555 Timer

Pin layout for the LM555 timer:

Datasheet for the LM555

Inside the 555 timer is a voltage divider the divides +V_CC into thirds. A voltage comparator compares the trigger input (pin 2) with 1/3 V_CC, and another comparator compares the threshold (pin 6) with 2/3 V_CC. The point at 2/3 V_CC on the voltage divider is connected to the control voltage.

Monostable and Astable mode

The 555 family of timer chips can be used in either monostable mode or astable' mode.