Useful Libraries & Demo Code
Arduino Tutorial Reference Search Tool - by Visual Micro
HCSR04 Ultrasonic Project - Wiring Guide on Indistructables
HCSR04 Ultrasionic Demo - Code on Indistructables
Playing sounds on a Speaker or Piezo Sounder
Playing Tones - Code & Wiring
Pitch Follower Project - Code & Wiring
Playing a Melody - Code & Wiring
