e105d41b_Circuit_Based_on_the_555_Timer__PURPOSE
[PIN] 5 [DESCRIPTION] Control Voltage [PURPOSE] 
The control voltage pin allows the input of external voltages to affect the timing of the 555 chip. When not used, it should be bypassed to ground through an 0.01uF capacitor.
DPRG: A Simple PWM Circuit Based on the 555 Timer
PURPOSE