“This Instructables show how to make a simple servo tester.
Servo is a motor gear box that you can control the arm turning angle by a duty signal. Most common servo can control the arm angle from 0 - 180 degree. Servo is a very common component to build robot.
However, not all servo behavior the same, especially the cheap one. Even you buy it in bulk, the turning angle is variant and very easy to get defect. And also some of them do not have lock at the angle 0 and 180 degree, you cannot know the current arm position before plug in the power and give the signal. So it is better test it before use it.
This tool help you test the servo before you screw up to the robot.”