Block Diagram of IC Timer 555 and Its Various Operating:

Modes

The block diagram of IC 555 is illustrated in Figure which may be utilized to explain its various modes of operation, as follows.

The several operating modes of the 555 timer may be explained as follows. Let + V = V_cc, - V = ground, now consider the following cases.

Case 1

When V_tr <1/3  V_cc, then C₂ → High, which sets the FF resulting in V₀₃ → HIGH.

Though, Q is low (≈ 0.2 volts for + 5 V supply) hence, Q₀ is off and pin 7 behaves like an OPEN circuit.

Figure: Block Diagram of IC Timer 555