1. Try the sample code with the circuit using an external LEDs rather than the
    on-board LED.

  2. Try the sample code with two external LEDs rather than 1. Let’s say
    you use the red LED and the green LED. Then modify the
    if...else statement so that when the red LED is turned on, the green LED
    is turned off, and when the green LED is turned on, the red LED is turned off.

  3. Modify the delay time every 5th time the Arduino goes through loop().
    To do this,

Now modify loop() to include another if...else statement
at the top. This statement should test the value of counter: