The principle behind this project is that a 555 timer can be configured in “Astable multivibrator mode” and used to generate the signal for the Servo Motor to work. The servo motors with pulse width modulated signal usually with a frequency of 25-50Hz. The angle of the servo varies according to the ON time  period of the signal(i.e, duration of the pulse). Hence, different servos have a different angle of rotation with respect to pulse widths mentioned in there in the datasheet. For example, a pulse of 1 ms, moves the servo towards 0 degrees while a pulse of 2 ms would take it to 180 degrees.

The 555 timer in Astable mode provides an oscillating pulse as output which is switching between high and low states at a certain frequency and pulse width. In Astable Mode the threshold pin and trigger pin of the timer are connected to each other which allows the output to toggle between high and low states. This can be analysed when we look at the internal structure of the 555 containing NPN transistors, and some voltage divider circuits,and flip-flops.