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Unit III
THE 555 TIMER

Multivibrators: IC 555-internal structure, working. Astable and Monostable

multivibrators–working, waveforms, mention of expression for frequency/pulse width.
The 555 Timer IC is an integrated circuit (chip) implementing a variety of timer and
multivibrator applications. The IC was designed by Hans R. Camenzind in 1970 and brought to
market in 1971 by Signetics (later acquired by Philips). The original name was the SE555 (metal
can)/NE555 (plastic DIP) and the part was described as "The IC Time Machine" It has been claimed
that the 555 gets its name from the three 5 kΩ resistors used in typical early implementations.
555 timer pin connections

The Internal structure of a 555 timer is shown in Figure

Pin1: Ground (0V) all the voltages are measured with respect to this terminal.

Pin2: Trigger. The output of the timer depends on the amplitude of the external trigger pulse applied
to this pin. The output is low if the voltage at this pin is greater than 2/3 VCC. When a negative going
pulse of amplitude greater than 1/3 VCC is applied to this pin, comparator 2 output goes low, which
intern switches the output of the timer high. The output remains high as long as the trigger terminal is
held at a low voltage.
Pin3: Output. There are two ways by which a load can be connected to the output terminal: either
between pin 3 and ground or between pin3 and supply voltage +VCC. When the output is low the load
current flows through the load connected between pin3 and +VCC into the output terminal and is called

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sink current. The current through the grounded load is zero when the output is low. For this reason the
load connected between pin 3 and +VCC is called the normally on load and that connected between pin
3 and ground is called normally off-load. On the other hand, when the output is high the current
through the load connected between pin 3 and +VCC is zero. The output terminal supplies current to
the normally off load. This current is called source current. The maximum value of sink or source
current is 200mA.
Pin4: Reset. The 555 timer can be reset (disabled) by applying a negative pulse to this pin. When the
reset function is not in use, the reset terminal should be connected to +VCC to avoid any possibility of
false triggering.
Pin5: Control Voltage. An external voltage applied to this terminal changes the threshold as well as
trigger voltage. Thus by imposing a voltage on this pin or by connecting a pot between this pin and
ground, the pulse width of the output waveform can be varied. When not used, the control pin should
be bypassed to ground with a 0.01µF Capacitor to prevent any noise problems.
Pin6: Threshold. This is the non-inverting input of comparator 1, which monitors the voltage across
the external capacitor. When the voltage at this pin is greater than or equal to the threshold voltage
2/3 VCC, the output of comparator 1 goes high, which intern switches the output of the timer low.
Pin7: Discharge. This pin is connected internally to the collector of transistor Q1. When the output is
high Q1 is OFF and acts as an open circuit to external capacitor C connected across it. On the other
hand, when the output is low, Q1 is saturated and acts as a short circuit, shorting out the external
capacitor C to ground.
Pin8: +VCC. The supply voltage of +4.5V to + 16V is applied to this pin with respect to ground.
Basic operation:
The Comparators are devices whose outputs are HIGH when the voltage at the Non
inverting input (+ input) is greater than the voltage on the inverting input ( - Input) and LOW when
the inverting input voltage is greater than the non inverting input voltage.
The voltage divider consisting three 5 K resistors provides a trigger level of VCC and a Threshold

level of VCC. The control voltage input (pin 5) can be used to externally adjust the trigger and
threshold levels to other values if necessary.
When the normally HIGH trigger input momentarily goes below VCC, the output of comparator
B switches from LOW to HIGH and sets the S-R Latch, causing the output (pin 3) to go HIGH and
turning the Discharge Transistor Q1 OFF.
The output will stay HIGH until the normally LOW threshold input goes above VCC
And causes the output of comparator A to switch from LOW to HIGH. This resets the latch, causing
the output to go back LOW and turning the discharge transistor ON.
The external reset input can be used to reset the latch independent of the threshold circuit. The
trigger and threshold inputs (pins 2 and 6) are controlled by external components connected to
produce either Monostable or astable action

ASTABLE MULTIVIBRATOR

A 555 Timer connected to operate as an Astable Multivibrator, is shown in figure.

Initially when the power is turned ON, the capacitor C 1 is uncharged and thus the trigger voltage
(pin 2) is at 0V. This causes the output of comparator B to be HIGH and the output of Comparator A
to be LOW forcing the output of S-R latch to go HIGH This makes the base of transistor Q 1 to go
LOW keeping the transistor in OFF state.
Now C1 begins charging through R1 and R2, as indicated in figure. When the capacitor voltage
reaches VCC, Comparator B switches to its LOW state and when the capacitor voltage reaches
VCC, comparator A switches to its HIGH output state. This resets the latch, causing the base of Q 1 to
go HIGH and turning on the Transistor. This creates a discharge path for the capacitor through R 2 &
the transistor as indicated. The discharge causes comparator A to go LOW. At the point where the
capacitor discharges down to VCC, comparator B switches HIGH. This sets the latch again making
the base of Q1 go LOW and turning off the transistor. Another charging cycle begins and the entire
process repeats.

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Notice that the threshold input is now connected to the trigger input. The external components
R1, R2 and C1 form the timing network that sets the frequency of oscillation.

The result is rectangular wave output whose duty cycle depends on the values of R 1
and R2. The frequency of oscillation is given by the following formula

f=

The period of oscillation is given by

T=

The ratio between the pulse duration (W) and the period (T) of a rectangular waveform is called as
the DUTY CYCLE (D). The expression for Duty cycle is given by

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D=

If R1>> R2, the duty cycle approaches 50%. An Astable 555 timer is often called free running
multivibrator because it produces a continuous train of rectangular pulses.
MONOSTABLE MULTIVIBRATOR: An external resistor and capacitor connected as shown in
figure are used to set up the 555 timer as a Monostable Multivibrator. The pulse width of the output is
determined by the time constant of R1 and C1 according to the following formula.
TP= 1.1 RAC1

TRIGGER INPUT, OUTPUT AND

CAPACITOR VOLTAGE WAVEFORM

INTERNAL STRUCTURE OF
MONOSTABLE MULTIVIBRATOR

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on
and capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls
below +1/3 VCC, the output of comparator goes high which resets the flip-flop and consequently the
transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable to
quasi-stable state, as shown in figure. As the discharge transistor is cutoff, the capacitor C begins
charging toward +VCC through resistance RA with a time constant equal to RAC. When the increasing
capacitor voltage becomes slightly greater than +2/3 VCC, the output of comparator 1 goes high, which
sets the flip-flop. The transistor goes to saturation, thereby discharging the capacitor C and the output
of the timer goes low, as illustrated in figure.
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USES: Monostable multivibrator is used to generate pulses of required time interval.