// Define the pins
const int pin1 = 9;
const int pin2 = 10;

// Frequency in Hertz
const float frequency = 2.0;

// Period of the wave in milliseconds
const float period = 1000.0 / frequency;

void setup() {
  // Initialize the digital pins as outputs
    pinMode(pin1, OUTPUT);
      pinMode(pin2, OUTPUT);
      }

      void loop() {
        // Turn pin1 on and pin2 off
          digitalWrite(pin1, HIGH);
            digitalWrite(pin2, LOW);
              
                // Wait for half the period (one quarter of the sine wave cycle)
                  delay(period / 2);

                    // Turn pin1 off and pin2 on
                      digitalWrite(pin1, LOW);
                        digitalWrite(pin2, HIGH);
                          
                            // Wait for the other half of the period
                              delay(period / 2);
                              }