e105d41b_Circuit_Based_on_the_555_Timer__PURPOSE
[PIN] 2 [DESCRIPTION] Trigger [PURPOSE] 
The trigger pin triggers the beginning of the timing sequence. When it goes LOW, it causes the output pin to go HIGH. The trigger is activated when the voltage falls below 1/3 of +V on pin 8.
DPRG: A Simple PWM Circuit Based on the 555 Timer
PURPOSE