Lights 2 and 3

Wire it

• Unplug the USB cable before you add or move any wires.
• Short red wire: relay 1 COM → relay 2 COM
• Relay 2 NO (top screw) → 220 Ω resistor
• Other end of the resistor → LED 2 long leg (+)
• LED 2 short leg (−) → battery − (black)
• Short red wire: relay 2 COM → relay 3 COM
• Relay 3 NO (top screw) → 220 Ω resistor
• Other end of the resistor → LED 3 long leg (+)
• LED 3 short leg (−) → battery − (black)
• Both LEDs: long leg (+) to the resistor, short leg (−) to battery −.
• Tip: several wires need to reach battery −. Use a spare breadboard row (not the Arduino's blue rail) so they can all meet the battery's black wire.

✔ All ticked. Now upload the code.

Upload the code

This is the same relay test sketch from Step 1. If it is still on the board, you don't need to upload it again.

1. Close the Serial Monitor if it is open.
2. Download the sketch below and open it in Arduino IDE (File → Open). If the IDE asks to put it in a folder, click OK.
3. Plug the USB cable back in. Check Tools → Board says Arduino UNO and Tools → Port shows your COM port.
4. Click Upload (the round → arrow button). Wait for Done uploading.

// Step 1: test the relay module.
// Turns relay 1, 2, 3, 4 ON one at a time (1 second each), then all OFF.
// Wiring: IN1 = pin 8, IN2 = pin 9, IN3 = pin 10, IN4 = pin 11.
// This relay module is "active-LOW": LOW turns a relay ON, HIGH turns it OFF.

const int RELAY_PINS[4] = {8, 9, 10, 11};
const int RELAY_ON  = LOW;
const int RELAY_OFF = HIGH;

void allOff()
{
  for (int i = 0; i < 4; i++) {
    digitalWrite(RELAY_PINS[i], RELAY_OFF);
  }
}

void setup()
{
  Serial.begin(9600);
  for (int i = 0; i < 4; i++) {
    pinMode(RELAY_PINS[i], OUTPUT);
  }
  allOff();
  Serial.println("Relay test starting");
}

void loop()
{
  for (int i = 0; i < 4; i++) {
    allOff();
    digitalWrite(RELAY_PINS[i], RELAY_ON);
    Serial.print("Relay ");
    Serial.print(i + 1);
    Serial.println(" ON");
    delay(1000);
  }
  allOff();
  Serial.println("All OFF");
  delay(1000);
}