V it resets itself, so a blinking status LED that keeps restarting points to the supply. Use a buck converter set to 4.0 V or a single lithium cell, with bulk capacitors such as 100 µF and 1000 µF near the module.

Check the network before you buy. SIM800L is a 2G module, and carriers have been closing 2G in many countries. Ask yours whether your SIM still gets it, and if a working module answers AT but never registers, this may be why. For a new design, consider an LTE module instead. It costs more, but it does not depend on an old network.

Connect it with crossed TX and RX and a common ground. Guides disagree on the logic level: one wires RXD and TXD directly, while a module listing gives a 2.8 V UART maximum, so divide the ESP32 TX line to be safe. The default speed is 9600 or 115200 baud. Test in this order: AT should answer OK, AT+CSQ gives signal strength, and AT+CREG? shows if it has registered. Send SMS only after those work. Fit the antenna before you transmit, and keep it away from the display's touch panel, since transmit bursts can cause interference. Display boards often have no spare UART, so check that first.

Power and logic levels

5 V signal: needs a divider or level shifter

Supply	3.4–4.4 V
Signal level	UART listed at 2.8 V maximum on one module
On an ESP32	Its TX suits an ESP32 RX. Divide the ESP32 TX line to about 2.8 V.

ESP32 pins, including those on a display board's connectors, work at 3.3 V and are not 5 V tolerant. Modules differ, so if in doubt, measure the signal before you connect it.

HardwareSerial gsm(1);
const int RX_PIN = 16, TX_PIN = 17;

void at(const char *cmd) {
  gsm.println(cmd);
  unsigned long t = millis();
  while (millis() - t < 1000) {
    while (gsm.available()) Serial.write(gsm.read());
  }
}

void setup() {
  Serial.begin(115200);
  gsm.begin(9600, SERIAL_8N1, RX_PIN, TX_PIN);
  delay(3000);
  at("AT");                           // should answer OK
  at("AT+CSQ");                       // signal strength
  at("AT+CREG?");                     // network registration
}

void loop() {}

Use this information at your own risk. It is for general guidance only: the example code compiles, but it has not been tested on every part, and modules differ between sellers. Check the manufacturer's data before you build. Product names belong to their owners, and LCD Ultra Pro is not affiliated with them. See our Terms.