Page 1128 - Basic Electrical Engineering

Comparator 2 compares the trigger voltage with reference voltage + V .
CC

Thus, the reference voltage of the comparators are one-third and two-third of
the supply voltage which may be between + 5 V and + 18 V. The output of

both the comparators are supplied to the RS flip-flop. The flip-flop changes
its state in accordance with the output of the comparators. The flip-flop

changes states when the trigger input at pin 2 is brought down below +

V CC . When this occurs the output (at pin 3) changes state to + V . However,
CC

if the threshold input (at pin 6) is now raised above + V CC , the output will

return to ground. The n–p–n transistor, T is the discharge transistor. Pin 7,
1
i.e., the discharge pin is connected to the collector of this transistor. The

emitter is connected to the ground. When this transistor is on, this pin, i.e.,
pin 7 gets connected to ground through a timing capacitor (not shown in

figure) connected externally between pin 7 and ground when the transistor is
on. The base of another transistor T is connected to the reset terminal. This
2
pin is used to make the output go low, by applying a pulse. The 555 timer can
produce a single pulse when triggered or can produce a continuous pulse train

as long as it remains powered.