Servo motors operate from the changes in the pulse width in a range between 1 and 2 ms. The separation between pulses is 20 ms. This circuit is used to produce variable pulses in the range needed to test these devices. Figure bellow shows the complete servo test. The circuit is formed by an oscillator that determined the central frequency of 50 Hz and a monostable that produces pulses with variable lengths. The pulse width is adjusted by the 47 k ohm potentiometer within the range that needs to be tested.