I have created an automatic guitar tuner and it works perfectly when the Arduino is plugged via USB into the computer. As soon as I use an external power supply like a 9V battery or 9V DC wall plug the servo motor starts to act up. In my code I have the motor not move when not in use (depending if a guitar signal is received or not). The motor moves and does funny things even though the code tells it not to move.
What is the main difference between the USB power and a 9V battery power? Anyone else ever experienced this problem or have any ideas for me to troubleshoot?
Thanks in advance,