Turn on the motion detection of TTL camera:

  cam.setMotionDetect(true); 

Verify if the motion is activated:

  Serial.print("Motion detection is "); 
  if (cam.getMotionDetect())  
    Serial.println("ON"); 
  else  
    Serial.println("OFF"); 
} 

What happens when motion is detected by the camera:

if (cam.motionDetected()) { 
   Serial.println("Motion!");    
   cam.setMotionDetect(false); 

If motion is detected, take the picture or display the message:

  if (! cam.takePicture())  
    Serial.println("Failed to snap!"); 
  else  
    Serial.println("Picture taken!");