Relay module with ESP32 DevKit V1 — 3.3 V trigger, no boot click

A relay module lets the ESP32 switch a circuit that is electrically separate from it — a lamp, a heater element, or the coil of a contactor. The DevKit V1 can’t power the coil from its 3.3 V pin, so the coil runs from VIN (5 V) and only the control line comes from a GPIO.

Two ESP32 details matter: pick a pin that stays quiet during boot (GPIO26 here, not a strapping pin), and set the OFF level before the pin becomes an output so the relay never clicks on at reset. Type on or off in the Serial Monitor to switch it.

The low-voltage side is safe to wire on the bench. Anything on the screw terminals at 230 V is electrician-only work — and motors or pumps always go through a contactor, never straight through this relay.

Wiring

Board quirks

• The sketch writes the OFF level before pinMode(OUTPUT). Otherwise an active-LOW relay can click on for a moment at every reset.
• Avoid D2, D5, D12, D14 and D15 for relays: they are strapping pins or output PWM during boot, so the relay chatters when the ESP32 starts.
• A 3.3 V HIGH may not fully switch off a module built for a 5 V trigger (the opto LED still glows faintly). Buy a module sold as “3.3 V trigger” / “high-low level trigger”, or drive IN through an NPN transistor.
• 230 V is lethal. Only the DC+, DC− and IN side is low voltage. The COM/NO/NC terminals carry mains: have an electrician wire them behind an MCB and RCCB, switch the live wire only, earth any metal enclosure, and keep the module in a closed plastic box.
• The “10 A 250 VAC” rating is for resistive loads. A pump, motor or compressor draws several times its running current at start — let the relay switch a contactor coil, and the contactor the motor.

Code

// 1-channel relay module (active-LOW) on an ESP32 DevKit V1.
// DC+ → VIN (5 V), DC− → GND, IN → D26 (GPIO26). Type "on" or "off" in the Serial Monitor.
// Low-voltage side only. The screw terminals (COM/NO) carry 230 V — electrician only.

constexpr uint8_t RELAY_PIN = 26;
constexpr bool ACTIVE_LOW = true;  // most blue relay modules: LOW = relay on

void setRelay(bool on) {
  digitalWrite(RELAY_PIN, (on ^ ACTIVE_LOW) ? HIGH : LOW);
  Serial.printf("Relay %s\n", on ? "ON  (COM–NO closed)" : "OFF (COM–NC closed)");
}

void setup() {
  // Drive the pin to the OFF level before it becomes an output, so the relay never
  // clicks on during boot.
  digitalWrite(RELAY_PIN, ACTIVE_LOW ? HIGH : LOW);
  pinMode(RELAY_PIN, OUTPUT);
  Serial.begin(115200);
  Serial.println("Relay on GPIO26 — type on / off");
  setRelay(false);
}

void loop() {
  if (!Serial.available()) return;
  String cmd = Serial.readStringUntil('\n');
  cmd.trim();
  cmd.toLowerCase();
  if (cmd == "on") setRelay(true);
  else if (cmd == "off") setRelay(false);
  else if (cmd.length()) Serial.println("Type on or off");
}

// 1-channel relay module (active-LOW) on an ESP32 DevKit V1.
// DC+ → VIN (5 V), DC− → GND, IN → D26 (GPIO26). Type "on" or "off" in the Serial Monitor.
// Low-voltage side only. The screw terminals (COM/NO) carry 230 V — electrician only.

constexpr uint8_t RELAY_PIN = 26;
constexpr bool ACTIVE_LOW = true;  // most blue relay modules: LOW = relay on

void setRelay(bool on) {
  digitalWrite(RELAY_PIN, (on ^ ACTIVE_LOW) ? HIGH : LOW);
  Serial.printf("Relay %s\n", on ? "ON  (COM–NO closed)" : "OFF (COM–NC closed)");
}

void setup() {
  // Drive the pin to the OFF level before it becomes an output, so the relay never
  // clicks on during boot.
  digitalWrite(RELAY_PIN, ACTIVE_LOW ? HIGH : LOW);
  pinMode(RELAY_PIN, OUTPUT);
  Serial.begin(115200);
  Serial.println("Relay on GPIO26 — type on / off");
  setRelay(false);
}

void loop() {
  if (!Serial.available()) return;
  String cmd = Serial.readStringUntil('\n');
  cmd.trim();
  cmd.toLowerCase();
  if (cmd == "on") setRelay(true);
  else if (cmd == "off") setRelay(false);
  else if (cmd.length()) Serial.println("Type on or off");
}

Expected serial output

Relay on GPIO26 — type on / off
Relay OFF (COM–NC closed)
Relay ON  (COM–NO closed)
Relay OFF (COM–NC closed)

Relay on GPIO26 — type on / off
Relay OFF (COM–NC closed)
Relay ON  (COM–NO closed)
Relay OFF (COM–NC closed)

Common errors