//exercise 3:Program the LED’s to alternate blinks i.e. when LED 1 turns on, LED 2 turns off, then when LED 2 turns on, LED 1 turns off.
int led1 = D2;
int led2 = D3;

void setup() {

    pinMode( led1, OUTPUT);
    pinMode( led2, OUTPUT);
}

void loop() {
        digitalWrite( led1, HIGH );
        digitalWrite( led2, LOW );
        delay( 1000 );
        digitalWrite( led1, LOW );
        digitalWrite( led2, HIGH );
        delay( 1000 );

}