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Circuit Diagram and Prototype Construction (Project 1)
slide L1-1
Objectives
How to read schematic Diagrams
 How to build a prototype board with schematic diagrams

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
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Reading schematics
 It
means the ability to look at a schematic and describe in detail
how the circuit works.
 Electronic circuits are presented in schematic form. A schematic is
really a map showing the path the current takes through the various
components.
 Each component is represented by a symbol, usually with either a
label or a value (or both).
 The arrangement of the components on paper is chosen to make
the function of the circuit clear, and usually only vaguely resembles
the actual construction of the device.
 The current path is shown with lines, again drawn for maximum
clarity, with little concern for the length or position of the real wires.
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-3
Conventions to all schematics
The layout of a schematic is designed to show the function,
usually with signal progressing from left to right.
 The actual layout of the circuit will be quite different.
 All points on a line are electrically identical.

 This
includes all branches of a line.
 When we discuss the properties of circuits, we will assume the
wires are perfect conductors, with no resistance or propagation
delays of any kind.
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-4
Symbols
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
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Ground

All ground points in the schematic are connected together.
 These
points represent places in the circuit that are at 0 volts for
reference in measurements.
 Often the ground includes the metal chasis of a device, but not
always.
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-6
When is a wire on a schematic
connected to another wire?

2 intersecting lines, no dot
 not

connected
2 intersecting lines, dot
 connected
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-7
Detail of the prototype

The function of the circuit


to convert TTL voltage level to RS232 level and vice versa
Major component
 MAX232A
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-8
RS-232 and TTL

Converter
 so
you can connect TTL/CMOS to an RS-232 compatible device,
such as the COM port on your PC.
RS-232" is pretty much the standard serial communications
interface found on all types of equipment such as computers,
modems, printers.
 Most microcontrollers run on a single supply voltage


voltage is +5 volts.
 In rough terms, logical 1 on these devices indicates that +5 is the
voltage on the output pin. Logical 0 specifies that 0 volts is on the
line.
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-9
RS-232 and TTL
The RS-232-C standard specifies that the voltage on the wire for
sending a logical 0 are from +5v to +15v.
 The voltage for sending a 1 are from -5v to -15v.
 Most microcontrollers not capable of generating these voltages.
So, to connect a microcontroller SCI port to a true RS-232
device, you need to convert the TTL voltages of 0 and +5 into
voltages between about -10 volts and +10 volts.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-10
RS232C standard
"Space" (logic 0) will be between +3 and +25 Volts.
 "Mark" (Logic 1) will be between -3 and -25 Volts.
 The region between +3 and -3 volts is undefined.
 An open circuit voltage should never exceed 25 volts

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-11
Serial Ports

Serial Ports come in two "sizes”
 There
are the D-Type 25 pin connector and the D-Type 9 pin
connector
 both of which are male on the back of the PC,

Thus you will require a female connector on your device.
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-12
MAX232A
It runs on a single chip supply (+5
volts), and requires a few external
capacitors.
 It shows a double charge pump
voltage doubler and a +10v to -10v
voltage inverter.
 The voltages output are used to
generate the RS-232 compliant
signals.
 The MAX232A has provisions for
two serial ports on the same
physical package.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-13
MAX232A
he MAX232 comes in several
varieties.
 MAX232A requires only .1uF
capacitors.
 The original MAX232 required
4.7 and 10uF capacitors, which
are bigger and cost more.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
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Schematic Diagram
Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-15
Prototype Building
This sample has been built
using prototype board
which has single isolated
copper pads on each hole.
 The interconnection uses
non-insulated copper wire
 The advantage of this
technique is that it
produces a robust product
that stands up to handling.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-16
Prototype Building
Looking at the underside, you can
see how the copper wire does not
cross itself.
 This means you must design your
layout very carefully to avoid
crossing.
 The effect is similar to a single sided
pcb layout.
 This technique is not suitable for
complex circuits, or ones which
require many interconnections.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-17
Prototype Building
For this type of circuit you should use insulated wire with
prototype board.
 However, jumper wires can be added to the component side of
the board to improve connection possibilities.
 Try to keep the power supply wires as short as possible and
avoid too many bends as these add inductance which will result
in voltage spikes on the power supply.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-18
Stripboard
This prototype uses stripboard
where holes are joined horizontally
by copper track (see underside
view).
 On the top, you can see how
copper wire has been used to join
together the horizontal copper
tracks underneath.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-19
Stripboard
Here you can see how the board
has copper track to join together
all holes in a horizontal line.
 If you don't want the holes to be
joined together, you can use a
'spot face cutter' to break the
track.
 Notice how the tracks carrying
the power rails (0V and 5V) have
been 'thickened' by adding
additional solder.
 This will reduce their resistance
and minimize voltage drop.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-20
Wirewrap
This prototype uses wirewrap
to achieve the connection
between the various
components of the circuit. IC
are mounted in special sockets
which have longer connection
pins
 The interconnecting wire is
insulated, tinned copper wire.
 About 1cm of the insulation is
stripped, and a special tool
used to wrap the wire around
the connection pin.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
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SOLDERING
Cleaning the bit with a damp sponge
 Tinning the bit
 Heat the biggest part of the joint for 2-3
seconds.
 Apply solder to the joint not to the iron.
 Allow the solder to run over the joint
without moving the iron.
 Remove the solder.
 Remove the iron.
 Avoid overheating.The process should
take only 2-3 seconds.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-22
SOLDERING
A good joint is smooth and shiny.
 If you have taken too long it will have have solder spikes.
 All soldered joints should be easily removable.This means a
minimum of lead wrapping.
 Do not carry solder on the bit to the joint. The smoke you see is
evaporating flux, which should normally clean the joint.
 Reflow soldering is tinning two pieces of wire separately and
then reheating to join them together.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-23
DESOLDERING COMPONENTS
Ensure the iron bit is well tinned.
 Heat up the joint to be de-soldered until the solder runs.
 Apply the solder sucker and remove the solder. This should only
take two or three seconds.
 In some cases de-soldering braid is useful. Ensure the iron is
tinned.
 Place the braid on top of the joint. Place the iron on top of the
braid and allow the solder from the joint to run up the braid.

Dept. of Information &
Communications Technology
Circuit Diagram and Prototype Construction (Project 1)
slide L1-24
CONTINUITY TESTING
Continuity testing means making sure that something is
continuous i.e. not broken.
 For example, copper tracks on PCB's or wires in cables must not
be open circuit but continuous.
 A test meter, set to measure resistance, can be used to measure
continuity.
 Ensure that the meter is set to read low resistance.
 Some testers give out a whistle when continuity is ok, so you can
keep your eye on the job and not keep looking at the meter

Dept. of Information &
Communications Technology