Lighting an LED when a HC-SR04 ultrasonic sensor is triggered

Projects Programming
2

For starters,

  Serial.println();
  void Trigger();

Why do you have a void in that last line? That line is a function prototype and does not belong in a function; get rid of the void keyword.

Secondly

void Trigger()
{
  if(sonar[SONAR_NUM].ping_cm() <= 59)
    {

You only check the non-existing 6th element of the array. Remember array index starts from 0, so 4 is the index of the last element in the 5 element array.