Astable multivibrator kit

Introduction
Practical work plays a very important role in the education of electronics
engineers. People working in the field of electronics engineering education recognize
practical work done by students that as a very important part of the studies. Even
though the students get strong theoretical background during studies, their engineering
skills are not complete without practical exercises. Well skilled graduated engineers
are preferred as the labour worldwide. Students themselves recognize the importance
of the practical training. Moreover, their comment „we need more practice“is rather a
request that just a comment. These are the reasons why special attention has been paid
to organization of student laboratory and practical work during the education of
students pursuing engineering. The goals of organizing the student practical work have
been to provide the students with the possibility of the following like working in
laboratory conditions, becoming familiar with the equipment and using standard
engineering software packages. There is saying "Failing to plan is planning to fail.”
Although the project plan changes frequently as the project moves forward, the project
plan remains quite important in controlling the project. The purpose of this project is
to prepare us to create an engaged learning project demonstrating the best use of
technology, specifically the PCB making, Soldering, and etc. This project has three
main components: the scenario, presentation page, and student project pages. Each
component gives a unique perspective of the project. The project has compiled a
number of case studies with relevance for electrical and electronics parts and
techniques, some of which can be found in this later in the project. The project also
provides an introduction to PCB making and the 555 timers.
2. Every project begins with a proposal, but not every proposal can or should
become a project. In a world of limited resources, choices have to be made. Not
every project has viability. And, amongst those that do, limited resources (people,
time, money and equipment), must be applied judiciously. The goal of the project
selection process is to analyze project viability, and to approve or reject project
proposals based on established criteria, following a set of structured steps and
checkpoints. We should have a clear idea of the project and the way it will be
managed. Without a documented and approved synopsis, you risk wasted time, missed
requirements, and unrealistic expectations. It is important for us to select and plan our
projects quickly to assure enough time to for the entire group to participate. Keeping
all this in mind we where in great fix on selecting our project. The main focus was to
make something which can in real sense be useful to us, to the collage and to the rest
of the students. The focus was also to make something which is cost effective and can
be constructed within a reasonable time. All these consideration were always in mind
while selecting our project. After intense research and discussion we finally came to
with a project in mind. The project that we decided was on Astable Multivibrator Kit.
Astable, multivibrator is a type of circuit in which the circuit is not stable in either
state, it continuously oscillates from one state to the other. The astable circuit has no
stable state. With no external signal applied, the transistors alternately switch from
cut-off to saturation at a frequency determined by the RC time constants of the
coupling circuits.
3. This project of ours required our knowledge in basic electronics and helped in
betterment of our skills in electronics. The various aspects of project were discussed
2
And all the ethical aspects of the project was always kept in consideration and done.
The project firstly the circuit was designed in simple plane paper and then the
specifications of the components for optimum result were obtained. Next the circuit
was checked in a bread board along with the components. On obtaining our desired
result in the CRO we next preceded to the PCB designing. .The first thing that we did
was to make a lay out of the circuit in the transparent paper using laser printers. Then
we proceeded by cutting a copper plate as per our required dimension as per our
circuit. Next comes the most important part of the designing, it is the working in dark
room. In the dark room the copper plate was first immersed in a photo resist coating.
The resist coated copper plate is placed in an oven for about five minutes and then the
photo resist complete glued to the copper plate. The next part the photo resist board
needs to be exposed to ultra-violet light through the artwork, using a UV exposure
box. UV exposure units can easily be made using standard fluorescent lamp ballasts
and UV tubes. A timer which switches off the UV lamps automatically is essential,
and should allow exposure times from 2 to 10 minutes in 30 second increments, after
exposure, you can usually see a faint image of the pattern in the photosensitive layer.
For developing we use a silicate based product, which comes as a liquid concentrate,
this is sodium met silicate pentahydrate Na2SiO3*5H2O. This leads to the end of our
dark room works. Now we come to the other important aspects of the PCB design
.After dark room works the next process is the etching. In etching process we use a
etchant It attacks ANY metal including stainless steel, so when setting up a PCB
etching area, thus it removes the unwanted metal and the developed layout part forms
the metal track. The process which is followed is known as tinplating. In this the pcb
is passed through the tinning machine. The process needs to be done with carefully
and requires minute supervision. The required precautions must be always kept in
mind while working in the tinning machine. Next we do drilling, in drilling the places
in the PCB which require the components to be mounted, there we make holes through
the drilling machine.
4. Now comes the main part that is the mounting of the components. For this we
need to use our soldering skill. And the soldering should be done as clearly and neatly
and should not make a mess in the PCB. After the component mounting is over the
major portion of our project is over. Now one more thing we had to keep in our
agenda was to provide a proper power supply to the main circuit. The power supply
consists of a step-down transformer, transistor and capacitor. The circuit consist of a
555 timer, resistors, capacitors and a diode. Using resistor-capacitor networks within
the circuit to define the time periods of the unstable states, the various types is
implemented. This project of ours that is the Astable Multivibrator kit produces
rectangular waves; on connecting the kit to the CRO we can obtain the rectangular
waves. Through this we can calculate the time period and the thus obtain the duty
cycle. This project has helped us to better our knowledge in the field of basic
electronics, during the course of working in this project we gained a lot specialy in the
field of PCB making and basic of project building .this project has helped to
understand the basic rules which a person should follow to perform and how he should
manage time and detailing he should have in his work regarding the various parts and
aspects of his project
 3
The 555 Timer IC
5. One of the most common linear integrated circuits is the 555 timer. SE 555/NE
555 IC was first introduced in early 1970 by Signetics Corporation and was called
"The IC Time Machine" and was also the very first and only commercial timer IC
available. It provided circuit designers and hobby tinkerers with a relatively cheap,
stable, and user-friendly integrated circuit for both monostable and astable
applications. Since this device was first made commercially available, a myrad of
novel and unique circuits have been developed and presented in several trade,
professional, and hobby publications. The past ten years some manufacturers stopped
making these timers because of competition or other reasons. Yet other companies,
like NTE (a subdivision of Philips) picked up where some left off. Some typical
application of the 555 timer is monostable and astable multivibrator, DC-DC
converter, digital logic probes, waveform generators, analog frequency meter and
tachometers, temperature measurement and control, infrared transmitters, burglar toxic
gas alarms, voltage regulators, etc. The 555 timer is a monolithic timing circuit that is
showing accurate and highly stable time delays and oscillations. The 555 timer is
reliable, easy to use and economical. The 555 timer is available as 8-pin metal can, 8-
pin mini DIP or 14-pin DIP. The SE 555 is having large operating temperature range
(-55 oC to 125 oC) whereas other version of timer IC, NE 555, is having small
operating temperature (0 oC to 70 oC).
 TABLE -1

TABLE -2
6. When the low signal input is applied to the reset terminal, the timer output
remains low regardless of the threshold voltage or the trigger voltage. Only when the
high signal is applied to the reset terminal, the timer's output changes according to
threshold voltage and trigger voltage. When the threshold voltage exceeds 2/3 of the
supply voltage while the timer output is high, the timer's internal discharge Tr. turns
on, lowering the threshold voltage to below 1/3 of the supply voltage. During this
time, the timer output is maintained low. Later, if a low signal is applied to the trigger
voltage so that it becomes 1/3 of the supply voltage, the timer's internal discharge Tr.
turns off, increasing the threshold voltage and driving the timer output again at high.
 TABLE -3

7. The table above shown is the Basic Operating Table of a 555 IC timer. 555
timers is a highly stable integrated circuit capable of functioning as an accurate time-
delay generator and as an astable multivibrator or free running multivibrator. When
used as an oscillator the frequency and duty cycle are accurately controlled by only
two external resistors and a capacitor. Some of its important features are timing from
micro-seconds through hours, monostable and astable operation, trigger and reset
inputs are logic compatible, and output compatible with CMOS, DTL and TTL (when
used with 5V supply).

Typical Performance Characteristics

6.
 555 Timers in Astable Multivibrator Mode
8. The 555 timer can generate a very wide frequency range, depending on the
values of R1, R2 and C. The following figure shows how to choose the timing
resistors. The designing equation is given as ,charge time (output high):
0.693*(R1+R2)*C Discharge time (output low): 0.693*(R2)*C,Period:
0.693*(R1+2*R2) ,Frequency: 1.44 / ((R1+2*R2)*C) .Duty cycle: Time High / Time
Low: (R1+R2) / R2 With a 5-volt supply, the resistors can range from 1KΩ (minimum
value of R1 or R2) through 3.3MΩ (maximum value of R1 and R2 in series)
7.
 FIG-555 Astable Free Running Frequency

FIG-555 connected as an astable multivibrator

. Best results are obtained with capacitors of 1000pF or larger, but smaller values can
be used with lower values of R1 and R2. The maximum operating frequency is around
1 MHz, but best operation is obtained below 300 kHz. The minimum operating
frequency is limited only by the size and leakage of the capacitor you use. For
instance, a 10μF capacitor and a 3.3 Ω resistor will give a time interval of 23.1
seconds if the leakage of the capacitor is low enough. By making R2 large with
respect to R1, we can get an essentially symmetrical square-wave output. For instance,
if R1 is 1KΩ and R2 is 1MΩ, the difference in charging and discharging resistance is
only 0.1%, and good symmetry results. Any symmetry you want from 50% through
99.9% can be obtained by a selection of the ratio of R1 and R2. Only a small
frequency variation occurs due to power supply variation but variation due to
temperature changes is large, so any precise instrumentation projects require more
stable crystal clock.

8.
9. An astable timer operation is achieved by adding resistor RB to and configuring
as shown. In the astable operation, the trigger terminal and the threshold terminal are
connected so that a self-trigger is formed, operating as a multi vibrator. When the
timer output is high, its internal discharging Tr turns off and the VC1 increases by
exponential function with the time constant (RA+RB)*C. When the VC1, or the
threshold voltage, reaches 2Vcc/3, the comparator output on the trigger terminal
becomes high, resetting the F/F and causing the timer output to become low. This in
turn turns on the discharging Tr. and the C1 discharges through the discharging
channel formed by RB and the discharging Tr. When the VC1 falls below Vcc/3, the
comparator output on the trigger terminal becomes high and the timer output becomes
high again. The discharging Tr. turns off and the VC1 rises again. In the above
process, the section where the timer output is high is the time it takes for the VC1 to
rise from Vcc/3 to 2Vcc/3, and the section where the timer output is low is the time it
takes for the VC1 to drop from 2Vcc/3 to Vcc/3. When timer output is high, the
equivalent circuit for charging capacitor C1 is as follows:

9.
10. Since the duration of the timer output low state (tL) is the amount of time it
takes for the VC1 (t) to reach Vcc/3,

Since RD is normally RB>>RD although related to the size of discharging Tr.,

tL=0.693RBC1 (10)
Consequently, if the timer operates in astable, the period is the same with
'T=Th+tL=0.693(RA+RB) C1+0.693RBC1=0.693(RA+2RB) C1’
Because the period is the sum of the charge time and discharge time. And since
frequency is the reciprocal of the period, the following applies.

10.
 Important Features
11. The 555 timer basically operates in one of the two modes either as a monostable
(one shot) multivibrator or as an astable (free running) multivibrator. In the one-shot
mode, the 555 acts like a monostable multivibrator. A monostable is said to have a
single stable state that is the off state. Whenever it is triggered by an input pulse, the
monostable switches to its temporary state. It remains in that state for a period of time
determined by an RC network. It then returns to its stable state. In other words, the
monostable circuit generates a single pulse of fixed time duration each time it receives
and input trigger pulse. Thus the name one-shot, One-shot multivibrators are used for
turning some circuit or external component on or off for a specific length of time. It is
also used to generate delays. When multiple one-shots are cascaded, a variety of
sequential timing pulses can be generated. Those pulses will allow you to time and
sequence a number of related operations.
12. The other basic operational mode of the 555 is as and astable multivibrator. An
astable multivibrator is simply and oscillator. The astable multivibrator generates a
continuous stream of rectangular off-on pulses that switch between two voltage levels.
The frequency of the pulses and their duty cycle are dependent upon the RC network
values. The important features of the 555 timer are as follows:
(a) Can operate on +5V to +18V supply voltage.
(b) Having adjustable duty cycle.
(c) Timing from micro-seconds to hours.
(d) Producing high current output.
(e) Having capacity to source or sink current of 200 mA.
(f) Output can drive TTL.
o
(g) Having temperature stability of 50 ppm per C change in temperature or
0.005% per oC.
(h) Is reliable, easy to use, and low cost.
13. The NE 555 timer is the bipolar version of timer. This primer is about this
fantastic timer which is after 30 years still very popular and used in many schematics.
Although these days the CMOS version of this IC, like the Motorola MC1455, is
mostly used, the regular type is still available; however there have been many
improvements and variations in the circuitry. But all types are pin-for-pin plug
compatible. This can operate over a supply voltage range of +2V to +18V and has
output current sinking and sourcing capabilities of 100 mA and 10 mA. Advantages of
CMOS version timer are low power requirement and very high input impedance.
Building Blocks of a 555 timer
14. This timer uses a maze of transistors, diodes and resistors and for this complex
reason I will use a more simplified (but accurate) block diagram to explain the internal
organizations of the 555. The functional diagram of the timer 555 is as shown in fig.
The 555 timer consists of two comparators, one flip-flop, two transistors and some
resistors. Voltage Comparators in many applications, it is necessary to cause a digital
switching action when an analog voltage rises above or drops below some value. An
example would be a case when we wanted a digital signal to turn on a “discharge”
light when the battery voltage dropped below a specific point, say 12.5 volts for an
automotive application. In this case, we would want logic high (or low) when the
battery voltage dropped below 12.5 volts. We have seen how diodes and BJT base-

11.
 emitter junctions as well as enhancement MOSFETs have thresholds where they begin
to conduct. It would be possible to construct a circuit to create an output transition
from low to high as the input voltage crossed the trigger point. As simple example
would be an appropriate voltage divider to drop the battery voltage down to an
appropriate level and feed it into the base of a transistor inverter followed by several
more inverters to provide gain and make the output rail-rail transition occur for a very
small transition of the input signal. A few years ago, discrete devices were used as
discussed to create this voltage comparison function. However, integrated circuits
allow the voltage comparator function to be done in a single integrated circuit chip.
These chips are based on operational amplifiers circuits with a switching transistor at
the output. A functional representation is shown in Figure 1.

FIG.: Functional representation of a voltage comparator and its symbol.

15. This circuit is simply an operational amplifier followed by a transistor whose
emitter is connected to ground and whose collector is open. A typical application is
shown in Figure 2. In this case a reference voltage is connected to the Vin- input or the
inverting input. When the signal voltage at the Vin+ input rises above the reference,
the output voltage goes high. What’s really happening is that the output of the op-amp
goes low, cutting off the transistor so the external pull-up resistor causes the output
voltage to go high. When the input voltage goes below the reference, the output of the
op-amp goes high, causing the transistor to saturate and pull the output low. If you
reversed the connection of the two input signals, the output function would switch
directions. All of the internal buffering and interfacing is taken care of within the op-
amp part of the circuit.

FIG- comparator circuit

12.
 Typical application of a voltage comparator and its voltage transfer characteristic. The
VTC shows an abrupt switch when the input voltage passes the reference voltage. The
transition is not instantaneous, but occurs over a few milli-volts of the input signal for
most comparators. The LM311, whose data is given on page 100 of the “‘Design
Compendium”, shows a minimum gain of 40 V/mV, or a gain of 40,000. Thus, for the
example above, if +V is 5 volts, the transition occurs with a change of 0.125 mV at the
input. The circuits shown above are typical of the LM319, and 339 voltage
comparators. However, the LM311 is a little different in that the emitter of the output
transistor is not connected to ground. It is left uncommitted, similar to the collector, so
the user can have switching between other voltage ranges. An interface between
positive voltage systems TTL or CMOS to the negative voltage ECL would be a
typical application. It should be noted, however, that the emitter must be connected to
a voltage within the +/- Vcc rails of the voltage comparator. Appropriate rails would
be +5 and -5.2 volts of the two logic systems. One problem using voltage comparators
with very slowly varying input signals is output signal oscillation when the inputs are
in close proximity of the switch point. All signals including power supplies have a
small amount of noise embedded on top of the desired signal. We saw earlier that the
switching transition occurs with less than a milli-volt change in the input voltage. If,
for example, the input signal was just at the switch point and there was a small noise
blip on it, the output would switch. This switching can, and often does, induce more
noise on the system. This additional noise can cause the input signal to drop, causing
another transition in the output. This second transition can then again induce noise
reversing the transition again. Such oscillations are frequently seen and cause many
problem in the circuits.
16. Hysteresis is One way to prevent the spontaneous oscillation of voltage
comparator circuits is to introduce hysteresis as shown in Figure 3. In this case, two
additional resistors are added to provide feedback. Typically, R2 >> R3 so only a
small amount of feedback is provided. As far as the comparator is concerned, the
voltage at the non-inverting input is the voltage that it sees and will control the output
state.

FIG- hysteresis circuit

If Vin+ is just slightly below Vref, VO ≈0, the output transistor is saturated.
Vnoninv = (R1/ (R2+R3))* Vin+
As long as R2 >> R3, Vnoninv = Vin+
If we now raise the input voltage slightly so the output switches state, then
Vnoninv = (R2+R1/ (R1+R2+R3))* Vin+ + (R3/ (R3+R2+R1))*VDD
13.
 Again, as long as R1 + R2 >> R3 then the first term is approximately Vin+. However,
the second term adds to the voltage at the non-inverting terminal. Because R3 is much
smaller than the sum of R1 and R2, this amount is quite small, but not zero. Typically,
you want this amount to be a few milli-volts to prevent oscillation.
Flip-Flop
17. In digital circuits, a flip-flop is a term referring to an electronic circuit that has
two stable states and thereby is capable of serving as one bit of memory. A flip-flop is
usually controlled by one or two control signals and/or a gate or clock signal. The
output often includes the complement as well as the normal output. As flip-flops are
implemented electronically, they require power and ground connections. The first
electronic flip-flop was invented in 1918 by William Eccles and F. W. Jordan. It was
initially called the Eccles–Jordan trigger circuit and consisted of two active elements
(electron tubes). The name flip-flop was later derived from the sound produced on a
speaker connected to one of the back-coupled amplifiers outputs during the trigger
process within the circuit. This original electronic flip-flop—a simple two-input
bistable circuit without any dedicated clock (or even gate) signal, was transparent, and
thus a device that would be labeled as a "latch" in many circles today. Flip-flops can
be either simple (transparent) or clocked. Simple flip-flops can be built around a pair
of cross-coupled inverting elements: vacuum tubes, bipolar transistors, field effect
transistors, inverters, and inverting logic gates have all been used in practical circuits
—perhaps augmented by some gating mechanism (an enable/disable input). The more
advanced clocked (or non-transparent) devices are specially designed for synchronous
(time-discrete) systems; such devices therefore ignore its inputs except at the
transition of a dedicated clock signal (known as clocking, pulsing, or strobing). This
causes the flip-flop to either change or retain its output signal based upon the values of
the input signals at the transition. Some flip-flops change output on the rising edge of
the clock, others on the falling edge. Clocked flip-flops are typically implemented as
master–slave devices where two basic flip-flops (plus some additional logic)
collaborate to make it insensitive to spikes and noise between the short clock
transitions; they nevertheless also often include asynchronous clear or set inputs which
may be used to change the current output independent of the clock. Flip-flops can be
further divided into types that have found common applicability in both asynchronous
and clocked sequential systems: the SR ("set-reset"), D ("data" or "delay"), T
("toggle"), and JK types are the common ones; all of which may be synthesized from
(most) other types by a few logic gates. The behavior of a particular type can be
described by what is termed the characteristic equation, which derives the "next" (i.e.,
after the next clock pulse) output, Qnext, in terms of the input signal(s) and/or the
current output, Q.
18. In astable multivibrator we use SR (set-reset) flip-flop. The SR flip-flop is the
simplest Flip-Flop of all types and also commonly known as RS flip-flop, where S and
R stand for set and reset, respectively. It can be constructed from a pair of cross-
coupled NAND or NOR logic gates. The stored bit is present on the output marked Q.
Normally, in storage mode, the S and R inputs are both low, and feedback maintains
the Q and Q outputs in a constant state, with Q the complement of Q. If S is pulsed
high while R is held low, then the Q output is forced high, and stays high even after S

14
 returns low; similarly, if R is pulsed high while S is held low, then the Q output is
forced low, and stays low even after R returns low.

FIG – showing sr flip flop

The next-state equation of the SR flip-flop is Qnext = S + R'Q;Where Q is the current

state and R' is the NOT of R. Qnext becomes Q (the stored value) at clock edge.
SR Flip-Flop operation (BUILT WITH
NOR GATES)
Characteristic
Excitation table
table
S R Action Q(t) Q(t+1) S R Action
No
0 0 Keep state 0 0 0 X
change
01 Q=0 1 0 0 1 reset
10 Q=1 0 1 1 0 set
Unstable race
11 1 1 X0
combination condition
('X' denotes “Don’t care condition”; meaning the signal is irrelevant).
The uses of flip flops
19. The uses of flip flops areas follows
(a) A single flip-flop can be used to store one bit, or binary digit, of data.
(b) Any one of the flip-flop types can be used to build any of the others.
(c) Many logic synthesis tools will not use any other type than D flip-flop and D latch.
(d) Level sensitive latches cause problems with Static Timing Analysis (STA) tools
and Design for Test (DFT). Therefore, their usage is often discouraged.
(e) Many FPGA devices contain only edge-triggered D flip-flops
(f) The data contained in several flip-flops may represent the state of a sequencer, the
value of a counter, an ASCII character in a computer's memory or any other piece
of information.
(g) One use is to build finite state machines from electronic logic. The flip-flops
remember the machine's previous state, and digital logic uses that state to calculate
the next state.

15.
 (h) The T flip-flop is useful for constructing various types of counters. Repeated
signals to the clock input will cause the flip-flop to change state once per high-to-
low transition of the clock input, if its T input is "1". The output from one flip-flop
can be fed to the clock input of a second and so on. The final output of the circuit,
considered as the array of outputs of all the individual flip-flops, is a count, in
binary, of the number of cycles of the first clock input, up to a maximum of 2n-1,
where n is the number of flip-flops used. One of the problems with such a counter
(called a ripple counter) is that the output is briefly invalid as the changes ripple
through the logic. There are two solutions to this problem. The first is to sample the
output only when it is known to be valid. The second, more widely used, is to use a
different type of circuit called a synchronous counter. This uses more complex
logic to ensure that the outputs of the counter all change at the same, predictable
time.
(i) Frequency division: a chain of T flip-flops as described above will also function to
divide an input in frequency by 2n, where n is the number of flip-flops used
between the input and the output.
20. A flip-flop in combination with a Schmitt trigger can be used for the
implementation of an arbiter in asynchronous circuits. Clocked flip-flops are prone to
a problem called Meta stability, which happens when a data or control input is
changing at the instant of the clock pulse. The result is that the output may behave
unpredictably, taking many times longer than normal to settle to its correct state, or
even oscillating several times before settling. Theoretically it can take infinite time to
settle down. In a computer system this can cause corruption of data or a program
crash. The Meta stability in flip-flops can be avoided by ensuring that the data and
control inputs are held valid and constant for specified periods before and after the
clock pulse, called the setup time (tsu) and the hold time (th) respectively. These times
are specified in the data sheet for the device, and are typically between a few
nanoseconds and a few hundred picoseconds for modern devices. Unfortunately, it is
not always possible to meet the setup and hold criteria, because the flip-flop may be
connected to a real-time signal that could change at any time, outside the control of the
designer. In this case, the best the designer can do is to reduce the probability of error
to a certain level, depending on the required reliability of the circuit. One technique
for suppressing Meta stability is to connect two or more flip-flops in a chain, so that
the output of each one feeds the data input of the next, and all devices share a common
clock. With this method, the probability of a meta-stable event can be reduced to a
negligible value, but never to zero. The probability of Meta stability gets closer and
closer to zero as the number of flip-flops connected in series is increased.So-called
meta-stable-hardened flip-flops are available, which work by reducing the setup and
hold times as much as possible, but even these cannot eliminate the problem entirely.
This is because Meta stability is more than simply a matter of circuit design. When the
transitions in the clock and the data are close together in time, the flip-flop is forced to
decide which event happened first. However fast we make the device, there is always
the possibility that the input events will be so close together that it cannot detect which
one happened first. It is therefore logically impossible to build a perfectly meta-stable-
proof flip-flop. Another important timing value for a flip-flop (F/F) is the clock-to-
output delay (common symbol in data sheets: tCO) or propagation delay (tP), which is
the time the flip-flop takes to change its output after the clock edge. The time for a
16
 High-to-low transition (tPHL) is sometimes different from the time for a low-to-high
transition (tPLH). When cascading F/Fs which share the same clock (as in a shift
register), it is important to ensure that the tCO of a preceding F/F is longer than the hold
time (th) of the following flip-flop, so data present at the input of the succeeding F/F is
properly "shifted in" following the active edge of the clock. This relationship between
tCO and th is normally guaranteed if the F/Fs are physically identical. Furthermore, for
correct operation, it is easy to verify that the clock period has to be greater than the
sum tsu + th.
21. Flip-flops can be generalized in at least two ways: by making them 1-of-N
instead of 1-of-2, and by adapting them to logic with more than two states. In the
special cases of 1-of-3 encoding, or multi-valued ternary logic, these elements may be
referred to as flip-flap-flops. In a conventional flip-flop, exactly one of the two
complementary outputs is high. This can be generalized to a memory element with N
outputs, exactly one of which is high (alternatively, where exactly one of N is low).
The output is therefore always a one-hot (respectively one-cold) representation. The
construction is similar to a conventional cross-coupled flip-flop; each output, when
high, inhibits all the other outputs. Alternatively, more or less conventional flip-flops
can be used, one per output, with additional circuitry to make sure only one at a time
can be true. Another generalization of the conventional flip-flop is a memory element
for multi-valued logic. In this case the memory element retains exactly one of the logic
states until the control inputs induce a change. In addition, a multiple-valued clock can
also be used, leading to new possible clock transitions.
Transistor
22. Transistors are at the very core of today's electronics technology. The
development of the transistor has resulted in many changes to the world. It has
resulted in everything from portable transistor radios, through to cellular phones, and
computers. All these and many more everyday items have all been made possible by
the invention of the transistor. All these developments have taken place since the
development of the first transistor. Transistor is a 3-layer, two junctions (J1 and J2),
and three terminal device. It is made up of either germanium or silicon. In a transistor,
current is carried by both holes and electrons; therefore, it is called as BI-POLAR
transistor. It is the back bone of every electronic circuit. In other words, A Transistor
can be thought of as a device that is active in only One Direction. The transistor can
draw more or less current through its load resistor. It can either Source Current or it
can Sink Current, it Cannot do Both. A bipolar junction transistor (BJT) can be in
three modes: first is the cut-off mode, in this mode the transistor acts like an open
switch between collector and emitter (i.e., collector–emitter “resistance” is infinite).
Second is the active mode, in this mode the transistor acts like a dynamic resistor
between collector and emitter that adjusts its resistance in order to keep collector
current at a set level (i.e., collector–emitter resistance is finite and positive). And the
last is the saturation mode, in this the transistor acts like a closed switch between
collector and emitter (i.e., collector–emitter “resistance” is very low). In the active
mode, the transistor adjusts the collector current to be a version of the base current
amplified by some constant > 0. If the base current falls to 0, the transistor enters cut-
off mode and shuts off. When the base current rises too far, the transistor loses its
ability to decrease the collector–emitter resistance to linearly increase the collector
current. In this case, the transistor enters saturation mode. To keep the transistor out of
17
 saturation mode, the collector and emitter should be separated by at least 0.2V. When
a transistor is connected in a common-emitter configuration, the input signal is
injected between the base and emitter, which is a low resistance, low-current circuit.
As the input signal swings positive, it also causes the base to swing positive with
respect to the emitter. This action decreases forward bias which reduces collector
current (IC) and increases collector voltage (making VC more negative). During the
negative alternation of the input signal, the base is driven more negative with respect
to the emitter. This increases forward bias and allows more current carriers to be
released from the emitter, which results in an increase in collector current and a
decrease in collector voltage (making VC less negative or swing in a positive
direction). The collector current that flows through the high resistance reverse-biased
junction also flows through a high resistance load (not shown), resulting in a high
level of amplification. Since the input signal to the common emitter goes positive
when the output goes negative, the two signals (input and output) are 180 degrees out
of phase. The common-emitter circuit is the only configuration that provides a phase
reversal. The common-emitter is the most popular of the three transistor
configurations because it has the best combination of current and voltage gain. The
term GAIN is used to describe the amplification capabilities of the amplifier. It is
basically a ratio of output versus input. Each transistor configuration gives a different
value of gain even though the same transistor is used. The transistor configuration
used is a matter of design consideration. However, as a technician you will become
interested in this output versus input ratio (gain) to determine whether or not the
transistor is working properly in the circuit.
Basic transistor structure
23. The transistor is a three terminal device and consists of three distinct layers.
Two of them are doped to give one type of semiconductor and then there is the
opposite type, i.e. two may be n-type and one p-type, or two may be p-type and one
may be n-type. They are arranged so that the two similar layers of the transistor
sandwich the layer of the opposite type. As a result transistor are designated either P-
N-P (PNP) types of N-P-N (NPN) types according to the way they are made up.

FIG- showing npn and pnp transistors

The centre region is called the base and gains its name from the fact that in the very
earliest transistors it formed the "base" for the whole structure. The other two
connections are called the emitter and collector. These names result from the way in
which they either emit or collect the charge carriers. It is also essential that the base

18
 region is very thin if the device is to be able to operate. In today's transistors the base
may typically be only about 1 µm [micrometre] across. It is the fact that the base
region of the transistor is thin that is the key to the operation of the device.
Transistor operation
24. A transistor can be considered as two P-N junctions placed back to back. One of
these, namely the base emitter junction is forward biased, whilst the other, the base
collector junction is reversing biased. It is found that when a current is made to flow in
the base emitter junction larger current flows in the collector circuit even though the
base collector junction is reverse biased. The example of an NPN transistor is taken.
The same reasoning can be used for a PNP device, except that holes are the majority
carriers instead of electrons. When current flows through the base emitter junction, the
electrons leave the emitter and flow into the base. However the doping in this region is
kept low and there are comparatively few holes available for recombination. As a
result most of the electrons are able to flow right through the base region and on into
the collector region, attracted by the positive potential.

FIG -showing a transistor operation

Only a small proportion of the electrons from the emitter combine with holes in the
base region giving rise to a current in the base-emitter circuit. This means that the
collector current is much higher. The ratio between the collector current and the base
current is given the Greek symbol . For most small signal transistors this may be in
the region 50 to 500. In some cases it can be even higher. This means that the collector
current is typically between 50 and 500 times that flowing in the base. For a high
power transistor the value of  is somewhat less: 20 is a fairly typical value.

19.
 PNP
25. Single N region is sandwiched between two P-regions in a PNP transistor as
shown above. Holes are majority carriers in a PNP transistor. A PNP transistor The
schematic representation of a transistor is shown below.

Note the arrow pointing down towards the emitter. This signifies it's an NPN transistor
(current flows in the direction of the arrow). A transistor is basically a current
amplifier. Say we let 1mA flow into the base. We may get 100mA flowing into the
collector. As we all know that the currents flowing into the base and collector exit
through the emitter (sum of all currents entering or leaving a node must equal zero).
The gain won't be identical even in transistors with the same part number. The gain
also varies with the collector current and temperature. Because of this we will add a
safety margin to all our base current calculations.

Resistors
26. A resistor is a two-terminal electronic component that produces a voltage across
its terminals that is proportional to the electric current through it in accordance with
Ohm's law: V = IR Resistors are elements of electrical networks and electronic circuits
and are ubiquitous in most electronic equipment. Practical resistors can be made of
various compounds and films, as well as resistance wire (wire made of a high-
20.
resistivity alloy, such as nickel/chrome). The primary characteristics of a resistor are
the resistance, the tolerance, maximum working voltage and the power rating. Other
characteristics include temperature coefficient, noise, and inductance. Less well-
known is critical resistance, the value below which power dissipation limits the
maximum permitted current flow, and above which the limit is applied voltage.
Critical resistance depends upon the materials constituting the resistor as well as its
physical dimensions; it's determined by design. Resistors can be integrated into hybrid
and printed circuits, as well as integrated circuits. Size, and position of leads (or
terminals) are relevant to equipment designers; resistors must be physically large
enough not to overheat when dissipating their power. Resistors in a parallel
configuration each have the same potential difference (voltage). To find their total
equivalent resistance (Req):

The parallel property can be represented in equations by two vertical lines "||" (as in
geometry) to simplify equations. For two resistors,

The current through resistors in series stays the same, but the voltage across each
resistor can be different. The sum of the potential differences (voltage) is equal to the
total voltage. To find their total resistance:

A resistor network that is a combination of parallel and series can be broken up into
smaller parts that are either one or the other. For instance,

21.
27. Carbon composition resistors consist of a solid cylindrical resistive element
with embedded wire leads or metal end caps to which the lead wires are attached. The
body of the resistor is protected with paint or plastic. Early 20th-century carbon
composition resistors had un-insulated bodies; the lead wires were wrapped around the
ends of the resistance element rod and soldered. The completed resistor was painted
for colour coding of its value. The resistive element is made from a mixture of finely
ground (powdered) carbon and an insulating material (usually ceramic). A resin holds
the mixture together. The resistance is determined by the ratio of the fill material (the
powdered ceramic) to the carbon. Higher concentrations of carbon, a weak conductor,
result in lower resistance. Carbon composition resistors were commonly used in the
1960s and earlier, but are not as popular for general use now as other types have better
specifications, such as tolerance, voltage dependence, and stress (carbon composition
resistors will change value when stressed with over-voltages). Moreover, if internal
moisture content (from exposure for some length of time to a humid environment) is
significant, soldering heat will create a non-reversible change in resistance value.
These resistors, however, if never subjected to overvoltage nor overheating were
remarkably reliable. They are still available, but comparatively quite costly. Values
ranged from fractions of an ohm to 22 mega ohms. A carbon film is deposited on an
insulating substrate, and a helix cut in it to create a long, narrow resistive path.
Varying shapes, coupled with the resistivity of carbon, (ranging from 90 to 400 nΩm)
can provide a variety of resistances.[1] Carbon film resistors feature a power rating
range of 0.125 W to 5 W at 70 °C. Resistances available range from 1 ohm to 10 mega
ohms. The carbon film resistor can operate between temperatures of -55 °C to 155 °C.
It has 200 to 600 volts maximum working voltage range.

COLOUR-CODING SCHEME ON RESISTORS

28. Most axial resistors use a pattern of colored stripes to indicate resistance.
Surface-mount resistors are marked numerically, if they are big enough to permit
marking; more-recent small sizes are impractical to mark. Cases are usually tan,
brown, blue, or green, though other colours are occasionally found such as dark red or
dark gray. Early 20th century resistors, essentially un-insulated, were dipped in paint
22.
to cover their entire body for colour coding. A second colour of paint was applied to

Colou st
1 band 2nd band 3rd band (multiplier) 4th band (tolerance) Temp. Coefficient
r

Black 0 0 ×100

Brown 1 1 ×101 ±1% (F) 100 ppm

Red 2 2 ×102 ±2% (G) 50 ppm

Orange 3 3 ×103 15 ppm

Yellow 4 4 ×104 25 ppm

Green 5 5 ×105 ±0.5% (D)

Blue 6 6 ×106 ±0.25% (C)

Violet 7 7 ×107 ±0.1% (B)

Gray 8 8 ×108 ±0.05% (A)

White 9 9 ×109

Gold ×10−1 ±5% (J)

Silver ×10−2 ±10% (K)

None ±20% (M)

element, and a colour dot (or band) in the middle provided the third digit. The rule was
"body, tip, dot", providing two significant digits for value and the decimal multiplier,
in that sequence. Default tolerance was ±20%. Closer-tolerance resistors had silver
(±10%) or gold-colored (±5%) paint on the other end. Four-band identification is the
 most commonly used colour-coding scheme on resistors. It consists of four colored
bands that are painted around the body of the resistor. The first two bands encode the

23.
first two significant digits of the resistance value, the third is a power-of-ten multiplier
or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of
the value. The first three bands are equally spaced along the resistor; the spacing to the
fourth band is wider. Sometimes a fifth band identifies the thermal coefficient, but this
must be distinguished from the true 5-color system, with 3 significant digits. For
example, green-blue-yellow-red is 56×104 Ω = 560 kΩ ± 2%. An easier description
can be as followed: the first band, green, has a value of 5 and the second band, blue,
has a value of 6, and is counted as 56. The third band, yellow, has a value of 104,
which adds four 0's to the end, creating 560,000Ω at ±2% tolerance accuracy.
560,000Ω changes to 560 kΩ ±2% (as a kilo- is 103). Each colour corresponds to a
certain digit, progressing from darker to lighter colours, as shown in the chart below.
29. Early resistors were made in more or less arbitrary round numbers; a series
might have 100, 125, 150, 200, 300, etc. Resistors as manufactured are subject to a
certain percentage tolerance, and it makes sense to manufacture values that correlate
with the tolerance, so that the actual value of a resistor overlaps slightly with its
neighbours. Wider spacing leaves gaps; narrower spacing increases manufacturing and
inventory costs to provide resistors that are more or less interchangeable. A logical
scheme is to produce resistors in a range of values which increase in a geometrical
progression, so that each value is greater than its predecessor by a fixed multiplier or
percentage, chosen to match the tolerance of the range. For example, for a tolerance of
±20% it makes sense to have each resistor about 1.5 times its predecessor, covering a
decade in 6 values. In practice the factor used is 1.4678, giving values of 1.47, 2.15,
3.16, 4.64, 6.81, 10 for the 1-10 decade (a decade is a range increasing by a factor of
10; 0.1-1 and 10-100 are other examples); these are rounded in practice to 1.5, 2.2,
3.3, 4.7, 6.8, 10; followed, of course by 15, 22, 33, … and preceded by … 0.47, 0.68,
1. This scheme has been adopted as the E6 range of the IEC 60063 preferred number
series. There are also E12, E24, E48, E96 and E192 ranges for components of ever
tighter tolerance, with 12, 24, 96, and 192 different values within each decade. The
actual values used are in the IEC 60063 lists of preferred numbers. A resistor of 100
ohms ±20% would be expected to have a value between 80 and 120 ohms; its E6
neighbours are 68 (54-82) and 150 (120-180) ohms. A sensible spacing, E6 is used for
±20% components; E12 for ±10%; E24 for ±5%; E48 for ±2%, E96 for ±1%; E192 for
±0.5% or better. Resistors are manufactured in values from a few milliohms to about a
giga ohm in IEC60063 ranges appropriate for their tolerance.
Operating Principle of the 555 timer
30. In the monolithic 555 timer all those components are fabricated on single chip.
The fig. shows the voltage divider made up of three series resistors (R). This circuit is
used to provide the reference voltages of the comparators named C1 and C2. The
comparator reference voltages are fixed at (2/3) Vcc for comparator C1 and (1/3) Vcc
for comparator C2.The outputs of the comparators C1 and C2 are used to drive the set
(S) and reset (R) terminals of a flip-flop, which in turn controls the ‘on; and ‘off’
cycles of the discharge transistor Q1. When a negative transistor of pulse is applied at
the trigger terminal of the 555 timer the voltage passes through the reference voltage
of comparator C2 which is (1/3) Vcc, the output of comparator changes its state. Since
 the output of the comparator C2 is connected to the set input (S) of the flip-flop, the
output of the flip-flop, Q, goes to low level. On the other hand, when the voltage is
applied at the threshold terminal of comparator C1, change its
24.
state. This change in output is applied to the reset input (R) of the flip-flop and the
output of flip-flop, Q, goes high. A separate pin is also provided in the timer IC to
reset the output. When reset voltage is applied externally, it will override the effect of
set input and reset the output through the clear input of the flip-flop. Normally, when
the reset terminal is not used, it should be connected to positive supply (Vcc). The
output of the timer IC is taken from the flip-flop output Q through the buffer. The
buffer is necessary to source current as high 200 mA.

FIG showing IC 555 timer circuit

Output
31. The supply current, when the output is 'high', is typically 1 milli-amp (mA) or
less. The initial monostable timing accuracy is typically within 1% of its calculated
value, and exhibits negligible (0.1%/V) drift with supply voltage. Thus long-term
supply variations can be ignored, and the temperature variation is only 50ppm/°C
(0.005%/°C). All IC timers rely upon an external capacitor to determine the off-on
time intervals of the output pulses. As you recall from your study of basic electronics,
it takes a finite period of time for a capacitor

25.
(C) to charge or discharge through a resistor (R). Those times are clearly defined and
can be calculated given the values of resistance and capacitance. The basic RC

charging circuit is shown in fig. 4.

Assume that the capacitor is initially discharged. When the switch is closed, the
capacitor begins to charge through the resistor. The voltage across the capacitor rises
from zero up to the value of the applied DC voltage. The charge curve for the circuit is
shown in fig. 6. The time that it takes for the capacitor to charge to 63.7% of the
applied voltage is known as the time constant (t). That time can be calculated with the
simple expression:
t=RXC
Assume a resistor value of 1 Mega-Ohm and a capacitor value of 1uF (micro-Farad).
The time constant in that case is:
t = 1,000,000 X 0.000001 = 1 second
 Assume further that the applied voltage is 6 volts. That means that it will take one
time constant for the voltage across the capacitor to reach 63.2% of the applied
voltage.Therefore, the capacitor charges to approximately 3.8 volts in one second.

26.
Change in the input pulse frequency allows completion of the timing cycle. As a
general rule, the monostable 'ON' time is set approximately 1/3 longer than the
expected time between triggering pulses. Such a circuit is also known as a 'Missing
Pulse Detector'.
PIN CONFIGURATION
32. The 555 pin configuration is shown in, in fig. 1 and fig. 2 above, come in two
packages, either the round metal-can called the 'T' package or the more familiar 8-pin
DIP 'V' package. About 20-years ago the metal-can type was pretty much the standard
(SE/NE types). The 556 timer is a dual 555 version and comes in a 14-pin DIP
package, the 558 is a quad version with four 555's also in a 14 pin DIP case.
Inside the 555 timer, at fig. 3, are the equivalent of over 20 transistors, 15 resistors,
and 2 diodes, depending of the manufacturer. The equivalent circuit in block diagram
providing the functions of control, triggering, level sensing or comparison, discharge,
and power output. Some of the more attractive features of the 555 timer are: Supply
voltage between 4.5 and 18 volt, supply current 3 to 6 mA, and a Rise/Fall time of 100
n Sec. It can also withstand quite a bit of abuse. The Threshold current determine the
maximum value of Ra + Rb. For 15 volt operation the maximum total resistance for R
(Ra +Rb) is 20 Mega-ohm. Refer to the internal 555 schematic of Fig. 4-2.

27.
33. The pin configuration is given by:-
(a) Pin 1(Ground): All voltages are measured with respect to this terminal. The
ground (or common) pin is the most-negative supply potential of the device, which
is normally connected to circuit common (ground) when operated from positive
supply voltages.
(b) Pin 2(Trigger): The signal applied to this pin decides the output of the 555 timer.
If a negative transition of pulse is applied at this terminal and the voltage passes
through (1/3) Vcc, the output of flip-flop, Q, goes to low level.This pin is the input
to the lower comparator and is used to set the latch, which in turn causes the output
to go high. This is the beginning of the timing sequence in monostable operation.
Triggering is accomplished by taking the pin from above to below a voltage level
of 1/3 V+ (or, in general, one-half the voltage appearing at pin 5). The action of the
trigger input is level-sensitive, allowing slow rate-of-change waveforms, as well as
pulses, to be used as trigger sources. The trigger pulse must be of shorter duration
than the time interval determined by the external R and C. If this pin is held low
longer than that, the output will remain high until the trigger input is driven high
again. One precaution that should be observed with the trigger input signal is that it
must not remain lower than 1/3 V+ for a period of time longer than the timing
cycle. If this is allowed to happen, the timer will re-trigger itself upon termination
of the first output pulse. Thus, when the timer is driven in the monostable mode
with input pulses longer than the desired output pulse width, the input trigger

28.
 should effectively be shortened by differentiation. The minimum-allowable pulse
Width for triggering is somewhat dependent upon pulse level, but in general if it is
greater than the 1uS (micro-Second), triggering will be reliable. A second
precaution with respect to the trigger input concerns storage time in the lower
comparator, this portion of the circuit can exhibit normal turn-off delays of several
microseconds after triggering; that is, the latch can still have a trigger input for this
period of time after the trigger pulse. In practice, this means the minimum
monostable output pulse width should be in the order of 10uS to prevent possible
double triggering due to this effect. The voltage range that can safely be applied to
the trigger pin is between V+ and ground. A dc current, termed the trigger current,
must also flow from this terminal into the external circuit. This current is typically
500nA (nano-amp) and will define the upper limit of resistance allowable from pin
2 to ground. For an astable configuration operating at V+ = 5 volts, this resistance
is 3 Mega-ohm; it can be greater for higher V+ levels.
(c) Pin 3 (Output): Output of timer is taken from this pin. There are two methods; a
load can be connected to the output terminal: either between pin 3 and pin 1
(ground), or between pin 3 and pin 8 (+Vcc).The output of the 555 comes from a
high-current totem-pole stage made up of transistors Q20 - Q24. Transistors Q21
and Q22 provide drive for source-type loads, and their Darlington connection
provides a high-state output voltage about 1.7 volts less than the V+ supply level
used. Transistor Q24 provides current-sinking capability for low-state loads
referred to V+ (such as typical TTL inputs). Transistor Q24 has a low saturation
voltage, which allows it to interface directly, with good noise margin, when driving
current-sinking logic. Exact output saturation levels vary markedly with supply
voltage, however, for both high and low states. At a V+ of 5 volts, for instance, the
low state Vce (sat) is typically 0.25 volts at 5 mA. Operating at 15 volts, however,
it can sink 200mA if an output-low voltage level of 2 volts is allowable (power
dissipation should be considered in such a case, of course). High-state level is
typically 3.3 volts at V+ = 5 volts; 13.3 volts at V+ = 15 volts. Both the rise and
fall times of the output waveform are quite fast, typical switching times being
100nS. The state of the output pin will always reflect the inverse of the logic state
of the latch, and this fact may be seen by examining Fig. 3. Since the latch itself is
not directly accessible, this relationship may be best explained in terms of latch-
input trigger conditions. To trigger the output to a high condition, the trigger input
is momentarily taken from a higher to a lower level. (See "Pin 2 - Trigger"). This
causes the latch to be set and the output to go high. Actuation of the lower
comparator is the only manner in which the output can be placed in the high state.
The output can be returned to a low state by causing the threshold to go from a
lower to a higher level (see "Pin 6 - Threshold"), which resets the latch. The output
can also be made to go low by taking the reset to a low state near ground [see "Pin
4 - Reset"]. The output voltage available at this pin is approximately equal to the
Vcc applied to pin 8 minus 1.7V.
(d) Pin 4 (Reset): This pin is used to reset the timer IC. When a negative pulse is
applied to this pin, output of the flip-flop gets reset; even the signal is also coming
from set input. When the reset function is not in use, the reset terminal should be
connected to +Vcc to avoid any possibility of false triggering. This pin is also used
to reset the latch and return the output to a low state. The reset voltage threshold
29
 level is 0.7 volt, and a sink current of 0.1mA from this pin is required to reset the
device. These levels are relatively independent of operating V+ level; thus the reset
input is TTL compatible for any supply voltage. The reset input is an overriding
function; that is, it will force the output to a low state regardless of the state of
either of the other inputs. It may thus be used to terminate an output pulse
prematurely, to gate oscillations from "on" to "off", etc. Delay time from reset to
output is typically on the order of 0.5 µS, and the minimum reset pulse width is 0.5
µS. Neither of these figures is guaranteed, however, and may vary from one
manufacturer to another. In short, the reset pin is used to reset the flip-flop that
controls the state of output pin 3. The pin is activated when a voltage level
anywhere between 0 and 0.4 volt is applied to the pin. The reset pin will force the
output to go low no matter what state the other inputs to the flip-flop are in. When
not used, it is recommended that the reset input be tied to V+ to avoid any
possibility of false resetting.
(e) Pin 5 (Control Voltage): Threshold voltage and trigger voltage of the timer IC can
be changed by this terminal control voltage. A voltage may be imposed on this pin
by connecting a potentiometer between this pin and ground. By applying this
voltage the pulse width of the output waveform can be varied. When not used, the
control voltage pin should be by passed to ground with 0.01µF capacitor to prevent
any noise disturbances. This pin allows direct access to the 2/3 V+ voltage-divider
point, the reference level for the upper comparator. It also allows indirect access to
the lower comparator, as there is a 2:1 divider (R8 - R9) from this point to the
lower-comparator reference input, Q13. Use of this terminal is the option of the
user, but it does allow extreme flexibility by permitting modification of the timing
period, resetting of the comparator, etc. When the 555 timer is used in a voltage-
controlled mode, its voltage-controlled operation ranges from about 1 volt less than
V+ down to within 2 volts of ground (although this is not guaranteed). Voltages
can be safely applied outside these limits, but they should be confined within the
limits of V+ and ground for reliability. By applying a voltage to this pin, it is
possible to vary the timing of the device independently of the RC network. The
control voltage may be varied from 45 to 90% of the Vcc in the monostable mode,
making it possible to control the width of the output pulse independently of RC.
When it is used in the astable mode, the control voltage can be varied from 1.7V to
the full Vcc. Varying the voltage in the astable mode will produce a frequency
modulated (FM) output. In the event the control-voltage pin is not used, it is
recommended that it be bypassed, to ground, with a capacitor of about 0.01uF
(10nF) for immunity to noise, since it is a comparator input. This fact is not
obvious in many 555 circuits since I have seen many circuits with 'no-pin-5'
connected to anything, but this is the proper procedure. The small ceramic cap may
eliminate false triggering.
(f) Pin 6 (Threshold): This pin is actually the non-inverting terminal of the
comparator C1 as shown in fig. When the voltage at this pin is greater than or equal
to (2/3) Vcc, the output of comparator C1 goes high, which in turn switches the
output of the timer, Q, high.Pin 6 is one input to the upper comparator (the other
being pin 5) and is used to reset the latch, which causes the output to go low.
Resetting via this terminal is accomplished by taking the terminal from below to
above a voltage level of 2/3 V+ (the normal voltage on pin 5). The action of the
30.
 threshold pin is level sensitive, allowing slow rate-of-change waveforms. The
voltage range that can safely be applied to the threshold pin is between V+ and
ground. A dc current, termed the threshold current, must also flow into this
terminal from the external circuit. This current is typically 0.1µA, and will define
the upper limit of total resistance allowable from pin 6 to V+. For either timing
configuration operating at V+ = 5 volts, this resistance is 16 Mega-ohm. For 15
volt operation, the maximum value of resistance is 20 Mega-Ohms.
(g) Pin 7 (Discharge): The collector terminal of the transistor, Q1, as shown in fig, is
the discharge pin of the 555 timer. An external capacitor is connected between this
pin and ground. When the output of flip-flop, Q, goes high, the transistor Q1 goes
in saturation region and capacitor is shorted out to the ground whereas for the low
output of flip-flop, the transistor switches to cut-off region and acts as an open
circuit to the external capacitor. This pin is connected to the open collector of an
NPN transistor (Q14), the emitter of which goes to ground, so that when the
transistor is turned "on", pin 7 is effectively shorted to ground. Usually the timing
capacitor is connected between pin 7 and ground and is discharged when the
transistor turns "on". The conduction state of this transistor is identical in timing to
that of the output stage. It is "on" (low resistance to ground) when the output is low
and "off" (high resistance to ground) when the output is high. In both the
monostable and astable time modes, this transistor switch is used to clamp the
appropriate nodes of the timing network to ground. Saturation voltage is typically
below 100mV (milli-Volt) for currents of 5 mA or less, and off-state leakage is
about 20nA (these parameters are not specified by all manufacturers, however).
Maximum collector current is internally limited by design, thereby removing
restrictions on capacitor size due to peak pulse-current discharge. In certain
applications, this open collector output can be used as an auxiliary output terminal,
with current-sinking capability similar to the output (pin 3).
(h) Pin 8 (+Vcc): The supply voltage of +5V to +18V is applied to this with respect to
pin 1 (ground).The V+ pin (also referred to as Vcc) is the positive supply voltage
terminal of the 555 timer IC. Supply-voltage operating range for the 555 is +4.5
volts (minimum) to +16 volts (maximum), and it is specified for operation between
+5 volts and +15 volts. The device will operate essentially the same over this range
of voltages without change in timing period. Actually, the most significant
operational difference is the output drive capability, which increases for both
current and voltage range as the supply voltage is increased. Sensitivity of time
interval to supply voltage change is low, typically 0.1% per volt. There are special
and military devices available that operate at voltages as high as 18 volts.
Applications
33. There are literally thousands of different ways that the 555 can be used in
electronic circuits. In almost every case, however, the basic circuit is either a one-shot
or an astable. The application usually requires a specific pulse time duration, operation
frequency, and duty-cycle. Additional components may have to be connected to the
555 to interface the device to external circuits or devices. In the remainder of this
experiment, we can build both the one-shot and astable circuits and learn about some
of the different kinds of applications that can be implemented.

31
 Physical dimension diagram

Applications
33. There are literally thousands of different ways that the 555 can be used in
electronic circuits. In almost every case, however, the basic circuit is either a one-shot
or an astable. The application usually requires a specific pulse time duration, operation
frequency, and duty-cycle. Additional components may have to be connected to the
555 to interface the device to external circuits or devices. In the remainder of this
experiment, we can build both the one-shot and astable circuits and learn about some
of the different kinds of applications that can be implemented.

Multivibrator
34. A form of electronic circuit that employs positive feedback to cross-couple two
devices so that two distinct states are possible, for example, one device ON and the
other device OFF, and in which the states of the two devices can be interchanged
either by use of external pulses or by internal capacitance coupling. The commonest
form is the astable or oscillating type, which generates a square wave - the high level
of harmonics in its output is what gives the mutivibrator its common name. When the
circuit is switched between states, transition times are normally very short compared
to the ON and OFF periods. Hence, the output waveforms are essentially rectangular
in form. Multivibrators may be classified as bistable, monostable, or astable. A
bistable multivibrator, often referred to as a flip-flop, has two possible stable states;
each with one device ON and the other OFF, and the states of the two devices can be
interchanged only by the application of external pulses. A monostable multivibrator,
sometimes referred to as a one-shot, also has two possible states, only one of which is
stable. If it is forced to the opposite state by an externally applied trigger, it will
recover to the stable state in a period of time usually controlled by a resistance-
capacitance (RC) coupling circuit. An astable multivibrator has two possible states,
neither of which is stable, and switches between the two states, usually controlled by
32.
 two RC coupling time constants. The astable circuit is one form of relaxation
oscillator, which generates recurrent waveforms at a controllable rate. In its simplest
form the multivibrator circuit consists of two cross-coupled transistors. Using resistor-
capacitor networks within the circuit to define the time periods of the unstable states,
the various types may be implemented. Multivibrators find applications in a variety of
systems where square waves or timed intervals are required. Simple circuits tend to be
inaccurate since many factors affect their timing, so they are rarely used where very
high precision is required. The working of the multivibrator depends on the positive
feedback.
35. Feedback is a mechanism, process or signal that is looped back to control a
system within itself. Such a loop is called a feedback loop. Intuitively many systems
have an obvious input and output; feeding back part of the output so as to increase the
input is positive feedback; feeding back part of the output in such a way as to partially
oppose the input is negative feedback. In more general terms, a control system has
input from an external signal source and output to an external load; this defines a
natural sense (or direction) or path of propagation of signal; the feed forward sense or
path describes the signal propagation from input to output; feedback describes signal
propagation in the reverse sense. When a sample of the output of the system is fed
back, in the reverse sense, by a distinct feedback path into the interior of the system, to
contribute to the input of one of its internal feed forward components, especially an
active device or a substance that is consumed in an irreversible reaction; it is called the
"feedback". The propagation of the signal around the feedback loop takes a finite time
because it is causal Positive feedback, sometimes referred to as "cumulative
causation", refers to a situation where some effect causes more of itself. A system
undergoing positive feedback is unstable, that is, it will tend to spiral out of control as
the effect amplifies itself. Technically, a system exhibiting positive feedback responds
to perturbation in the same direction as the perturbation. That is, "A produces more of
B which in turn produces more of A". In contrast, a system that responds to the
perturbation in the opposite direction is said to exhibit negative feedback. These
concepts were first recognized as broadly applicable by Norbert Wiener in his 1948
work on cybernetics. The effect of a positive feedback loop is not necessarily
"positive" in the sense of being desirable. Positive refers to the direction of change
rather than the desirability of the outcome. A negative feedback loop tends to reduce
or inhibit a process, while a positive feedback loop tends to expand or promote it.
36. Before the advent of low-cost integrated circuits, chains of multivibrators found
use as frequency dividers. A frequency divider is an electronic circuit that takes an
input signal with a frequency, fin, and generates an output signal with a frequency =
fn/f; where n is an integer. Phase-locked loop frequency synthesizers make use of
frequency dividers to generate a frequency that is a multiple of a reference frequency.
Frequency dividers can be implemented for both analog and digital applications. A
regenerative frequency divider, also known as a Miller frequency divider, mixes the
input signal with the feedback signal from the mixer. The feedback signal is fin / 2.
This produces sum and difference frequencies fin / 2, 3fin / 2 at the output of the mixer.
A low pass filter removes the higher frequency and the fin / 2 frequencies is amplified
and fed back into mixer. Steady state examination seems simple enough however
startup is more complicated. In order to establish a stable 1/2 frequency feedback, the

33.
 amplifier gain at the half frequency must be greater than unity. The phase shift must
also be an integer multiple of 2pi.

Figure: frequency divider circuit

A free-running multivibrator with a frequency of one-half to one-tenth of the reference
frequency would accurately lock to the reference frequency. This technique was used in
early electronic organs, to keep notes of different octaves accurately in tune. Other
applications included early television systems, where the various line and frame
frequencies were kept synchronized by pulses included in the video signal. A free-
running oscillator which has a small amount of a higher-frequency signal fed to it will
tend to oscillate in step with the input signal. Such frequency dividers were essential in
the development of television. An integrated circuit multivibrator, the 555, is very
common in electronics. It uses a more sophisticated design to overcome some of the
precision issues with the simpler circuits. There are three types of multivibrator circuit:
(a) Astable, in which the circuit is not stable in either state - it continuously
oscillates from one state to the other. Another name for this type of circuit is
relaxation oscillator.
(b) Monostable, in which one of the states is stable, but the other is not - the circuit
will flip into the unstable state for a determined period, but will eventually
return to the stable state. Such a circuit is useful for creating a timing period of
fixed duration in response to some external event.
(c) Bistable, in which the circuit will remain in either state indefinitely. The
circuit can be flipped from one state to the other by an external event or trigger.
Such a circuit is important as the fundamental building block of a register or
memory device.
37. Monostable is one in which one of the states is stable, but the other is not—the
circuit will flip into the unstable state for a determined period, but will eventually
return to the stable state. Such a circuit is useful for creating a timing period of fixed
duration in response to some external event. This circuit is also known as a one shot.
A common application is in eliminating switch bounce. The monostable circuit has
one stable state; one transistor conducts while the other is cut off. A signal must be
applied to change this condition. After a period of time, determined by the internal RC
components, the circuit will return to its original condition where it remains until the
next signal arrives.

34.
 Figure: monostable multivibrator circuit
38. Astable is a circuit in which the circuit is not stable in either state; it
continuously oscillates from one state to the other. The astable circuit has no stable
state. With no external signal applied, the transistors alternately switch from cut-off to
saturation at a frequency determined by the RC time constants of the coupling circuits
39. Bistable: In which the circuit will remain in either state indefinitely. The circuit
can be flipped from one state to the other by an external event or trigger. Such a circuit
is important as the fundamental building block of a register or memory device. This
circuit is also known as a flip-flop. The bistable multivibrator has two stable states. It
remains in one of the stable states until a trigger is applied. It then FLIPS to the other
stable condition and remains there until another trigger is applied. The multivibrator
then changes back (FLOPS) to its first stable state.

Figure: astable multivibrator

35.
 ASTABLE MULTIVIBRATOR
40. An astable multivibrator is also known as a FREE-RUNNING
MULTIVIBRATOR. It is called free-running because it alternates between two
different output voltage levels during the time it is on. The output remains at each
voltage level for a definite period of time. If you looked at this output on an
oscilloscope, you would see continuous square or rectangular waveforms. The astable
multivibrator has two outputs, but NO inputs.
A multivibrator is an electronic circuit used to implement a variety of simple two-state
systems such as oscillators, timers and flip flops. An astable multivibrator has two
states, neither one stable. The circuit therefore behaves as an oscillator with the time
spent in each state controlled by the charging or discharging of a capacitor through a
resistor. The astable multivibrator may be created directly with transistors or with use
of integrated circuits such as operational amplifiers (op amps) or the 555 timer. Most
operational amplifiers are powered by a positive and negative rail voltage, the output
never able to exceed these rail voltages. Depending upon initial conditions, the op
amp’s output will drive to either positive or negative rail. Upon this occurrence, the
capacitor will either charge or discharge through the resistor R2, its voltage slowly
rising or falling. As soon as the voltage at the op amp’s inverting terminal reaches that
at the non-inverting terminal (the op amp’s output voltage divided by R1 and R2), the
output will drive to the opposing rail and this process will repeat with the capacitor
discharging if it had previously charged and vice versa. Once the inverting terminal
reaches the voltage of the non-inverting terminal the output again drives to the
opposing rail voltage and the cycle begins again. Thus, the astable multivibrator
creates a square wave with no inputs. Period of astable multivibrator displayed.

Figure: Astable
Multivibrator
41. An astable multivibrator generates a string of pulses. When we build an astable
multivibrator using a 555 timer, we used two resistors and a capacitor to control the
timing of the circuit. It is what happens inside of the timer, however, that creates the
steady stream of pulses. The block diagram in Figure 2 shows the R-R-C combination
(Ra, Rb and C1 in the animation) and it also shows the way the signal travels through
the inside the 555 timer to create the desired output. Before you start building, identify

36.
 the parts of the circuit in the animation that correspond to each of the blocks in the
diagram.

The rate of the output pulses is determined by the rate at which the capacitor, C1,
charges and discharges. The middle trace shows the voltage across the capacitor.
When the voltage across the capacitor exceeds 2/3 of the source voltage, Vcc, the
output of comparator 1 goes high and causes the flip flop to set. The output flip flop
will go high, call this Q. The output of the 555 is the inverse of the output of the flip
flop, so call this Q. Important point – for this model, the Q of the flip flop is the output
of the 555 timer. When Q goes high, the discharge transistor switch closes, and pin 7
is grounded. The capacitor begins to discharge through this new path to ground. When
the capacitor discharges down to 1/3 of the source voltage, the output of comparator 2
goes high and causes the flip flop to reset. The output at Q goes high (and Q goes
low). When Q goes low, the discharge transistor switch opens, the path to ground is
broken, and the capacitor begins to charge again. Before we can fully understand the
function of the multivibrator, we must understand the components we are using to
build it. There the following components that can be used for building it such as: an
RC circuit, a voltage divider, two comparators, a flip flop, a transistor, and an
inverting amplifier.
42. The phase shifting circuit has a first branch connected between the first and
second input ports, the first branch including a first resistor-capacitor (RC) segment
connected in series via a first mid-point to a second resistor-capacitor (RC) segment
wherein the second RC segment is configured, with reference to the first mid-point, in
complete mirror symmetry relative to the first RC segment. The phase shifting circuit
further includes a second branch connected between the first and second input ports in
parallel to the first branch The second branch includes a first resistor segment
connected in series via a second mid-point to a second resistor segment wherein the
second resistor segment is in total mirror symmetry relative to the first resistor
segment with reference to the second mid-point.. The phase shifting circuit has at least
two output ports each connecting to the first RC segment and the second RC segment
maintaining a constant phase difference between them. The first mid-point is
connected to the second mid-point thus establishing a virtual ground potential for the
phase shifting circuit wherein the virtual ground potential having a definite voltage

37.
 correlation with the input signal. It is an advantage of the present invention that it
provides a method and circuit architecture for a phase shifting/splitting network such
that aforementioned difficulty encountered in the prior art may be resolved. Here’s a
circuit diagram of a very basic voltage divider. It consists of two resistors, Ra and Rb.
Those two resistors are in series. An input voltage is applied to the series combination
and an output voltage is measured across one of the two resistors.

43. The output voltage will be some fraction of the input voltage, and the fraction
is controlled by the values of the two resistors. Our first goal is to understand how the
output voltage, Vout, depends upon the input voltage, Vin, and the values of the two
resistors, Ra and Rb. We'll examine this problem and solve for the output voltage by
using what we know about resistors and Kirchhoff's Laws. Once we find that current,
then we can calculate the output voltage using Ohm's Law. You have to notice that Ra
and Rb are two resistors in series and divide the input voltage by the series equivalent.
Then, once you have the current you can find the voltage across either of the resistors
because you know Ohm's Law. Doing that you should find the following expression
for the output voltage of the voltage divider. Iseries = Vin / (Ra + Rb) because the two
resistors are in series if no current is drawn from the voltage divider circuit. Vout = Vin
Rb/ (Ra + Rb) because the voltage across the resistor can be obtained using Ohm's law.
A comparator circuit compares two voltage signals and determines which one is
greater. The result of this comparison is indicated by the output voltage: if the op-
amp's output is saturated in the positive direction, the non-inverting input (+) is a
greater, or more positive, voltage than the inverting input (-), all voltages measured
with respect to ground. If the op-amp's voltage is near the negative supply voltage (in
this case, 0 volts, or ground potential), it means the inverting input (-) has a greater
voltage applied to it than the non-inverting input (+). This behaviour is much easier
understood by experimenting with a comparator circuit than it is by reading someone's
verbal description of it. In this experiment, two potentiometers supply variable
voltages to be compared by the op-amp. The output status of the op-amp is indicated
visually by the LED. By adjusting the two potentiometers and observing the LED, one
can easily comprehend the function of a comparator circuit. Comparator circuits are
widely used to compare physical measurements provided those physical variables can
be translated into voltage signals. For instance, if a small generator were attached to an
anemometer wheel to produce a voltage proportional to wind speed that wind speed
signal could be compared with a "set-point" voltage and compared by an op-amp to
drive a high wind speed alarm.
44. Inverting Amplifier circuit the operational amplifier is connected with
feedback to produce a closed loop operation. There are two very important rules to
remember about inverting amplifiers are that, "no current flows into the input
38.
 terminal" and that "V1 equals V2". This is because the junction of the input and
feedback signal (X) is at the same potential as the positive (+) input which is at zero
volts or ground then, the junction is a "Virtual Earth". Because of this virtual earth
node the input resistance of the amplifier is equal to the value of the input resistor, R in
and the closed loop gain of the inverting amplifier can be set by the ratio of the two
external resistors. We said above that there are two very important rules to remember
about Inverting Amplifiers or any operational amplifier for that matter and they are:
first is that, No Current Flows into the Input Terminals, second is that, the Differential
Input Voltage is Zero as V1 = V2 = 0 (Virtual Earth).
45. The transistor is a semiconductor device than can function as a signal amplifier
or as a solid-state switch. .A transistor is an electronically controlled switch that will
close the connection between the collector and the emitter when the difference
between the base voltage and the emitter voltage (VBE) exceeds about 0.7V. In this
circuit, the emitter voltage is ground (0V) and the base is connected to the output of a
flip flop. The output of the transistor circuit is measured at the collector. This is the
discharge pin of our flip flop model. When the flip flop outputs a logic low voltage
(theoretically 0V), the base voltage of the transistor does not exceed 0.7V above the
emitter voltage (VBE = 0V - 0V) and the switch is open. In this instance, the discharge
pin of our timer model is not connected. When the flip flop outputs a logic high
voltage (theoretically the supply voltage Vcc; 4V or 5V, depending on the IO Board),
the difference between the base voltage and the emitter voltage (VBE = Vcc - 0V) is
greater than 0.7V and the switch closes. This grounds the collector and forces the
discharge pin to 0V. It is through this path to ground that the capacitor in the R-R-C
combination discharges.
46. Flip-flops are synchronous bistable devices. The term synchronous means the
output changes state only when the clock input is triggered. That is, changes in the
output occur in synchronization with the clock. Flip-flop is a term referring to an
Electronic circuit that has two stable states and thereby is capable of serving as one bit
of memory. A flip-flop is usually controlled by one or two control signals and/or a
gate or clock signal. The output often includes the complement as well as the normal
output. Flip-flop is the common name given to two-state devices which offer basic
memory for sequential logic operations. Flip-flops are heavily used for digital data
storage and transfer and are commonly used in banks called "registers" for the storage
of binary numerical data. As flip-flops are implemented electronically, they require
power and ground connections.
Another important aspect of this project is the equations that you use to calculate the
on and off times of the output pulses. The R-R-C combination is directly related to the
timing of the pulses generated by the multivibrator. You have been given two
equations that can be used to find the rate and duty cycle of the pulses this circuit
generates:

We know that the charge/discharge time of a capacitor is determined using the time
constant, T = RC where C is the capacitance of the capacitor and R is the total
resistance that the capacitor is charging or discharging through. For the on cycle, the
capacitor charges through Ra and Rb. For the off cycle, the capacitor discharges
through Rb only. Therefore,

39.
 The constant, 0.693, is related to the portion of the charge cycle over which the
capacitor charges and discharges. We can find this using the charge and discharge
equations. The equation for a capacitor charging is:

The equation for the discharge is:

We can use these equations to determine how much voltage is on the capacitor at any
given time. For instance, at t = 0, an uncharged capacitor will start with an initial
change of:

And a fully charged capacitor will have an initial voltage of:

If the capacitor is charging up from 0V, we can find how much time it will take to gain
1/3 of its total charge, as follows:

The capacitor in the astable multivibrator charges and discharges between 2/3 and 1/3
of the source voltage.
47. Astable Multivibrator is a two stage switching circuit in which the output of the
first stage is fed to the input of the second stage and vice versa. This free running
multivibrator generates square wave without any external triggering pulse. The circuit
has two stable states and switches back and forth from one state to another, remaining
in each state for a time depending upon the discharging of the capacitive circuit.
Figure 6 shows the astable multivibrator, square wave outputs V1 and V2can be
obtained from the collector point of Q1 or Q2, respectively.

40.
 Imperfection in hardware will cause the first cycle, for our purpose we can ignore this
detail and simply assume that currently Q1 is in state on. Q1 holds the bottom of
R1 (and the left side of C1) near ground (0V). The right side of C1 (and the
base of Q2) is being charged by R2 from below ground to 0.7V. R3 is pulling the base
of Q1 up, but its base-emitter diode prevents the voltage from rising above 0.7V.
R4 is charging the right side of C2 up to the power supply voltage (+VCC). Because
R4 is less than R2, C2 charges faster than C1. When the base of Q2 reaches
0.7V, Q2 turns on, and the following positive feedback loop occurs. Q2 abruptly pulls
the right side of C2 down to near (0V). Because the voltage across a capacitor
cannot suddenly change, this causes the left side of C2 to suddenly fall to almost
−VCC, well below 0V. Q1 switches OFF due to the sudden disappearance of its base
voltage. R1 and R2 work to pull both ends of C1 toward +VCC, completing
Q2’s turn on. The process is stopped by the B − E diode of Q2, which will not let the
right side of C1 rise very far. This now takes us to State 2, the mirror image of the
initial state, where Q1 is switched OFF and Q2 is switched on. Then R1 rapidly pulls
C1’s left side toward +VCC, while R3 more slowly pulls C2’s left side toward +0.7V.
When C2’s left side reaches 0.7V, the cycle repeats. Voltage of capacitor C1 is VC1=
VFINAL + (VINIT − VFINAL) • e−t/RC. Voltage of C1 discharges from +VCC to –VCC by
conditions of circuit. The limiting point of its discharge is approximately zero volts for
which system changes state. Time required for voltage reach this zero is t1.
CIRCUIT DESCRIPTION
48. Astable operation is enabled by a high level on the ASTABLE input. The period
of the square wave at the Q and Q Outputs in this mode of operation is a function of
the external components employed. "True" input pulses on the ASTABLE input or
"Complement" pulses on the ASTABLE input allow the circuit to be used as a gatable
multivibrator. The OSCILLATOR output period will be half of the Q terminal output
in the astable mode. However, a 50% duty cycle is not guaranteed at this output. In the
monostable mode, positive-edge triggering is accomplished by application of a
leading-edge pulse to the +TRIGGER input and a low level to the –TRIGGER input.
For negative-edge triggering, a trailing-edge pulse is applied to the -TRIGGER and a
high level is applied to the +TRIGGER. Input pulses may be of any duration relative
to the output pulse. The multivibrator can be retriggered (on the leading edge only) by
applying a common pulse to both the RETRIGGER and +TRIGGER inputs. In this
mode the output pulse remains high as long as the input pulse period is shorter than
the period determined by the RC components. An external countdown option can be
implemented by coupling "Q" to an external "N" counter and resetting the counter
with the trigger pulse. The counter output pulse is fed back to the ASTABLE input
and has duration equal to N times the period of the multivibrator. A high level on the
EXTERNAL RESET input assures no output pulse during an "ON" power condition.
This input can also be activated to terminate the output pulse at any time. In the
monostable mode, a high-level or power-on reset pulse must be applied to the
EXTERNAL RESET whenever VDD is applied. The capacitor used in the circuit
should be non-polarized and have low leakage (i.e. the parallel resistance of the
capacitor should be an order of magnitude greater than the external resistor used).
There is no upper or lower limit for either Rt or Ct value to maintain oscillation.
However, in consideration of accuracy, Ct must be much larger than the inherent stray
capacitance in the system unless this capacitance can be measured and taken into
41.
 account). Rt must be much larger than the LOCMOS ‘ON’ resistance in series with it,
which typically is hundreds of ohms. The recommended values for Rt and Ct to
maintain agreement with previously calculated formulae without trimming should be:
Ct≥100 pF, up to any practical value,10 kΩ ≤ Rt ≤ 1 MΩ.
49. Frequency stability. - some astable multivibrators must have a high degree of
frequency stability. One way to obtain a high degree of frequency stability is to apply
triggers. Figure 3-9, view (A), shows the diagram of a triggered, astable multivibrator.
At time T0, a negative input trigger to the base of Q1 (through C1) causes Q1 to go
into saturation, which drives Q2 to cutoff. The circuit will remain in this condition as
long as the base voltage of Q2 is positive. The length of time the base of Q2 will
remain positive is determined by C3, R3, and R6. Observe the parallel paths for C3 to
discharge.

Figure. - Triggered astable multivibrator and output.

View (B) of figure 3-9 shows the waveforms associated with the circuit. At time T1,
Q2 comes out of cutoff and goes into saturation. Also, Q1 is caused to come out of
saturation and is cut off. The base voltage waveform of Q1 shows a positive potential
that is holding Q1 at cutoff. This voltage would normally hold Q1 at cutoff until a
point between T2 and T3. However, at time T2 another trigger is applied to the base of
Q1, causing it to begin conducting. Q1 goes into saturation and Q2 is caused to cut off.
This action repeats each time a trigger (T2, T4, T6) is applied.

Figure. - Triggered astable multivibrator and output

42.
50. The prt of the input triggers must be shorter than the natural free-running prt of
the astable multivibrator, or the trigger prf must be slightly higher than the free-
running prf of the circuit. This is to make certain the triggers control the prt of the
output. PCB (Printed circuit Board). A printed circuit board, or PCB, is used to
mechanically support and electrically connect electronic components using conductive
pathways, tracks, or traces, etched from copper sheets laminated onto a non-
conductive substrate. It is also referred to as printed wiring board (PWB) or etched
wiring board. A PCB populated with electronic components is a printed circuit
assembly (PCA), also known as a printed circuit board assembly (PCBA). PCBs are
inexpensive, and can be highly reliable. They require much more layout effort and
higher initial cost than either wire-wrapped or point-to-point constructed circuits, but
are much cheaper and faster for high-volume production. Much of the electronics
industry's PCB design, assembly, and quality control needs are set by standards that
are published by the IPC organization. The inventor of the printed circuit was the
Austrian engineer Paul Eisler (1907–1995) who, while working in England, made one
circa 1936 as part of a radio set. Around 1943 the USA began to use the technology on
a large scale to make rugged radios for use in World War II. After the war, in 1948,
the USA released the invention for commercial use. Printed circuits did not become
commonplace in consumer electronics until the mid-1950s, after the Auto- Assembly
process was developed by the United States Army. Before printed circuits (and for a
while after their invention), point-to-point construction was used. For prototypes, or
small production runs, wire wrap or turret board can be more efficient. Originally,
every electronic component had wire leads, and the PCB had holes drilled for each
wire of each component. The components' leads were then passed through the holes
and soldered to the PCB trace. This method of assembly is called through-hole
construction. Soldering could be done automatically by passing the board over a
ripple, or wave, of molten solder in a wave-soldering machine. However, the wires
and holes are wasteful since drilling holes is expensive and the protruding wires are
merely cut off. In recent years, the use of surface mount parts has gained popularity as
the demand for smaller electronics packaging and greater functionality has grown.
Printed circuit boards are found virtually in all electronic equipments. They are also
the most custom-designed part of any electronic equipment. PCBs provide mechanical
support apart from functional electrical interconnection between components. A PCB
is a dielectric substrate with metallic circuitry photo-chemically formed upon that
substrate. Typically, there are two major types of board materials (dielectric
substrates) that are used for the baseboard, fibre glass (glass-epoxy) and phenolic
paper. The baseboard is also known as copper clad. There are three types of printed
circuit boards available today. Printed Circuit Board implementation has been
described as the most expensive and time-consuming effort in the product design
cycle. Experience has proven modern design automation systems are capable of
significant reductions in design time. This does not say that all PCB design should be
done on design automation systems. This paper addresses three classes of PCB's, and
describes proven techniques for methods of classification and cost effective solutions.
The oral presentation will be accompanied by visual aids (slides) of actual designs in
each classification they are:
(a) Single sided boards: Where the entire circuit is laid on one side of the board
and there may or may not be holes on the board for mounting of components, or

43.
 for interconnection of components. Normally the wire-leaded components must
be mounted on only one side of the PCB, with all the leads through holes,
soldered and clipped. You can also mount the components on the track surface
using Surface Mount Technology (SMT) or Surface Mount devices (SMD).
Surface mount circuitry is generally smaller than conventional. Surface mount
is generally more suited to automated assembly than conventional. In practice,
most boards are a mix of surface mount and conventional components. This can
have its disadvantages as the two technologies require different methods of
insertion and soldering. Conventional circuitry is generally easier to debug and
repair.
(b) Double sided boards: With the circuit on both sides of the board and electrical
connection is established by drilling holes through the board and plating copper
through the holes. Two layers of copper, one each side of the board. The
components must be mounted on only one side of the PCB but you can also
mount components on both sides of the PCB. Normally only surface mounts
circuitry would be mounted on both sides of a PCB. The components must be
mounted using both through holes technology or Surface Mount Technology
(SMT) or Surface Mount devices (SMD). Conventional circuitry is generally
easier to debug and repair.
(c) Multilayer boards: Two or more pieces of dielectric materials with circuitry
formed upon them and are stacked up and bonded together. Electrical
connections are established from one side to the other, and to the inner layer
circuitry by drilled holes which are subsequently plated through with copper. A
PCB Laminate may be manufactured with more than two layers of copper tracks
by using a sandwich construction. The cost of the laminate reflects the number
of layers. The extra layers may be used to route more complicated circuitry,
and/or distribute the power supply more effectively.

Figure – showing a multilayer PCB

51. A multi layer PCB is much more expensive and difficult to manufacture than a
single or double sided board, but it really does give you a lot of extra density to route
power and signal tracks. By having your signals running on the inside of your board,
you can pack your components more tightly on your board to give you a more
compact design. Deciding to go from double sided to multi layer can be a big decision,
so make sure that a multi layer board is warranted on the grounds of board size and
complexity. You can forget about making multi layer boards yourself, it requires a

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 commercial manufacturer. Most of the hobby board suppliers will not do multi layer
boards. Multi layer boards come in even number of layers. With 4, 6, and 8 layer
being the most common. You can go many layers above this, but now you are in the
realm of the very specialised. Technically you can get an odd number of layers
manufactured, like a 3 layer board for instance. But it really won’t save you any cost
over a 4 layer board. In fact a 3 layer board might even be more expensive than a 4
layer board because it calls for a non-standard manufacturing process. If you decide to
go multi layer then make sure you make use all of your layers, there is no point
leaving one completely blank. With a multi layer board, you would typically dedicate
one complete layer to a ground plane, and another to your power. With perhaps a few
signal tracks thrown on the power layer if you need to. If you have a digital only
board, then you’d often dedicate the entire power layer also. If you have room on the
top or bottom layer, you can route any additional power rail tracks on there. Power
layers are almost always in the middle of the board, with the ground closer to the top
layer. Once you have your power taken care of on the inner layers, you’ll be surprised
at the room you now have available for your signal tracks. It really does open up a
whole new dimension to routing. If power planes are vital, and you have a lot of
connections to route, then you may have to move from 4 to 6 layers. Six layers will
give you four full signal routing layers and two layers dedicated to power. You can
really do some advanced routing with 6 layers. Eight layers and above is basically
more of the same. With multi layer design comes the options of using different types
of vias to improve your routing density. There are three types of vias - standard, blind,
and buried. Standard vias go through the whole board, and can connect any of the top,
bottom or inner layers. These can be wasteful of space on layers which aren’t
connected. “Blind” vias go from the outside surface to one of the inner layers only.
The hole does not protrude through the other side of the board. The via is in effect
“blind” from the other side of the board. “Buried” vias only connect two or more inner
layers, with no hole being visible on the outside of the board. So the hole is
completely buried inside your board. Blind and buried vias cost more to manufacture
than standard vias. But they are very useful, and almost mandatory for very high
density designs like those involving Ball Grid Array (BGA) components.
PCB Construction
52. After we have designed your circuit, perhaps even bread boarded a working
prototype, and now it's time to turn it into a nice Printed Circuit Board (PCB) design.
For some designers, the PCB design will be a natural and easy extension of the design
process. But for many others the process of designing and laying out a PCB can be a
very daunting task. There are even very experienced circuits designers who know very
little about PCB design, and as such leave it up to the "expert" specialist PCB
designers. Many companies even have their own dedicated PCB design departments.
This is not surprising, considering that it often takes a great deal of knowledge and
talent to position hundreds of components and thousands of tracks into an intricate
(some say artistic) design that meets a whole host of physical and electrical
requirements. Proper PCB design is very often an integral part of a design. In many
designs (high speed digital, low level analog and RF to name a few) the PCB layout
may make or break the operation and electrical performance of the design. It must be
remembered that PCB traces have resistance, inductance, and capacitance, just like

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your circuit does. Once the PCB layout has been completed, one can start thinking of
transferring the layout from the CAD software or even the layout from electronics
magazines. The author would like to suggest that one jump over the steps of using
transparency, photo-resist chemical, developer chemical and the use of Ultra Violet
light. The use of Printed Circuit Board Transfer Film will eliminate the steps
mentioned. If your layout is in the CAD software, just print the layout onto this
transfer file using a laser printer. If your layout is in the form of hardcopy like
magazines or artwork, all you need to do is to photocopy the layout into the PCB
transfer Film. During this project helped me to take some of the mystery out of PCB
design. It gives some advice and “rules of thumb” on how to design and lay out your
PCBs in a professional manner. It is, however, quite difficult to try and “teach” PCB
design. There are many basic rules and good practices to follow, but apart from that
PCB design is a highly creative and individual process. It is like trying to teach
someone how to paint a picture. Everyone will have their own unique style, while
some people may have no creative flair at all! Indeed, many PCB designers like to
think of PCB layouts as works of art, to be admired for their beauty and elegance. “If
it looks good, it’ll work good.” is an old catch phrase. This procedure is to guide to
producing consistently high quality PCBs quickly and efficiently, particularly for
professional prototyping of production boards. Unlike most other PCB homebrew
guides, emphasis is placed on quality, speed and repeatability rather than minimum
You need to generate a positive (i.e. black = copper) UV translucent artwork film
You'll never get a good board without good artwork, so it is important to get the best
possible quality at this stage. The most important thing is to get a clear sharp image
with a very solid opaque black. Nowadays, artwork will almost always be drawn
using either a dedicated PCB CAD program, or a suitable drawing / graphics package.
The merits of various software packages will not be discussed here, other than to say
that it is absolutely essential that your PCB software prints holes in the middle of pads,
to act as centre-marks when drilling. It is virtually impossible to accurately hand-drill
boards without these holes. If you're looking to buy PCB software at any cost level,
and want to be able to do hand-prototyping of boards before production, check that
this facility is available. If you're using a general purpose CAD or graphic package,
define pads as either a grouped object containing a black filled circle with a smaller
concentric white filled circle on top of it, or as an unfilled circle with a thick black line
style (i.e. a black ring). When defining pad and line shapes, the following minimum
sizes are recommended for reliable results: Vias (through-linking holes): 50 mil (1 mil
= 1/1000th of an inch, 50 mil = 0.05"), assuming 0.8mm drill size (but stick to 65 mil
if you can to make drilling accuracy less critical). You can go smaller with smaller
drill sizes, but through-linking will be harder. Pads for normal components and DIL
ICs: 65 mil round or square pads, with 0.8mm hole. These will allow a 12.5 mil track
to pass between pins. Normal minimum line width 12.5 mil down to 10 mil, if you
really need to. Centre to centre spacing of 12.5 mil tracks: 25 mil- slightly less may be
possible if your printer can manage it. Take care to preserve the correct diagonal track-
to-track spacing on metered corners (pictured right, grid is 25 mil, track width 12.5
mil). The artwork must be printed such that the printed side will be in contact with the
PCB surface when UV exposing, to avoid blurred edges. In practice this means that if
you design the board as seen from the component side, the bottom (solder side) layer
should be printed the 'correct' way round, and the top side of a double-sided board

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must be printed mirrored. Artwork quality is very dependent on both the output device
and the media used, both of which will now be discussed. Media Contrary to what you
may think, it is NOT necessary to use a transparent artwork medium - as long as it is
reasonably translucent to UV, its fine - less translucent materials may need a slightly
longer exposure time. Line definition, black opaqueness and toner/ink retention are
much more important. Possible print media include the following:
(a) Clear acetate OHP transparencies - these may seem like the most obvious
candidate, but are expensive, tend to crinkle or distort from laser printer
heating, and toner/ink can crack off or get scratched very easily.
(b) Polyester drafting film is good but expensive, the rough surface holds ink
or toner well, and it has good dimensional stability. If used in a laser
printer, use the thickest stuff you can get, as the thinner film tends to crinkle
too much due to the fusing heat. Even thick film can distort slightly with
some laser printers.
(c) Tracing paper Get the thickest you can find - at least 90gsm (thinner stuff
can crinkle), 120gsm is even better but harder to find. It's cheap, easily
available from office or art suppliers (usually in pads the same size as
normal paper sizes), has good enough UV translucency and is nearly as good
as drafting film for toner retention, and stays flatter under laser-printer heat
than polyester or acetate film. The stuff I use is a "Gateway Tracing", Output
devices.
(d) Pen plotters - very fiddly and slow, you have to use expensive polyester
drafting film (tracing paper is no good as ink flows along the fibers) and you
need special inks and expensive ink pens with grooved tips to get acceptable
results. Pens need frequent cleaning and clog very easily.
(e) Ink-jet printers - Not tried them I, but I hear very mixed reports from
"perfect" to "useless"! The main problem will be getting an opaque enough
black. They are so cheap that it's certainly worth a try, and with as many
different media types as you can find, but don't expect the same quality you
can get from lasers. It may also be worth trying an inkjet print onto paper,
which can then be photocopied onto tracing paper with a good quality
photocopier. I have had good reports from several people using tracing paper
with HP DeskJet’s, but my Epson Stylus Photo750 inkjet is useless on
tracing paper. If you plot largish ground planes directly from inkjet, both
90gsm and 112gsm tracing papers crinkle slightly in these areas (the 90 more
than the 112). I find that the best procedure is to allow the inkjet plot to dry
thoroughly (on an HP DeskJet 670C or 895CXi set to normal - best print
quality is not necessary) and then flatten out the plot under a clean sheet of
paper placed under a big heavy book - I use A4 tracing paper that I get in
pad form from my local artist materials shop. I find that thoroughly dried and
flattened plots are perfectly re-usable. With either HP DeskJet (670C or
895CXi), I can consistently obtain 0.005 inch exposed and developed
resolution.
(f) Typesetters - for the best quality artwork, generate a Postscript or PDF file
and take it to a DTP or typesetting service, and ask them to produce a
positive film of it. This will usually have a resolution of at least 2400DPI,
absolutely opaque black and perfect sharpness. The cost is usually 'per page'

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 regardless of area used (UK£5 for A4 last time I did one), so if you can fit
multiple copies of the PCB, or both sides onto one sheet, you'll save money.
This is also a good way to do the occasional large PCB that won't fit your
laser printer - sizes up to A3+ are widely available, and larger ones can also
be done by more specialised services. Also a useful alternative for the
highest-resolution boards that won't quite make it with other methods.
Typeset artworks are good enough for production PCBs, but most PCB
houses nowadays only accept Gerber data, as it's easier for them to post-
process for step & repeat etc.
(g) Laser printers - Easily the best all-round solution. Very affordable, fast and
good quality. The printer used must have at least 600dpi resolution for all but
the simplest PCBs, as you will usually be working in multiples of 0.025" (40
tracks per inch). 300DPI does not divide into 40; 600DPI does, so you get
consistent spacing and line width. It is very important that the printer
produces a good solid black with no toner pinholes (pinholes in larger fill
areas are acceptable). If you're planning to buy a printer for PCB use, do
some test prints on tracing paper to check the quality first. If the printer has a
density control, set it to 'blackest'. Even the best laser printers don't generally
cover large areas (e.g. ground planes) well, but this isn't usually a problem as
long as fine tracks are solid. Note that the blackness of the printing on paper
doesn't always mean a good opaque result on tracing paper so always check
with tracing paper if you're buying a printer for PCB work. When using
tracing paper or drafting film, always use manual paper feed, and set the
straightest possible paper output path, to keep the artwork as flat as possible
and minimize jamming. For small PCBs, remember you can usually save
paper by cutting the sheet in half (e.g. cut A4 to A5), you may need to
specify a vertical offset in your PCB software to make it print on the right
part of the page. Some laser printers have poor dimensional accuracy, which
can cause problems for large PCBs, but as long as any error is linear (e.g.
does not vary across the page), it can be compensated by scaling the printout
in software. The only time that print accuracy is likely to be a noticeable
problem is when it causes misalignment of the sides on double-sided PCBs -
this can usually be avoided by careful arrangement of the plots on the page
to ensure the error is the same on both layers, for example choosing whether
to mirror horizontally or vertically when reversing the top-side artwork. I use
a Lexmark Optra R+ which does 1200DPI, although I only use this
resolution for really fine surface mount stuff - 600DPI is usually good
enough, and also feeds faster so heat distortion is reduced. When manually
feeding tracing paper in this printer, you must crease the leading edge
slightly downwards to avoid jamming.
52. Photo-resist PCB lamination always use good quality pre-coated positive
photo-resist fibre glass (FR4) board. Check carefully for scratches in the protective
covering, and on the surface after peeling off the covering. You don't need darkroom
or subdued lighting when handling boards, as long as you avoid direct sunlight,
minimum unnecessary light exposure, and develop immediately after UV exposure.
I've always used 'Microtrak' from Mega (formerly Instagraphic) board (SS euro card
Mega order code 03-5108-1) - it develops really quickly, gives excellent resolution,

48.
 and is available in thin (0.8mm) and heavy-copper flavours. I've never had any luck
using spray-on photo-resist, as you always get dust settling on the wet resist, and
coating thickness is both critical and very hard to get even. I wouldn't recommend it
unless you have access to a very clean area or drying oven, or only want to make low-
resolution PCBs. Even then you probably don't really want to bother with it - life's too
short to daff about coating your own laminate.
53. Exposure of the photo-resist board needs to be exposed to ultra-violet light
through the artwork, using a UV exposure box. UV exposure units can easily be made
using standard fluorescent lamp ballasts and UV tubes. For small PCBs, two or four 8
watt 12" tubes will be adequate, for larger (A3) units, four 15" 15 watt tubes are ideal.
To determine the tube to glass spacing, place a sheet of tracing paper on the glass and
adjust the distance to get the most even light level over the surface of the paper. Even
illumination is a lot easier to obtain with 4-tube units. The UV tubes you need are
those sold either as replacements for UV exposure units or insect killers. I've heard
reports that 'black light' tubes for disco lighting etc. don't work very well (these have a
black or dark purple appearance when off). The tubes you want look white when off
(just like normal white lamps), and light up with a light purple, which makes
fluorescent paper etc. glow brightly. DO NOT use short-wave UV lamps like EPROM
eraser tubes or germicidal lamps, which have clear glass - these emit short-wave UV
which can cause eye and skin damage, and are not suitable for PCB exposure. Mega in
the UK do cheap UV bulbs as replacements for their UV boxes. RS also stock a wide
range of UV tubes, including U shaped ones - search for 'insect killer' on their site.
Electrical suppliers like TLC also sell UV insect-killer tubes. A timer which switches
off the UV lamps automatically is essential, and should allow exposure times from 2
to 10 minutes in 30 second increments. It is very useful if the timer has an audible
indication (e.g. goes 'ping') when the timing period has completed. A mechanical or
electronic timer from a scrap microwave oven would be ideal. Dead scanners make
ideal cases for homemade UV boxes, but make sure the case is deep enough - a nice
old clunky one, not a modern slim line thing ( unless you don't mind using a lot of
tubes to get even illumination). Although it is probably possible to make a UV box
with UV LEDs, you'd need so many to get a decent exposure area that it is almost
certainly not worth even thinking about unless you happen to have a few hundred of
them and nothing more interesting to use them for. Short-term eye exposure to the
correct type of UV lamp is not harmful, but can cause discomfort, especially with
bigger units. Use glass sheet rather than plastic for the top of the UV unit, as it will
flex less and be less prone to scratches. Normal window glass works fine. I made up a
combined unit, with switchable UV and white tubes, so it doubles as an exposure unit
and a light-box for lining up double-sided artworks. If you do a lot of double-sided
PCBs, it may be worth making a double-sided exposure unit, where the PCB can be
sandwiched between two light sources to expose both sides simultaneously. You will
need to experiment to find the required exposure time for a particular UV unit and
laminate type - expose a test piece in 30 second increments from 2 to 8 minutes,
develop and use the time which gave the best image. Generally speaking,
overexposure is better than underexposure. (it's easier to add the odd wire-bridge than
hack off a load of unwanted copper with a Dermal or deal with lots of hairline shorts
on fine-pitch tracking) For a single-sided PCB, place the artwork, toner side up, on the
UV box glass, peel off the protective film from the laminate, and place it sensitive side

49.
 down on top of the artwork. The laminate must be pressed firmly down to ensure good
contact all over the artwork, and this can be done either by placing weights on the
back of the laminate (I use a few dead gel-cell lead-acid batteries for this), or by fitting
the UV box with a hinged lid lined with foam rubber, which can be used to clamp the
PCB and artwork. If you are using an old Scanner as a case, the lid will of course
already be there. To expose double-sided PCBs, print the solder side artwork as
normal, and the component side mirrored. Place the two sheets together with the toner
sides facing each other, and carefully line them up, checking all over the board area
for correct alignment, using the holes in the pads as a guide. A light box is very handy
here, but it can be done with daylight by holding the sheets on the surface of a
window. If printing errors have caused slight miss-registration, align the sheets to
'average' the error across the whole PCB, to avoid breaking pad edges or tracks when
drilling. When they are correctly aligned, staple the sheets together on two opposite
sides (3 sides for big PCBs), about 10mm from the edge of the board, forming a sleeve
or envelope. The gap between the board edge and staples is important to stop the paper
distorting at the edge. Use the smallest stapler you can find, so the thickness of the
staple is not much more than that of the PCB. Expose each side in turn, covering up
the top side with a reasonably light-proof soft cover when exposing the underside -
rubber mouse mats are ideal for this. Be very careful when turning the board over, to
avoid the laminate slipping inside the artwork envelope and ruining the alignment.
After exposure, you can usually see a feint image of the pattern in the photosensitive
layer.

Figure -showing UV ray exposure machine

54. Developing is the main thing to say here is does not use SODIUM
HYDROXIDE for developing photo-resist laminates. Use of Sodium hydroxide is the

50.
 primary reason people complain about poor results when trying to photo-etch PCBs.
It is completely and utterly dreadful stuff for developing PCBs - apart from its
causticity, it's very sensitive to both temperature and concentration, and made-up
solution doesn't last long. Too weak and it doesn't develop at all, too strong and it
strips all the resist off. It's almost impossible to get reliable and consistent results,
especially so if making PCBs in an environment with large temperature variations
(garage, shed etc), as is often the case for such messy activities as PCB making. A
much better developer is a silicate based product, which comes as a liquid
concentrate. I'm told this is sodium meta-silicate penta-hydrate Na2SiO3*5H2O (RS-
Components data sheet item 690-849 and Safety data sheet). See sources below for
method for making this developer. This stuff has huge advantages over sodium
hydroxide, most importantly is very hard to over-develop. You can leave the board in
for several times the normal developing time without noticeable degradation. This also
means it's not temperature critical - no risk of stripping at warmer temperatures. Made-
up solution also has a very long shelf-life, and lasts basically until it's worn out (and
even then you can just top up with more concentrate) - the concentrate lasts essentially
forever. The lack of over-developing problems allows you to make the solution up
really strong for very fast developing The recommended mix is 1 part developer to 9
parts water, but I usually make it stronger to develop Micro-Track laminate in about 5-
10 seconds (yes, seconds - dip, rinse and it's done!) without the risk of over-
development damage. You can check for correct development by dipping the board in
the ferric chloride very briefly (or dribbling a few drops onto the surface) - the
exposed copper should turn dull pink almost instantly, leaving the track pattern
sharply defined. If any shiny copper colored areas remain, or the gaps between tracks
are 'blurry', rinse and develop for a few more seconds. If the board was under-exposed,
you tend to get a thin layer of resist which isn't removed by the developer. You can
often remove this by gently wiping with dry paper towel (Kitchen roll, preferably none
coloured/patterned!) - The dry paper towel is just abrasive enough to remove the film
without damaging the resist. You can either use a photographic developing tray or a
vertical tank for developing - a tray makes it easier to see the progress of the
development. You don't need a heated tray or tank unless the solution is really cold
(<15°C). A defrost tray from a small scrap refrigerator is a possible alternative (I have
been known to use the tray from my fridge to develop & etch a particularly large
PCB).
55. Etching always used ferric chloride etchant - its messy stuff, but easier to get
and cheaper than most alternatives I've seen. It attacks ANY metal including stainless
steel, so when setting up a PCB etching area, use a plastic or ceramic sink, with plastic
fittings & screws wherever possible, and seal any metal screw heads etc. with silicone-
rubber sealant. If copper water pipes may get splashed or dripped-on, sleeve or cover
them in plastic (heat-shrink sleeving is great if you're installing new pipes). Fume
extraction is not normally required, although a cover over the tank or tray when not in
use is a good idea. If there is an easy way to vent fumes outside (e.g. a cover over the
tanks) this can make the fumes less objectionable but it's really not worth the hassle of
setting up a powered extractor unless you have a particularly sensitive nose/throat.
Power extraction is also rather problematic to do due to corrosion issues. You should
always use the hexa-hydrate type of ferric chloride, which is light yellow, and comes
as powder or big globular granules, which should be dissolved in warm water until no

51.
 more will dissolve, giving a typically muddy brown solution. Adding a teaspoon of
table salt helps to make the etchant clearer (looks like very strong tea), for easier
inspection. Anhydrous ferric chloride is sometimes encountered, which is a dark
green-brown crystalline powder. Avoid this stuff if at all possible Use extreme
caution, as it creates a lot of heat when dissolved - always add the powder very slowly
to water, do not add water to the powder, and use gloves and safety glasses. You may
well find that solution made from anhydrous FeCl doesn’t etch at all, if so, you need to
add a small amount of hydrochloric acid and leave it for a day or two. Don't add too
much acid though as it will produce very corrosive and choking fumes when warmed
for etching. Sorry, I don't know what constitutes ' too much' as its many years since I
used anhydrous ferric chloride. Always take extreme care to avoid splashing when
dissolving either type of FeCl - it tends to clump together in the container due to
absorbing moisture, and you often get big chunks coming out of the container &
splashing into the solution. It will damage eyes and permanently stain clothing and
pretty much anything else - use gloves and safety glasses and wash off any skin
splashes immediately. If you're making PCBs in a professional environment, where
time is money, you really should get a heated bubble-etch tank. With fresh hot ferric
chloride, a PCB will etch in well under 5 minutes, compared to up to an hour without
heat or agitation. Fast etching also produces better edge quality and consistent line
widths. If you aren't using a bubble tank, you need to agitate frequently to ensure even
etching. Warm the etchant by putting the etching tray inside a larger tray filled with
boiling water - you want the etchant to be at least 30-50ºC for sensible etch times.
56. For Tin Plating I've come to the conclusion that tin-plating is not really worth
the hassle - just strip the resist, rub with wire-wool, and immediately coat with a flux
pen. Although you can get special stripping solutions and hand applicators, most
resists can be dissolved off more easily and cleanly using methanol (methylated spirit).
Hold the (rinsed and dried) PCB horizontal, and dribble few drops of methanol on the
surface, tilting the PCB to allow it to run over the whole surface. Wait about 10
seconds, and wipe off with a paper towel dipped in methanol. Repeat if any resist
remains. For flux-coating, use a Chemtronics CW8200 flux pen. I've found that the
spray-on stuff is too sticky and thick - the pen is much cleaner and easier, and also
very handy for general rework use. I've left the old section on plating below, but I
don't really think it's worth it except maybe in situations where you need a finish that
lasts longer than the life of a typical prototype, e.g. for edge connectors or test-point
pads, or for better cosmetic appearance. Tin-plating a PCB makes it a lot easier to
solder, and is pretty much essential for surface mount boards. Unless you have access
to a roller-tinning machine, chemical tinning is the only option. Unfortunately, tin-
plating chemicals are expensive, but the results are usually worth it. If you don't tin-
plate the board, either leave the photo-resist coating on (most resists are intended to
act as soldering fluxes), or spray the board with rework flux to prevent the copper
oxidising. A 'flux pen be used to coat smaller PCBs. I use room-temperature tin
plating crystals, which produce a good finish in a few minutes. There are other tinning
chemicals available, some of which require mixing with acid or high-temperature use -
I've not tried these. Made-up tinning solution deteriorates over time, especially in
contact with air, so unless you regularly make a lot of PCBs, make up small quantities
at a time (just enough to cover a PCB in the tinning tray) keep the solution in a sealed
bottle (ideally one of those concertina-type bottles used for some photographic

52.
 solutions to exclude air), and return it to the bottle immediately after use - a few days
in an open tray and it can deteriorate badly. Also take care to avoid contamination,
which can very easily render the solution useless. Thoroughly rinse and dry the PCB
before tinning, keep a special tray and pair of tongs specifically for tinning (to avoid
contamination), and rinse them after use. Do not top-up used solution if it stops
tinning - discard it, clean & rinse the tray, and make up a fresh solution. Ensure the
temperature of the tinning solution is at least 25ºC, but not more than 40ºC - if
required, either put the bottle in a hot water bath, or put the tinning tray in a bigger
tray filled with hot water to warm it up. Putting a PCB in cold tinning solution will
usually prevent tinning, even if the temperature is subsequently raised. Preparation is
important for a good tinned finish - strip the photo-resist thoroughly - although you
can get special stripping solutions and hand applicators, most resists can be dissolved
off more easily and cleanly using methanol (methylated spirit). Hold the (rinsed and
dried) PCB horizontal, and dribble few drops of methanol on the surface, tilting the
PCB to allow it to run over the whole surface. Wait about 10 seconds, and wipe off
with a paper towel dipped in methanol. Repeat if any resist remains. Rub the copper
surface all over with wire wool (which gives a much better finish than abrasive paper
or those rubber 'eraser blocks') until it is bright and shiny all over, wipe with a paper
towel to remove the wire wool fragments, and immediately immerse the board in the
tinning solution. Take care not to touch the copper surface after cleaning, as finger
marks will impair plating. The copper should turn a silver colour within about 30
seconds, and you should leave the board for about 5 minutes, agitating occasionally
(do not use bubble agitation). For double-sided PCBs, prop the PCB at an angle to
ensure the solution can get to both sides. Rinse the board thoroughly, and rub dry with
paper towel to remove any tinning crystal deposits, which can spoil the finish. If the
board isn't going to be soldered for a day or two, coat it with flux, either with a rework
flux spray or a flux pen.
57. For Drilling If we're using fibre glass (FR4) board, which you almost certainly
will be, you MUST use tungsten carbide drill bits – fibre glass eats normal high-speed
steel (HSS) bits very rapidly, although HSS drills are OK for odd larger sizes (>2mm)
that you only use occasionally where the expense of a carbide isn't justified. Carbide
drill bits are expensive, and the thin ones snap very easily. When using carbide drill
bits below 1mm, you MUST use a good vertical drill stand - you WILL break drill bits
very quickly without one, and at UK£2-3 a pop, a drill stand will quickly pay for itself.
Carbide drill bits are available as straight-shank (i.e. the whole bit is the diameter of
the hole), or thick shank (also called 'turbo' or 'reduced' shank), where a standard size
(typically about 3.5mm or 1/8") shank tapers down to the hole size. I much prefer the
straight-shank type for sizes below about 1mm because they break less easily, the
longer thin section providing more flexibility. Straight-shank drills are also usually
cheaper, but sometimes less easy to obtain. When drilling with carbide bits, it's
important to hold the PCB down firmly, as the drill bit can snatch the board upwards
as it breaks through, and this will usually break the drill bit if the board isn't held
down. Small drills for PCB use usually come with either a set of collets of various
sizes or a 3-jaw chuck - sometimes the 3-jaw chuck is an optional extra, and is worth
getting for the time it saves changing collets. For accuracy, however, 3-jaw chucks
aren't brilliant, and small drill sizes below 1mm quickly form grooves in the jaws,
preventing good grip. Below 1mm you should use collets, and buy a few extra of the

53.
 smallest ones, keeping one collect per drill size, as using a larger drill in a collect will
open it out so it no longer grips smaller drills well. Some cheap drills come with
plastic collets - throw them away and get metal ones.

Figure – showing a drilling machine

58. You need a good strong light on the board when drilling to ensure accuracy. I
use a 12V diachronic halogen lamp (under-run at 9V to reduce brightness) mounted on
a microphone gooseneck for easy positioning (shown right). It can be useful to raise
the working surface about 6" above normal desk height for more comfortable
viewing. Dust extraction is nice, but not essential - an occasional blow does the trick!
Note that fibre glass dust & drill swarf is very abrasive and also irritating to the skin.
A foot-pedal control to switch the drill off and on is a very useful addition, especially
when frequently changing bits. Typical hole sizes: ICs, resistors etc. 0.8mm. Larger
diodes (1N4001 etc.), square-pin headers, D connectors, IDC connectors, TO-220
leads etc.: 1.0mm, terminal blocks, trimmers etc. 1.2 to 1.5mm. Avoid hole sizes less
than 0.8mm unless you really need them. Always keep at least two spare 0.8mm drill
bits, as they always break just when you need a PCB really urgently. 1.0mm and larger
are more resilient, but one spare is always a good idea. When making two identical
boards, it is possible to drill them both together to save time. To do this, carefully drill
a 0.8mm hole in the pad nearest each corner of each of the two boards, taking care to
get the centre as accurate as possible. For larger boards, drill a hole near the centre of
each side as well. Lay the boards on top of each other, and insert a 0.8mm track pin
(pictured below, under 'Through Plating') in 2 opposite corners, using the pins as pegs
to line the PCBs up. Squeeze (with pliers or a vice) or hammer the pins into the
boards, and then insert and squeeze pins into the remaining holes. The two PCBs will
now have been 'nailed' together accurately, and can be drilled together. Standard track
pins are just the right length to fix standard 1.6mm PCBs together without protruding
below the bottom board. On PCBs with several hole sizes, I'd suggest drilling the
larger sizes first, as this reduces the chance of accidentally under-drilling a hole -
something you typically only notice when the PCB is half-assembled, making it
awkward to re-drill. For occasions like this, and other occasional odd sizes, I'd highly
recommend getting a micro hand-reamer This one covers sizes 1 to 5mm, made by
G&J Hall (Sheffield UK), For hole sizes over about 3mm, I'd recommend pilot-

54.
 drilling at 1.0mm, then drilling to size with a conventional electric drill (preferably a
cordless one with speed control) and standard HSS drill bit.
59. For Cutting if you do any serious amount of PCB work, a small guillotine (cost
about £150) is very useful, as it's by far the easiest way to cut fibre glass laminate
Mega Electronics (see sources) do a very nice one. Ordinary saws (band
saws, jigsaws, hacksaws) will be blunted quickly unless they are carbide tipped, and
the dust can cause skin irritation. Although tempting if available, I would particularly
advise against using a band saw as it will not only wreck the expensive blade quickly,
the inevitable fibre glass dust is likely to do long-term damage to bearings etc. If
using a hacksaw, use a long-frame type i.e. not junior) with adjustable tension, and a
medium or fine metal-cutting blade, with plenty of tension (as tight as you can without
snapping the blade). Clamp the PCB firmly, using a strip of wood to clamp the entire
length of the board, close to the cut, with thin cardboard on each side of the board to
avoid scratching the photo resist. Keep the saw blade angle as shallow as possible -
this keeps the cut nice and straight. A carbide tile-saw blade in a jigsaw might be
worth a try, but bear in mind it's easy to accidentally scratch through the protective
film when sawing, causing photo resist scratches and broken tracks on the finished
board - if using a jigsaw I'd suggest adding a layer of parcel tape to increase protection
.If you have access to a sheet-metal guillotine, this is also excellent for cutting boards,
providing the blade is fairly sharp. O make cut-outs, drill a series of small holes,
punch out the blank and file to size. Alternatively use a fretsaw or small hacksaw, but
be prepared to replace blades often. With practice it's possible to do corner cut outs
with a guillotine but you have to be very careful not too over-cut! A cheap nibbling
tool like this one (pictured right) is very useful for making cut outs and shaping the
board edge. If you use a saw to cut the board, take care to ensure the edges are square,
as burrs on the board will raise it enough from the artwork for the UV light to get
between the artwork and the board check for burrs again once you have removed the
backing sheet just before exposure.

Figure- showing a cutting tool

60. In Through-plating when laying out double-sided boards, give some thought
to how top connections will be made. Some components (e.g. resistors, un-socketed
ICs) are much easier to top-solder than others (e.g. radial capacitors), so where there is

55.
a choice, make the top connection to the 'easier' component. For socketed ICs, use
turned-pin sockets, preferably the type with a thick pin section under the socket body.
Lift the socket slightly off the board, and solder a couple of pins on the solder side to
tack it in place, and adjust so the socket is straight. Solder all the solder side pins, then
solder the required top-side pins by reheating the joint on the solder side, while
applying solder to the pin and track on the component side, waiting until the solder has
flowed all round the pin before removing the heat (pictured right). On dense boards,
think carefully about the best order in which to insert sockets to make access to top-
side pins easier. When you have finished assembling the PCB, double-check that you
have top-soldered all the required top pads, as unsoldered top-side pins can cause
intermittent contact and be very hard to track down. Then when you can't get the board
working, check again for top-side pins you forgot to solder - there's always at least
one..! For vias (holes which link the two sides, without component pins in them), use
0.8mm snap-off linking pins (shown right), available from many electronics suppliers.
(See Sources) These are much quicker than using pieces of wire. Just insert the
bottom of the stick into the hole, bend over to snap off the bottom pin, repeat for other
holes, and then solder both sides. If you need 'proper' through-plated holes, for
example to connect to inaccessible top-side pins, or for underneath surface mount
devices (linking pins stick out too much for use here), Multi-core's "Copper set"
system works well, but the kit is very expensive (£190). It uses 'bail bars' (pictured
right), which consist of a rod of solder, with a copper/tin sleeve plated on the outside.
The sleeve is scored at 1.6mm intervals, corresponding to the PCB thickness. The bar
is inserted into the hole using a special applicator, and bent over to snap off the single
bail in the hole. It is then punched with a modified automatic centre-punch, which
causes the solder to splay over the ends of the plated sleeve, and also pushes the sleeve
against the side of the hole. The pads are soldered each side to join the sleeve to the
pads, and then the solder is removed with braid or a solder sucker to leave a clear
plated hole. Fortunately, it is possible to use this system for plating standard 0.8mm
holes without buying the full kit. You can buy the bail bars separately as refills (£24
for 500). For the applicator, use a 0.9mm automatic pencil, (the type which has a tip
like the one pictured right, e.g. Berol PCL2000), which actually works much better
than the original applicator, as you get one bail for every press of the button, and it has
a metal nose instead of the original plastic one. Get a small automatic centre-punch,
and grind the tip off so it's completely flat - this works fine for punching the bails. For
an anvil, use a thick flat piece of metal - the back of a large heat sink is perfect for this
- plate all the holes before fitting any components so the bottom surface is completely
flat. Holes must be drilled with a sharp 0.85mm carbide drill to get the hole size right
for the plating process. Note that if your PCB package draws pad holes the same size
as the drill size, the pad hole can come out slightly larger than the drilled hole (e.g.
from over-etching or non-centred drilling), causing connection problems with the
plating. Ideally, the pad holes should be about 0.5mm (regardless of drill size) to make
an accurate centre mark. I usually set the hole sizes to exactly half the drill size, so I
know what the 'real' sizes should be when sending NC drill data for production PCBs
.Through-plating using Rivets is another way to do through-plating on dense PCBs.
The rivets can be used quite easily on their own without the punch tool, just a pair of
fine tweezers (and a steady hand...). The 0.4mm rivets (pictured) fit a 0.6mm hole and
so can be used on quite dense groups of 0.05" pad dia. vias.

56.
61. Recommended equipment IS A Three-tank unit comprising heated bubble
etch, spray wash and developer tank. As a bare minimum, a bubble-etch tank and
some way of rinsing boards. Photographic developing trays are adequate for
developing and tinning. Different sized photographic developing trays for
tinning.PCB guillotine or small sheet-metal guillotine. A Jigsaw is an alternative but
you will get through blades quickly - use medium to fine metal-cutting blades and use
paper or card between the shoe-plate and the board to prevent the edge of the show
damaging the resist. PCB drill precision drill with metal collets and good quality
stand. A foot pedal on/off control is a very useful addition. If running water is not
available, get a hand-held spray bottle (as sold for garden insect sprays etc.) for rinsing
PCBs.

Figure -showing components in a PCB

62. Important safety precautions: The sodium hydroxide solid must NEVER be
handled, use disposable gloves. When the sodium hydroxide dissolves in water it
produces a great deal of heat so it must be added a little at a time and each portion
allowed dissolving before more is added. If the solution becomes very hot leave it to
cool before adding more sodium hydroxide. The solution is VERY caustic and it is
particularly damaging to the eyes, use eye protection when making and handling it. It
also ruins clothing. Water glass is also known as "sodium silicate solution" and "egg
preserver", it used in fire proofing fabrics, for waterproofing walls and making
"chemical gardens". Caustic soda is used for clearing drains and available from any
chemist. The solution cannot be made by dissolving solid sodium silicate.This
solution will be the same strength as the concentrate, and so will need diluting - about
1 part concentrate to 4 to 8 parts water, depending on the photo-resist used and
temperature.
Conclusion
63. The project that we opted for our is astable multivibrator kit, for this project we
have made into consideration various aspects of project making. We have done
detailing on various ethical parts of project making. In the course of making the
project we gradually are coming into terms of the various parts of project making.
Starting from the selection of the project which was really a difficult one, we wanted
 57.

to make a project that was useful not only to us but also to our collage, we thought of
making a project that can be used in practical purpose that will be useful for the
betterment of our skills in electronics and in a way will be useful for our collage as
well. We also had in mind that the project should be affordable economy wise. So by
keeping all this various considerations we finally decided to make a astable
multivibrator kit. After we were decided with our project we had other various issues
that had to be solved. The most important and difficult part was the designing part of
our project. We needed to have a good and efficient lab where we can do our various
preliminary tests and other important developments in our project. We are fortunate
enough to have good and equipped labs where we were able to perform the
preliminaries of our project. We were able see that whether we were able to get the
desired output from our project.
64. This project has been very useful for us in many ways. It has enabled us to gain
knowledge in various fields of electronics such as integrated circuits, digital circuits
and the most important, the designing of the printed circuit boards (PCB). We came to
know the various steps which were involved in making a PCB starting from the
selecting the components for the circuit then making of prototypes of our desired
circuit in transparent papers and making the negative of the desired PCB layout.
Finally after the required procedures we obtain our required PCB for our project. We
also got the knowledge of soldering and the essential components that constitutes a
circuit. This project has helped us to get informed about the various semiconducting
components and other devices use full in the fabrication of the PCB. In other word we
can say that we got a firsthand knowledge of the real working of a circuit. Those
aspects of electronics that we were studying we got the knowledge of those things
practically.
65. Through this project we conclude that multivibrator is very useful device in
various electronic and digital circuits. The astable multivibrator is a free running
regenerative circuit used for square waves generation that are used in digital circuits.
The astable multivibrator has no stable states or in other words, this circuit has two
quasi stable states. Since there is no stable states, the out voltage level changes from
one quasi stable state to other quasi stable state. No external signal or trigger is
required in this type of multivibrator. From this project we also came to know about
the various useful aspects of IC 555 timer. The IC 555 timer is a monolithic timing
circuit that is showing accurate and highly stable time delays and oscillations. Te 55
timer is reliable, easy to use and economical. The 555 timer can work on +5 volts to +
18volts supply voltage. It also has adjustable duty cycle and timing from
microseconds to hours. It also has the capacity to produce high current output and
capacity to source or sink current of 200 mill amperes. The 555 timer is bipolar
version of timer. A CMOS version of the 555 timer is also available. Finally we can
say that with a growing worldwide demand for professionals with strong technical
skills and an advanced knowledge of predictive analytics, companies are increasingly
turning to institutions of higher learning for well-rounded trained graduate. So we
think that in a way our project will help us and our college to provide good technical
and basic knowledge in the field of electronics and digital circuits.