SIM800L with NodeMCU ESP8266 — power, SoftwareSerial, AT test

A SIM800L lets a NodeMCU send SMS alerts and make missed calls when Wi-Fi is down — handy for power-cut alarms and farm pump controllers. It is also the module that most often “doesn’t work”, almost always because of power.

This guide gets the basics right: a separate 4.0 V, 2 A supply, a common ground, and the module’s serial lines on D5/D6 with SoftwareSerial, so the USB Serial Monitor stays free for testing. The sketch checks the SIM and network, then lets you type AT commands.

Wiring

Board quirks

• Power is everything. The SIM800L pulls up to 2 A in bursts when it transmits. The NodeMCU’s 3V3 regulator can’t supply that and its VIN (USB 5 V) is too high for the module (3.4–4.4 V). Use a buck converter set to 4.0 V, or a single Li-ion cell, with a big capacitor at the module.
• The ESP8266’s only full hardware UART is the USB serial on TX/RX. This guide uses SoftwareSerial (EspSoftwareSerial, bundled with core 3.1.2 — no install needed) on D5/D6. Keep it at 9600 baud: Wi-Fi interrupts can corrupt bytes at higher software-serial speeds.
• Alternative for a finished build: Serial.swap() moves the hardware UART to D7 (RX) / D8 (TX). It is rock-solid, but you lose the USB Serial Monitor, and the module’s TXD must not hold D8 high at boot — that is why this guide doesn’t start there.
• Avoid D3/D4/D8 for the module’s TXD: it idles HIGH, which would hold D8 (GPIO15) high and stop the board booting.
• India: the SIM800L is 2G-only. Jio is 4G-only, so Jio SIMs never register. Airtel, Vi and BSNL still run 2G in many areas, but check coverage first; for new designs consider a 4G module (A7670, SIM7600).
• Waits for module replies use yield() — on the ESP8266 a long wait without it can end in a soft WDT reset.

Code

// SIM800L GSM module on a NodeMCU (ESP8266) — checks the module and network, then bridges
// the Serial Monitor to the SIM800L so you can type AT commands.
// SIM800L VCC/GND → its own 4.0 V 2 A supply; that supply's − also → NodeMCU GND.
// TXD → D5 (GPIO14, SoftwareSerial RX), RXD → D6 (GPIO12, SoftwareSerial TX).
#include <SoftwareSerial.h>  // EspSoftwareSerial, bundled with the ESP8266 core

SoftwareSerial sim(D5, D6);  // RX, TX

void at(const char* cmd, unsigned long waitMs = 1000) {
  sim.println(cmd);
  unsigned long t = millis();
  while (millis() - t < waitMs) {
    while (sim.available()) Serial.write(sim.read());
    yield();  // let Wi-Fi and the watchdog run
  }
}

void setup() {
  Serial.begin(115200);
  sim.begin(9600);  // SIM800L auto-bauds; 9600 is reliable for SoftwareSerial on the ESP8266
  Serial.println();
  Serial.println("SIM800L check - set the Serial Monitor to 'Both NL & CR'");
  delay(3000);      // give the module time to start after power-up
  at("AT");         // expect OK
  at("AT+CPIN?");   // SIM inserted? expect +CPIN: READY
  at("AT+CSQ");     // signal 0-31 (99 = none)
  at("AT+CREG?");   // 0,1 = home network, 0,5 = roaming
  at("AT+COPS?");   // operator name
  Serial.println("Type AT commands now");
}

void loop() {
  while (Serial.available()) sim.write(Serial.read());
  while (sim.available()) Serial.write(sim.read());
}

// SIM800L GSM module on a NodeMCU (ESP8266) — checks the module and network, then bridges
// the Serial Monitor to the SIM800L so you can type AT commands.
// SIM800L VCC/GND → its own 4.0 V 2 A supply; that supply's − also → NodeMCU GND.
// TXD → D5 (GPIO14, SoftwareSerial RX), RXD → D6 (GPIO12, SoftwareSerial TX).
#include <SoftwareSerial.h>  // EspSoftwareSerial, bundled with the ESP8266 core

SoftwareSerial sim(D5, D6);  // RX, TX

void at(const char* cmd, unsigned long waitMs = 1000) {
  sim.println(cmd);
  unsigned long t = millis();
  while (millis() - t < waitMs) {
    while (sim.available()) Serial.write(sim.read());
    yield();  // let Wi-Fi and the watchdog run
  }
}

void setup() {
  Serial.begin(115200);
  sim.begin(9600);  // SIM800L auto-bauds; 9600 is reliable for SoftwareSerial on the ESP8266
  Serial.println();
  Serial.println("SIM800L check - set the Serial Monitor to 'Both NL & CR'");
  delay(3000);      // give the module time to start after power-up
  at("AT");         // expect OK
  at("AT+CPIN?");   // SIM inserted? expect +CPIN: READY
  at("AT+CSQ");     // signal 0-31 (99 = none)
  at("AT+CREG?");   // 0,1 = home network, 0,5 = roaming
  at("AT+COPS?");   // operator name
  Serial.println("Type AT commands now");
}

void loop() {
  while (Serial.available()) sim.write(Serial.read());
  while (sim.available()) Serial.write(sim.read());
}

Library: TinyGSM (Volodymyr Shymanskyy).

Expected serial output

SIM800L check - set the Serial Monitor to 'Both NL & CR'
AT
OK
AT+CPIN?
+CPIN: READY
OK
AT+CSQ
+CSQ: 18,0
OK
AT+CREG?
+CREG: 0,1
OK
AT+COPS?
+COPS: 0,0,"airtel"
OK
Type AT commands now

SIM800L check - set the Serial Monitor to 'Both NL & CR'
AT
OK
AT+CPIN?
+CPIN: READY
OK
AT+CSQ
+CSQ: 18,0
OK
AT+CREG?
+CREG: 0,1
OK
AT+COPS?
+COPS: 0,0,"airtel"
OK
Type AT commands now

Common errors