Capacitor Charge and Discharge in 555 Timer - Blog details

The Voltages for the two comparators inside the 555 are developed across a voltage divider consisting of three equal resistors R of 5K ohms each. The threshold comparator is referenced at 2/3 Vcc and the trigger comparator is referenced at 1/3 Vcc. The two comparators control the flip-flop, which, in turn controls the state of the output. When the timer is in the quiescent state, the internal transistor TI is conducting and represents a short circuit across timing capacitor C. The level of the output terminal is low. In most practical circuits, the-voltage on pin. 2 is held above the trigger point by a resistor connected to Vcc.

When a negative going trigger pulse on pin 2 causes the potential at this point to fall below 1/3 Vcc, the trigger comparator switches the flip-flop, cutting off T1 and forcing the output level high to a value slightly below Vcc- Capacitor CT now starts to charge and the voltage across it rises exponentially until it reaches 2/3 Vcc- At this point, the threshold comparator resets the flip-flop and the output returns to its low state-just slightly above ground. Transistor T1 is turned ON, discharging C so that it is ready for the next timing period. Once triggered, the circuit cannot respond to additional triggering until the timed interval has elapsed.
When the triggered input is in quiescent state, the corresponding comparator is at 0 state and thus the flip-flops remains in its initial 0 state. But when the triggered input changes to 1 state, the flip-flop outputs to 1. The Base of transistor is connected to Q output of flip-flop. When it changes to 1, it allow the capacitor to discharge.