Dept. of Information & Communications Technology Circuit Diagram and Prototype Construction (Project...

Dept. of Information &Communications Technology

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



Reading schematicsIt 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.



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.



Symbols



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.



When is a wire on a schematic connected to another wire?

2 intersecting lines, no dot not connected

2 intersecting lines, dot connected



Detail of the prototype The function of the circuit

to convert TTL voltage level to RS232 level and vice versa Major component

MAX232A



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.



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.



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



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.



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.



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.



Schematic Diagram



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.



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.



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.



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.



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.



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.



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.



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.



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.



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

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