555 Timer LED Flasher - Block Diagram of IC 555 Timer

Block Diagram of IC 555

Inside the 555 Timer

The voltage divider (blue) has three equal 5K resistors. It divides the input voltage (Vcc) into three equal parts.
The two comparators (red) are op-amps that compare the voltages at their inputs and saturate depending upon which is greater.

The Threshold Comparator saturates when the voltage at the Threshold pin (pin 6) is greater than (2/3)Vcc.
The Trigger Comparator saturates when the voltage at the Trigger pin (pin 2) is less than (1/3)Vcc
The flip-flop (green) is a bi-stable device. It generates two values, a “high” value equal to Vcc and a “low” value equal to 0V.
When the Threshold comparator saturates, the flip flop is Reset (R) and it outputs a low signal at pin 3.
When the Trigger comparator saturates, the flip flop is Set (S) and it outputs a high signal at pin 3.
The transistor (purple) is being used as a switch, it connects pin 7 (discharge) to ground when it is closed.
When Q is low, Qbar is high. This closes the transistor switch and attaches pin 7 to ground.
When Q is high, Qbar is low. This open the switch and pin 7 is no longer grounded