Relay module with ESP32-C3 SuperMini — safe wiring and code

A relay lets the SuperMini switch a separate circuit — a 12 V pump through a contactor, a fan, a small lamp. This guide covers the low-voltage side: coil power from 5V, the trigger on GPIO7, and a relay that stays off while the C3 boots.

Type 1 or 0 in the Serial Monitor to switch it. What you connect to COM/NO is the part that can hurt: keep to low-voltage loads, use a contactor for motors, and leave mains wiring to an electrician.

Wiring

Board quirks

• GPIO7 is quiet during boot. Avoid GPIO2/8/9 (strapping pins), and GPIO21, which carries the ROM’s boot messages and makes a relay chatter at reset.
• The sketch writes the pin HIGH (relay off) before making it an output, so the relay never clicks on while the board starts.
• Feed the coil from 5V, not 3V3: the SuperMini’s 3.3 V regulator is small and a relay coil plus Wi-Fi bursts can brown it out.
• Many 5 V modules need more than 3.3 V on IN to switch fully off. Buy one marked “3.3 V trigger”, or one with a JD-VCC jumper.
• COM/NO/NC are the switched side. Use the relay for low-voltage loads or a small lamp within its rating; pumps and motors go through a contactor, and any 230 V wiring (with MCB, RCCB and earth) is for an electrician.

Code

// 1-channel relay module (3.3 V-trigger, active-LOW) on an ESP32-C3 SuperMini.
// DC+ → 5V (USB 5 V), DC− → GND, IN → GPIO7.
// GPIO7 is not a strapping pin (2/8/9) and is quiet during boot, unlike GPIO21 (UART0 TX),
// so the relay doesn't chatter at power-up.
// Type 1 / 0 in the Serial Monitor. 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 = 7;
constexpr bool RELAY_ACTIVE_LOW = true;

void setRelay(bool on) {
  digitalWrite(RELAY_PIN, (on != RELAY_ACTIVE_LOW) ? HIGH : LOW);
  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);
  Serial.begin(115200);
  delay(1500);  // let the native USB serial port enumerate
  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);
  }
}

// 1-channel relay module (3.3 V-trigger, active-LOW) on an ESP32-C3 SuperMini.
// DC+ → 5V (USB 5 V), DC− → GND, IN → GPIO7.
// GPIO7 is not a strapping pin (2/8/9) and is quiet during boot, unlike GPIO21 (UART0 TX),
// so the relay doesn't chatter at power-up.
// Type 1 / 0 in the Serial Monitor. 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 = 7;
constexpr bool RELAY_ACTIVE_LOW = true;

void setRelay(bool on) {
  digitalWrite(RELAY_PIN, (on != RELAY_ACTIVE_LOW) ? HIGH : LOW);
  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);
  Serial.begin(115200);
  delay(1500);  // let the native USB serial port enumerate
  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

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

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

Common errors