Page 1131 - Basic Electrical Engineering

Figure 17.10 Functional block diagram of IC 555 timer (same diagram as in Fig. 17.9 redrawn)

It can be seen from the waveforms that the output remains low until the
trigger signal is applied. With the application of negative trigger voltage, the

output goes high while the capacitor gets charged and then goes back to low,
and it remains low until another trigger pulse is received. The name single
shot is appropriately given because multiple trigger shots during charging of

the capacitor has no effect on the output. However, the trigger voltage must
return to the high level again before the flip-flop can be reset by the threshold

voltage signal. The time t of the output wave depends on the time taken by
the capacitor to get charged up from a discharged state from near zero voltage

to V . The capacitor has been shown getting discharged instantaneously;
CC

however, the rate of discharge will depend upon the discharge transistor.