UNIT – V

Timers: Functional block diagram of 555, Applications: Astable and

Monostable multi-vibrators, Ramp generator.
Phase locked loops: Introduction, Basic principles, phase
detector/comparator, voltage controlled oscillator (VCO).

5.1 Introduction
One of the most versatile linear integrated circuits is the 555 timer. A sample of these
applications includes mono-stable and astable multivibrators, dc-dc converters, digital logic
probes, waveform generators, analog frequency meters and tachometers, temperature
measurement and control, infrared transmitters, burglar and toxic gas alarms, voltage
regulators, electric eyes, and many others.
The 555 is a monolithic timing circuit that can produce accurate and highly stable time
delays or oscillation. The timer basically operates in one of the two modes: either as monostable
(one-shot) multivibrator or as an astable (free running) multivibrator. The device is available
as an 8-pin metal can, an 8-pin mini DIP, or a 14-pin DIP.

5.2 Internal Architecture of 555 Timer

Pin 1: Ground.
All voltages are measured with respect to this terminal.
Pin 2: 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. However,when
a negative-going pulse of amplitude larger than 1/3 VCC is applied to this pin, the comparator
2 output goes low, which in turn switches the output of the timer high. The output remains high
as long as the trigger terminal is held at a low voltage.
Pin 3: Output.
There are two ways a load can be connected to the output terminal: either between pin
3 and ground (pin 1) or between pin 3 and supply voltage + VCC (pin 8). When the output is
low, the load current flows through the load connected between pin 3 and + VCC into the output
terminal and is called the sink current.
However, 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 the normally off load.
On the other hand, when the output is high, the current through the load connected
between pin 3and + VCC (normally on load) is zero. However, the output terminal supplies
current to the normally off load. This current is called the source current. The maximum value
of sink or source current is 200 mA.

Fig :5 .1 Pin diagram of 555Timer

Pin 4: 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.

Fig 5.2 Block Diagram

 Pin 5: Control voltage.
An external voltage applied to this terminal changes the threshold as well as the trigger
voltage . In other words, 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 0.01-μF capacitor to prevent any
noise problems.
Pin 6: Threshold. This is the non-inverting input terminal of comparator 1, which monitors the
voltage across the external capacitor. When the voltage at this pin is threshold voltage 2/3 V,
the output of comparator 1 goes high, which in turn switches the output of the timer low.
Pin 7: Discharge. This pin is connected internally to the collector of transistor Q1, as shown in
Figure 2.1(b). When the output is high, Q1 is off and acts as an open circuit to the 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.
Pin 8: + VCC.
The supply voltage of +5 V to +18 is applied to this pin with respect to ground (pin1).

5.3 FUNCTIONAL BLOCK DIAGRAM OF 555 TIMER:

Fig 5.3 Block diagram of timer

5.3.1 THE 555 AS A MONOSTABLE MULTIVIBRATOR
A monostable multivibrator, often called a one-shot multivibrator, is a pulse- generating
circuit in which the duration of the pulse is determined by the RC network connected externally
to the 555 timer.
 In a stable or standby state the output of the circuit is approximately zero or at logic-
low level. When an external trigger pulse is applied, the output is forced to go high ( ≈VCC).
The time the output remains high is determined by the external RC network connected to the

timer. At the end of the timing interval, the output automatically reverts back to its logic-low
stable state. The output stays low until the trigger pulse is again applied. Then the cycle repeats.
The monostable circuit has only one stable state (output low), hence the name mono-
stable. Normally, the output of the mono- stable multivibrator is low. Fig 2.2 (a) shows the
555 configured for monostable operation. To better explain the circuit’s operation, the internal
block diagram is included in Fig 2.2(b).

Figure 5.4 IC555 as monostable multivibrator

a) Mono-stable operation:
According to Fig 5.4 , initially when the output is low, that is, the circuit is in a
stable state, transistor Q is on and capacitor C is shorted out to ground. However, upon
application of a negative trigger pulse to pin 2, transistor Q is turned off, which releases the
short circuit across the external capacitor C and drives the output high. The capacitor C now
starts charging up toward Vcc through RA.
However, when the voltage across the capacitor equals 2/3 Va., comparator I ‘s output
switches from low to high, which in turn drives the output to its low state via the output of the
flip-flop. At the same time, the output of the flip-flop turns transistor Q on, and hence capacitor
C rapidly discharges through the transistor.
The output of the monostable remains low until a trigger pulse is again applied. Then
the cycle repeats. Figure 4-2(c) shows the trigger input, output voltage, and capacitor voltage
 waveforms. As shown here, the pulse width of the trigger input must be smaller than
the expected pulse width of the output waveform. Also, the trigger pulse must be a negative-
going input signal with amplitude larger than 1/3 the time during which the output remains
high is given by where

Fig.5.5 (b) 555 connected as a Monostable Multivibrator (c) input and output waveforms
Where RA is in ohms and C is in farads. Figure 2.2(c) shows a graph of the various
combinations of RA and C necessary to produce desired time delays. Note that this graph can
only be used as a guideline and gives only the approximate value of RA and C for a given time
delay. Once triggered, the circuit’s output will remain in the high state until the set time 1,
elapses. The output will not change its state even if an input trigger is applied again during this
time interval T. However, the circuit can be reset during the timing cycle by applying a negative
pulse to the reset terminal. The output will then remain in the low state until a trigger is again
applied.
Often in practice a decoupling capacitor (10 F) is used between + (pin 8) and ground
(pin 1) to eliminate unwanted voltage spikes in the output waveform. Sometimes, to prevent
 any possibility of mistriggering the monostable multivibrator on positive pulse edges, a wave
shapingcircuit consisting of R, C2, and diode D is connected between the trigger input pin 2
and pin 8, as shown in Figure 4-3. The values of R and C2 should be selected so that the time
constant RC2 is smaller than the output pulse width.

Fig.5.6 Monostable Multivibrator with wave shaping network to prevent +ve pulse edge
triggering
5.4 Monostable Multivibrator Applications
(a) Frequency divider: The monostable multivibrator of Figure 2.2(a) can be used as a
frequency divider by adjusting the length of the timing cycle tp, with respect to the tine
period T of the trigger input signal applied to pin 2. To use monostable multivibrator as a
divide-by-2 circuit, the timing interval tp must be slightly larger than the time period T of
the trigger input signal, as shown in Figure 2.4. By the same concept, to use the monostable
multivibrator as a divide-by-3 circuit, tp must be slightly larger than twice the period of
the input trigger signal, and so on. The frequency-divider application is possible because
the monostable multivibrator cannot be triggered during the timing cycle.

Fig 5.7 input and output waveforms of a monostable multi vibrator as a divide-by-2 network
(b) Pulse stretcher: This application makes use of the fact that the output pulse width (timing
interval) of the rnonostable multivibrator is of longer duration than the negative pulse width of
the input trigger. As such, the output pulse width of the monostable multivibrator can be viewed
as a stretched version of the narrow input pulse, hence the name pulse stretcher. Often, narrow-
pulse- width signals are not suitable for driving an LED display, mainly because of their very
narrow pulse widths. In other words, the LED may be flashing but is not visible to the eye
because its on time is infinitesimally small compared to its off time. The 555 pulse stretcher
can be used to remedy this problem

VTUPulse.com Fig 5.8 Monostable multi vibrator as a Pulse stretcher

Figure 5.8 shows a basic monostable used as a pulse stretcher with an LED indicator at
the output. The LED will be on during the timing interval tp = 1.1RAC, which can be varied
by changing the value of RA and/or C.
5.5 THE 555 AS AN ASTABLE MULTIVIBRATOR:
The 555 as an Astable Multivibrator, often called a free-running multivibrator, is a
rectangular- wave-generating circuit. Unlike the monostable multivibrator, this circuit does not
require an external trigger to change the state of the output, hence the name free running.
However, the time during which the output is either high or low is determined by the two
resistors and a capacitor, which are externally connected to the 555 timer. Fig 4-6(a) shows the
555 timer connected as an astable multivibrator. Initially, when the output is high, capacitor C
starts charging toward V through RA and R8. However as soon as voltage across the capacitor
equals 2/3 Vcc, comparator I triggers the flip flop, and the output switches low. Now capacitor
C starts discharging through R8 and transistor Q. When the voltage across C
equals 1/3 comparator 2’s output triggers the flip-flop, and the output goes high. Then the
cycle repeats.

Fig 5.9 The 555 as a Astable Multivibrator (a)Circuit(b)Voltage across Capacitor and O/P
waveforms.
The output voltage and capacitor voltage waveforms are shown in Figure 2.6(b). As shown in
this figure, the capacitor is periodically charged and discharged between 2/3 Vcc and 1/3 V,
respectively. The time during which the capacitor charges from 1/3 V to 2/3 V. is equal to the
time the output is high and is given by

where RA and R3 are in ohms and C is in farads. Similarly, the time during which the
capacitor discharges from 2/3 V to 1/3 V is equal to the time the output is low and is given by
 where RB is in ohms and C is in farads. Thus the total period of the output waveform is

This, in turn, gives the frequency of oscillation as

Above equation indicates that the frequency fo is independent of the supply voltage V. Often
the term duty cycle is used in conjunction with the astable multivibrator . The duty cycle is the
ratio of the time t during which the output is high to the total time period T. It is generally
expressed as a percentage. In equation form,

5.6 Astable Multivibrator Applications:

5.6.1 Square-wave oscillator: Without reducing RA = 0 , the astable multivibrator can be used to
produce a square wave output simply by connecting diode D across resistor RB, as shown in Figure
4-7. The capacitor C charges through RA and diode D to approximately 2/3 Vcc and discharges
through RB and terminal 7 until the capacitor voltage equals approximately 1/3
Vcc; then the cycle repeats. To obtain a square wave output (50% duty cycle), RA must be a
combination of a fixed resistor and potentiometer so that the potentiorneter can be adjusted for
the exact square wave.

Fig 5.10 Astable Multivibrator as a Square wave generator

5.6.2 Free-running ramp generator: The astable multivibrator can be used as a free-running
ramp generator when resistors RA and R3 are replaced by a current mirror. Figure 2.8(a) shows
an astable multivibrator configured to perform this function. The current mirror starts charging
capacitor C toward Vcc at a constant rate.

When voltage across C equals 2/3 Vcc, comparator 1 turns transistor Q on, and C
rapidly discharges through transistor Q. However, when the discharge voltage across C is
approximately equal to 1/3 Vcc, comparator 2 switches transistor Q off, and then capacitor C
starts charging up again. Thus the charge— discharge cycle keeps repeating. The discharging
time of the capacitor is relatively negligible compared to its charging time; hence, for all
practical purposes, the time period of the ramp waveform is equal to the charging time and is
approximately given by

Where I = (Vcc — VBE)/R = constant current in amperes and C is in farads. Therefore, the
free running frequency of the ramp generator is
 Fig 5.11 Free Running ramp generator (b) Output waveform.

5.7 PHASE-LOCKED LOOPS

The phase-locked loop principle has been used in applications such as FM (frequency
modulation) stereo decoders, motor speed controls, tracking filters, frequency synthesized
transmitters and receivers, FM demodulators, frequency shift keying (FSK) decoders, and a
generation of local oscillator frequencies in TV and in FM tuners.
Today the phase-locked loop is even available as a single package, typical examples of
which include the Signetics SE/NE 560 series (the 560, 561, 562, 564, 565, and 567). However,
for more economical operation, discrete ICs can be used to construct a phase- locked loop.
5.7.1 Block Schematic and Operating Principle
Figure 2.10 shows the phase-locked loop (PLL) in its basic form. As illustrated in this
figure, the phase-locked loop consists of (1) a phase detector, (2) a low-pass filter, and, (3) a
voltage controlled oscillator

Fig 5.12 Block Diagram of Phase Locked Loop

 The phase detectors or comparator compares the input frequency fIN with the
feedback frequency fOUT.. The output voltage of the phase detector is a dc voltage and

therefore is often referred to as the error voltage. The output of the phase is then applied to the
low-pass filter, which removes the high-frequency noise and produces a dc level.
This dc level, in turn, is the input to the voltage-controlled oscillator (VCO). The filter
also helps in establishing the dynamic characteristics of the PLL circuit. The output frequency
of the VCO is directly proportional to the input dc level. The VCO frequency is compared with
the input frequencies and adjusted until it is equal to the input frequencies. In short, the phase-
locked loop goes through three states: free- running, capture, and phase lock. Before the input
is applied, the phase-locked loop is in the free-running state. Once the input frequency is
applied, the VCO frequency starts to change and the phase-locked loop is said to be in the
capture mode. The VCO frequency continues to change until it equals the input frequency, and
the phase- locked loop is then in the phase-locked state. When phase locked, the loop tracks
any change in the input frequency through its repetitive action. Before studying the specialized
phase-locked-loop IC, we shall consider the discrete phase-locked loop, which may be
assembled by combining a phase detector, a low-pass filter, and a voltage-controlled oscillator.
(a) Phase detector:
The phase detector compares the input frequency and the VCO frequency and generates
a dc voltage that is proportional to the phase difference between the two frequencies.
Depending on the analog or digital phase detector used, the PLL is either called an analog or
digital type, respectively. Even though most of the monolithic PLL integrated circuits use
analog phase detectors, the majority of discrete phase detectors in use are of the digital type
mainly because of its simplicity.
A double-balanced mixer is a classic example of an analog phase detector. On the other
hand,examples of digital phase detectors are these:
1. Exclusive-OR phase detector
2. Edge-triggered phase detector
3. Monolithic phase detector (such as type 4044)
The following fig 2.11 shows Exclusive-OR phase detector:
Fig 5.13 (a) Exclusive-OR phase detector: connection and logic diagram. (b) Input and output
waveforms. (c) Average output voltage versus phase difference between fIN and fOUT curve.
(b) Low-pass filter.
The function of the low-pass filter is to remove the high-frequency components in the
output of the phase detector and to remove high-frequency noise.
More important, the 1ow-pass filter controls the dynamic characteristics of the phase-
locked loop. These characteristics include capture and lock ranges, bandwidth, and transient
response. The lock range is defined as the range of frequencies over which the PLL system
follows the changes in the input frequency fIN. An equivalent term for lock range is tracking
range. On the other hand, the capture range is the frequency range in which the PLL acquires
phase lock. Obviously, the capture range is always smaller than the lock range.
(c) Voltage-controlled oscillator:
A third section of the PLL is the voltage-controlled oscillator. The VCO generates an
output frequency that is directly proportional to its input voltage. Typical example of VCO is
Signetics NE/SE 566 VCO, which provides simultaneous square wave and triangular wave
outputs as a function of input voltage. The block diagram of the VCO is shown in Fig 2.12.
 The frequency of oscillations is determined by three external R1 and capacitor C1 and
the voltage VC applied to the control terminal 5.

The triangular wave is generated by alternatively charging the external capacitor C1 by

one current source and then linearly discharging it by another. The charging and discharging
levels are determined by Schmitt trigger action. The schmitt trigger also provides square wave
output. Both the wave forms are buffered so that the output impedance of each is 50 ohms.
In this arrangement the R1C1 combination determines the free running frequency and
the control voltage VC at pin 5 is set by voltage divider formed with R2 and R3. The initial
voltage VC at pin 5 must be in the range

Where +V is the total supply voltage.The modulating signal is ac coupled with the capacitor
C and must be <3 VPP. The frequency of the output wave forms is approximated by

where R1should be in the range 2KΩ < R1< 20KΩ. For affixed VC and constant C1, the
frequency fO can be varied over a 10:1 frequency range by the choice of R1 between 2KΩ <

V
R1< 20KΩ.

Fig 5.14: VCO Block Diagram

VCO
A Voltage-Controlled Oscillator (VCO) is a circuit that provides a varying output
signal (typically of square-wave or triangular-wave form) whose frequency can be
adjusted over a range controlled by an externally applied DC voltage.
The VCO provides a linear relationship between the applied voltage and the oscillation
frequency. The applied voltage is called control voltage.
The control of frequency with the help of control voltage is known as voltage to
frequency conversion. Hence VCO is otherwise known as Voltage to frequency
converter.
Practically VCO is available in IC form. IC 566 (LM566/SE566) from Signetics is a
popular VCO. IC 566 contains circuitry to generate both squarewave and triangular-
wave signals whose frequency is set by an external resistor and capacitor and then
varied by an applied dc voltage.
IC 566 is an 8 pin IC,
Two output pins for Square and Triangular outputs.
The frequency of the Square and Triangular waves are function of the input voltage
applied at Pin 5. This input voltage is also called as Modulating Input voltage.
The Frequency of the output voltage is determined by R1 , C1 and Control Voltage
Vc

Figure 1 shows that the 566 contains current sources to charge and discharge an
external capacitor C1 at a rate set by external resistor R1 and the control dc input
voltage.
A Schmitt trigger circuit is used to switch the current sources between charging
and discharging the capacitor, and the triangular voltage developed across the
capacitor and square wave from the Schmitt trigger are provided as outputs
through buffer amplifiers
Principle of Operation
The op-amp A1 is used a buffer.
The op-amp A2 is used as Schmitt trigger.
The op-amp A3 is used a an Inverter.
The Voltage Vc is applied to the modulation input pin which is a control voltage.
The capacitor C1 is linearly charged or discharged by a constant current source.
The charging current can be controlled by controlling the voltage Vc at pin 5 or by
varying the resistance R1 which is external to the IC.
The charging and discharging levels are determined by the Schmitt trigger
The output voltage of Schmitt trigger is designed to swing between +V and 0.5V
For Ra= Rb, the voltage at non-inverting terminal swings between 0.5(+V) to
0.25(+V).
Thus the triangular wave is generated due to alternate charging and discharging of
the capacitor C1 in linear manner.
When C1 voltage increases beyond 0.5(+V), the ST output goes low, and the
capacitor starts discharging.
When the voltage becomes less than 0.25(+V), the output of the ST goes high.
Due to similar current sources used for charging and discharging, the time taken
by C1 to charge and discharge is same. This produces exact triangular wave.
The output of the ST is step response, which is a square wave output.
Fig. Wave forms of VCO
Features:
1. Wide supply voltage range 10-24V
2. Very Linear Modulation characteristics
3. High temperature stability
4. Excellent power supply rejection
5. 10 to 1 Frequency range with fixed C1
6. The frequency can be controlled by means
Example
5.7.2 MONOLITHIC PHASE LOCK LOOPS IC 565:
Monolithic PLLs are introduced by signetics as SE/NE 560 series and by national
semiconductors LM 560 series.
 Fig 5.15 Pin configuration of IC 565

VTUPulse.com

Fig 5.16 Block Diagram of IC 565

Fig 5.15 and 5.16 shows the pin diagram and block diagram of IC 565 PLL. It
consists of phase detector, amplifier, low pass filter and VCO.As shown in the block diagram
the phase locked feedback loop is
 not internally connected. Therefore, it is necessary to connect output of VCO to the
phase comparator input, externally. In frequency multiplication applications a digital frequency
divider is inserted into the loop i.e., between pin 4 and pin 5. The centre frequency of the PLL
is determined by the free-running frequency of the VCO and it is given by

Where R1 and C1 are an external resistor and capacitor connected to pins 8 and 9, respectively.
The values of R1 and C1 are adjusted such that the free running frequency will be at the centre
of the input frequency range. The values of R1 are restricted from 2 kΩ to 20 kΩ,but a capacitor
can have any value. A capacitor C2 connected between pin 7 and the positive supply forms a
first order low pass filter with an internal resistance of 3.6 kΩ. The value of filter capacitor C2
should be larger enough to eliminate possible demodulated output voltage at pin 7 in order to
stabilize the VCO frequency
The PLL can lock to and track an input signal over typically ±60% bandwidth w.r.t fo
as the center frequency. The lock range fL and the capture range fC of the PLL are given by
the following equations.

Where fo=free running frequency

V=(+V)-(-V)Volts
And

From above equation the lock range increases with an increase in input voltage but decrease
with increase in supply voltage. The two inputs to the phase detector allows direct coupling of
an input signal, provided that there is no dc voltage difference between the pins and the dc
resistances seen from pins 2 and 3 are equal.