The 555 Timer IC got its name from the three 5KΩ5KΩ resistors that are used in its
voltage
divider network. This IC is useful for generating accurate time delays and oscillations. This
chapter explains about 555 Timer in detail.
Pin Diagram and Functional Diagram
In this section, first let us discuss about the pin diagram of 555 Timer IC and then its functional
diagram.
Pin Diagram
The 555 Timer IC is an 8 pin mini Dual-Inline Package (DIP). The pin diagram of a 555 Timer IC is
shown in the following figure −
The significance of each pin is self-explanatory from the above diagram. This 555 Timer IC can
be operated with a DC supply of +5V to +18V. It is mainly useful for generating non-
sinusoidal wave forms like square, ramp, pulse & etc
Functional Diagram
The pictorial representation showing the internal details of a 555 Timer is known as functional
diagram.
The functional diagram of 555 Timer IC is shown in the following figure −
Observe that the functional diagram of 555 Timer contains a voltage divider network, two
comparators, one SR flip-flop, two transistors and an inverter. This section discusses about the
purpose of each block or component in detail −
Voltage Divider Network
The voltage divider network consists of a three 5KΩ5KΩ resistors that are connected in
series between the supply voltage VccVcc and ground.
This network provides a voltage of Vcc3Vcc3 between a point and ground, if there exists
only one 5KΩ5KΩ resistor. Similarly, it provides a voltage of 2Vcc32Vcc3 between a point
and ground, if there exists only two 5KΩ5KΩ resistors.
Comparator
The functional diagram of a 555 Timer IC consists of two comparators: an Upper
Comparator (UC) and a Lower Comparator (LC).
Recall that a comparator compares the two inputs that are applied to it and produces an
output.
If the voltage present at the non-inverting terminal of an op-amp is greater than the
voltage present at its inverting terminal, then the output of comparator will
be +Vsat+Vsat. This can be considered as Logic High ('1') in digital representation.
If the voltage present at the non-inverting terminal of op-amp is less than or equal to the
voltage at its inverting terminal, then the output of comparator will be −Vsat−Vsat. This
can be considered as Logic Low ('0') in digital representation.
SR Flip-Flop
Recall that a SR flip-flop operates with either positive clock transitions or negative clock
transitions. It has two inputs: S and R, and two outputs: Q(t) and Q(t). The outputs, Q(t)
& Q(t) are complement to each other.
The following table shows the state table of a SR flip-flop
S R Q(t+1)
0 0 Q(t)
0 1 0
1 0 1
1 1 -
Here, Q(t) & Q(t+1) are present state & next state respectively. So, SR flip-flop can be
used for one of these three functions such as Hold, Reset & Set based on the input
conditions, when positive (negative) transition of clock signal is applied.
The outputs of Lower Comparator (LC) and Upper Comparator (UC) are applied as inputs
of SR flip-flop as shown in the functional diagram of 555 Timer IC.
Transistors and Inverter
The functional diagram of a 555 Timer IC consists of one npn transistor Q1Q1 and one
pnp transistor Q2Q2. The npn transistor Q1Q1 will be turned ON if its base to emitter
voltage is positive and greater than cut-in voltage. Otherwise, it will be turned-OFF.
The pnp transistor Q2Q2 is used as buffer in order to isolate the reset input from SR flip-
flop and npn transistor Q1Q1.
The inverter used in the functional diagram of a 555 Timer IC not only performs the
inverting action but also amplifies the power level.
The 555 Timer IC can be used in mono stable operation in order to produce a pulse at the
output. Similarly, it can be used in astable operation in order to produce a square wave at the
output.
