Q.Explain briefly each part of block diagram of 555timer.

The functional block diagram of a 555 timer contains two comparators, an RS flip-flop, an npn discharge transistor and a divider of three equal 5 kΩ resistors. …

[!TLDR]

The 555 block diagram = a three-$5\,k\Omega$ voltage divider, two comparators, an RS flip-flop and a discharge transistor; the comparators set/reset the flip-flop, whose output drives pin 3 and the discharge transistor.

The functional block diagram of the 555 timer (Figure 6.5.2) consists of two comparators, an RS flip-flop, an npn (discharge) transistor and three equal $5\,k\Omega$ resistors. Its parts are:

1. Resistive voltage divider — three equal $5\,k\Omega$ resistors are connected in series between $+V_{cc}$ (pin 8) and ground (pin 1). They divide the supply and fix two reference voltages: $\tfrac{2}{3}V_{cc}$ at the upper node and $\tfrac{1}{3}V_{cc}$ at the lower node. The control-voltage pin (pin 5) is brought out at the upper node so the $\tfrac{2}{3}V_{cc}$ reference can be changed externally.

2. Comparator 1 (upper comparator) — its non-inverting input is the reference $\tfrac{2}{3}V_{cc}$ and its inverting input is the threshold (pin 6). When the threshold voltage exceeds $\tfrac{2}{3}V_{cc}$, its output goes high and sets (S) the flip-flop.

3. Comparator 2 (lower comparator) — its inverting input is the reference $\tfrac{1}{3}V_{cc}$ and its non-inverting input is the trigger (pin 2). When the trigger voltage falls below $\tfrac{1}{3}V_{cc}$, its output goes high and resets (R) the flip-flop.

4. RS flip-flop — it receives the Set input from comparator 1 and the Reset input from comparator 2. Its outputs are Q and its complement $\overline{Q}$; the output at pin 3 is derived from $\overline{Q}$.

5. Discharge transistor $T_2$ — an npn transistor whose collector is brought out to the discharge pin (pin 7). It is driven by the flip-flop output: it saturates (turns on) or cuts off (turns off) according to the flip-flop state, providing a discharge path for the external timing capacitor. …