Example solution for the Public Lab activity: "Blink a LED - Write a 'Sketch' and build a breadboard circuit for the Riffle" at https://publiclab.org/n/13632

blink_external.ino

// Riffle example by pdhixenbaugh
// on the Riffle, pin 9 is connected to an on-board LED.

const int built_in = 9; //Built-in LED
const int hdr_pwr_enable = 8; //enable "3V3" pin for header power


// the setup function runs once when you press reset or power the board
void setup() {
  //initialize the built-in LED
  pinMode(built_in, OUTPUT); 

  // put the 3V3 pin in the right mode, and turn it off
  pinMode(hdr_pwr_enable, OUTPUT);
  digitalWrite(hdr_pwr_enable, HIGH); //Turn off power

  
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(built_in, HIGH);   // turn the built-in LED on (HIGH is the voltage level)

  // turn on external power
  digitalWrite(hdr_pwr_enable, HIGH); //Turn off external power
  
  delay(1000);              // wait for a second
  
  digitalWrite(built_in, LOW);    // turn the built-in LED off by making the voltage LOW
  
  // turn off external power
  digitalWrite(hdr_pwr_enable, LOW); //Turn on external power
  
  delay(3000);              // wait for three seconds in "off" mode
}