Rc servos are electro-mechanical devices that respond to a control signal, which instructs them to move their output shaft to a certain position. A servo is normally plugged into a radio receiver with a three pin connector. The three wires are power (usually 4.8v to 6.0v), a ground wire, and a signal wire. The signal wire carries a pwm (pulse-width modulation) signal consisting of a 1-2ms pulse repeated 50 times a second. A 1.5ms pulse will tell the servo to move to its output shaft to the center position, 0 degrees. For a servo with a 90 degree range of motion, a 1ms pulse will move the servo to -45 degrees, and a 2ms pulse will move the servo to +45 degrees. Full packaged -servo1pcs -servo arm1 bag - carton box -gw: 90g