Relay module with Arduino Uno — active-LOW wiring and safe switching

A relay module lets the Uno switch something it could never power itself — a lamp, a fan, a solenoid valve. The module has a transistor, an optocoupler and a flyback diode on board, so the Uno’s pin only lights a small LED inside the opto.

On the Uno this is easier than on an ESP32: the board’s 5 V HIGH turns the common active-LOW modules fully off, so there is none of the “relay never releases” trouble that 3.3 V boards have. What catches Uno users out is the click at reset. The pin floats for a moment before setup() runs, and an active-LOW module reads that as “on”. The sketch fixes it by writing HIGH before switching the pin to an output.

The wiring below switches a 12 V DC lamp, which is the right way to test. If you later switch 230 V, read the safety notes first — the relay part is the easy bit.

Wiring

Board quirks

• Most blue relay modules are active-LOW: LOW turns the relay on. The sketch writes HIGH before pinMode(OUTPUT), so the relay doesn’t click on during reset or upload.
• The relay’s contacts may be printed 10 A / 250 VAC, but the PCB tracks and screw terminals on cheap modules are not. Switch a small lamp, a fan within rating, or the coil of a contactor — never a pump or motor directly.
• Learn with a 12 V DC load first. Anything on 230 V needs an MCB and RCCB upstream, an earthed metal or closed plastic enclosure, and wiring by a licensed electrician.
• A coil switching off can reset the Uno through a dip on 5 V. Modules with the JD-VCC jumper let you power the coil side separately: remove the jumper, feed JD-VCC from a separate 5 V supply, and keep only VCC/IN from the Uno.

Code

// 1-channel 5 V relay module on an Arduino Uno R3.
// DC+ → 5V, DC− → GND, IN → D7. Most modules are active-LOW: LOW = relay ON.
// Type 1 / 0 in the Serial Monitor (9600 baud) to switch it; it also toggles every 5 s.
const uint8_t RELAY_PIN = 7;
const bool ACTIVE_LOW = true;  // set false for a high-level-trigger module

bool relayOn = false;
unsigned long lastToggle = 0;

void setRelay(bool on) {
  relayOn = on;
  digitalWrite(RELAY_PIN, (on ^ ACTIVE_LOW) ? HIGH : LOW);
  Serial.println(on ? F("Relay ON  (COM-NO closed)") : F("Relay OFF (COM-NC closed)"));
}

void setup() {
  // Write the OFF level before making the pin an output, so the relay
  // doesn't click on for a moment at every reset.
  digitalWrite(RELAY_PIN, ACTIVE_LOW ? HIGH : LOW);
  pinMode(RELAY_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println(F("Relay on D7. Send 1 = on, 0 = off"));
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();
    if (c == '1') setRelay(true);
    if (c == '0') setRelay(false);
    lastToggle = millis();  // manual control pauses the demo toggle
  }
  if (millis() - lastToggle >= 5000) {
    lastToggle = millis();
    setRelay(!relayOn);
  }
}

// 1-channel 5 V relay module on an Arduino Uno R3.
// DC+ → 5V, DC− → GND, IN → D7. Most modules are active-LOW: LOW = relay ON.
// Type 1 / 0 in the Serial Monitor (9600 baud) to switch it; it also toggles every 5 s.
const uint8_t RELAY_PIN = 7;
const bool ACTIVE_LOW = true;  // set false for a high-level-trigger module

bool relayOn = false;
unsigned long lastToggle = 0;

void setRelay(bool on) {
  relayOn = on;
  digitalWrite(RELAY_PIN, (on ^ ACTIVE_LOW) ? HIGH : LOW);
  Serial.println(on ? F("Relay ON  (COM-NO closed)") : F("Relay OFF (COM-NC closed)"));
}

void setup() {
  // Write the OFF level before making the pin an output, so the relay
  // doesn't click on for a moment at every reset.
  digitalWrite(RELAY_PIN, ACTIVE_LOW ? HIGH : LOW);
  pinMode(RELAY_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println(F("Relay on D7. Send 1 = on, 0 = off"));
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();
    if (c == '1') setRelay(true);
    if (c == '0') setRelay(false);
    lastToggle = millis();  // manual control pauses the demo toggle
  }
  if (millis() - lastToggle >= 5000) {
    lastToggle = millis();
    setRelay(!relayOn);
  }
}

Expected serial output

Relay on D7. Send 1 = on, 0 = off
Relay ON  (COM-NO closed)
Relay OFF (COM-NC closed)
Relay ON  (COM-NO closed)

Relay on D7. Send 1 = on, 0 = off
Relay ON  (COM-NO closed)
Relay OFF (COM-NC closed)
Relay ON  (COM-NO closed)

Common errors