Relay module or relay shield with LOLIN S2 Mini

Because the S2 Mini shares the D1 mini footprint, the cheap Wemos relay shield plugs straight on — its default pin lands on GPIO35. A loose relay module works the same way: coil on VBUS, IN on GPIO35.

Type 1 or 0 in the Serial Monitor. Keep what you switch within the relay’s rating, and leave any mains wiring to an electrician.

Wiring

Every connection
LOLIN S2 Mini	1-channel relay module (5 V coil)	Wire	Notes
VBUS	DC+	red	USB 5 V for the coil (~70 mA).
GND	DC−	black
35	IN	yellow	D1 mini “D1” position — the relay shield’s default.

5 V coil part, 3.3 V board. The coil runs on 5 V, but the IN pin only drives an optocoupler LED. Pick a module rated for a 3.3 V trigger when using ESP32 or Pico — some 5 V-only modules never fully release from a 3.3 V HIGH.

Board quirks

GPIO35 is where D1 sits on a D1 mini, so the LOLIN/Wemos relay shield (default jumper D1) works on this pin. The shield is active-HIGH — set RELAY_ACTIVE_LOW = false for it.
GPIO35 is also the default I2C SCL. If you add an I2C sensor, move the relay to another pin such as GPIO37 or GPIO39.
The pin is set to its “off” level before it becomes an output, so it doesn’t click on during the S2’s boot. The blue LED (GPIO15) mirrors the relay.
COM/NO/NC carry the load. Keep to low-voltage loads or a small lamp within the relay’s rating; pumps and motors go through a contactor, and 230 V wiring with MCB, RCCB and earth is for an electrician.

Code

relay-s2mini.inoArduino C++Open in Playground Flash

// Relay on a LOLIN S2 Mini — a 1-channel relay module, or the LOLIN/Wemos relay shield.
// DC+ → VBUS (USB 5 V), DC− → GND, IN → GPIO35. GPIO35 sits where D1 is on a D1 mini,
// which is the relay shield's default pin. That shield is active-HIGH (transistor driver);
// most blue opto modules are active-LOW — set RELAY_ACTIVE_LOW to match yours.
// Low-voltage or small resistive loads only; motors and pumps go through a contactor,
// and any 230 V wiring is for an electrician.
constexpr uint8_t RELAY_PIN = 35;
constexpr bool RELAY_ACTIVE_LOW = true;  // false for the LOLIN relay shield

void setRelay(bool on) {
  digitalWrite(RELAY_PIN, (on != RELAY_ACTIVE_LOW) ? HIGH : LOW);
  digitalWrite(LED_BUILTIN, on);  // blue LED mirrors the relay
  Serial.println(on ? "Relay ON" : "Relay OFF");
}

void setup() {
  digitalWrite(RELAY_PIN, RELAY_ACTIVE_LOW ? HIGH : LOW);  // off before the pin becomes an output
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(115200);
  uint32_t t0 = millis();
  while (!Serial && millis() - t0 < 3000) {}
  Serial.println("Send 1 = on, 0 = off");
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();
    if (c == '1') setRelay(true);
    if (c == '0') setRelay(false);
  }
}

// Relay on a LOLIN S2 Mini — a 1-channel relay module, or the LOLIN/Wemos relay shield.
// DC+ → VBUS (USB 5 V), DC− → GND, IN → GPIO35. GPIO35 sits where D1 is on a D1 mini,
// which is the relay shield's default pin. That shield is active-HIGH (transistor driver);
// most blue opto modules are active-LOW — set RELAY_ACTIVE_LOW to match yours.
// Low-voltage or small resistive loads only; motors and pumps go through a contactor,
// and any 230 V wiring is for an electrician.
constexpr uint8_t RELAY_PIN = 35;
constexpr bool RELAY_ACTIVE_LOW = true;  // false for the LOLIN relay shield

void setRelay(bool on) {
  digitalWrite(RELAY_PIN, (on != RELAY_ACTIVE_LOW) ? HIGH : LOW);
  digitalWrite(LED_BUILTIN, on);  // blue LED mirrors the relay
  Serial.println(on ? "Relay ON" : "Relay OFF");
}

void setup() {
  digitalWrite(RELAY_PIN, RELAY_ACTIVE_LOW ? HIGH : LOW);  // off before the pin becomes an output
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(115200);
  uint32_t t0 = millis();
  while (!Serial && millis() - t0 < 3000) {}
  Serial.println("Send 1 = on, 0 = off");
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();
    if (c == '1') setRelay(true);
    if (c == '0') setRelay(false);
  }
}

Expected serial output

Serial Monitor · 115200Output

Send 1 = on, 0 = off
Relay ON
Relay OFF

Send 1 = on, 0 = off
Relay ON
Relay OFF

Common errors

Relay is on when it should be off

Wrong polarity setting: the relay shield is active-HIGH, blue opto modules are active-LOW. Flip RELAY_ACTIVE_LOW.

Clicks but the load doesn’t switch

Load on NC. Full fix.