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Electronic Circuit Designing: Modular Approach (Part 1)

 

A person’s inquisitiveness and curiosity can do wonders. It can open a realm of ideas and imaginations never thought before and create something worthwhile. When I was a child I used to read lot of electronic articles and magazines regularly. I’ve seen a lot of interesting circuits and tried as many of them as possible at that time. Whenever a new circuit catches my attention I can’t resist myself trying it out. Every time I buy a new general purpose PCB and the required components listed with the circuit diagram and spending a lot of time soldering the components in the general purpose board. I used to implement the developed board for its practical purpose but it won’t last too long in its place and eventually end up in my small junk yard. As a child, I couldn’t afford the cost of all these hardware and now with the passing years I have a large collection of useless circuit boards with costly components soldered into it. I’ve successfully de-soldered few components but most of the time the process end up with broken leads or damaged component due to overheating and lack of skill. I was familiar with the bread-board but even today I don’t find it quiet useful for practical implementation other than testing circuits before soldering into a circuit board. Above all I find it a waste a lot of time is wasted first in soldering then testing, de-soldering and again soldering and testing; just because multiple mistakes were made while soldering the complex looking circuits.

After few years of building circuits from other designer’s article I tried to design my own circuit projects. I had my ideas and made circuit diagrams, but when it comes to the prototyping of the circuit idea I used to find it very difficult and most of the time I give up without any results even though the circuit diagram seems to be perfect. I realized that I was missing the knowledge of key technique or method which was keeping me away from realizing my circuit ideas. I don’t need just a circuit diagram or a breadboard full of components with ugly looking wires and loose connections, but I need a fully working and ready to implement circuit board   developed within a short span of time.

 The story might be same for almost all electronic hobbyists and beginners. With this idea in mind I started  thinking about a better technique to prototype electronic circuit projects. The technique that would bring an end to the wastage of hardware and help  to finish the project or prototype within a short period of time. The technique should save time, money and increase efficiency by reducing chances of mistakes. Finally it seems that I’ve found the answer which I was looking for a long time.

 

I would like to call the technique as Electronic Modular Design and Assembly.

This is not a new technique or method and is not defined anywhere, but I’ve observed the experienced guys following this method for developing electronic prototypes. The technique is to design a complex circuit from the very basic known circuit and then build these basic circuits into modules, test them individually and finally assemble the entire modules in a single board to complete the prototype. This is a technique that most of the designers use to build the prototype of a project within a short period of time and sometimes in actual manufacturing process as well. Since we are dividing the entire circuit into simplest modules we can test them individually and also reduces the complexity during the time of assembly. The chance of misconnection is highly reduced since we are actually building only the simplest known circuit each and every time even though the original circuit is highly complex. The most attractive part is that once we build a module we can reuse them in another circuit project.

In this article my intention is to familiarize the reader with the technique of electronic Modular Design and Assembly.

The technique and its steps are explained based on a project which I’ve prototyped myself using this very technique.

Even though all the details of the reference project are explained in this article, I request the reader not to consider this article simply as an explanation of the project idea or circuit. The article discusses the steps and techniques to develop the circuit itself and the method of prototyping the same using the electronic Modular Design and Assembly.

The way of explanation in this article might not be comfortable for those who just want to know the overview of the reference project only. But in the course of explaining the modular design method I’ve given special care to go in depth of the working details of the reference project

When I start the process I have nothing but some requirements of the project, not even a circuit diagram. I will design the circuit diagram somewhere during the course of this process and finally with this method itself I will build and assemble the hardware.

You can call this method as modularization technique, fast prototyping, hardware reuse etc. since this technique is not defined anywhere as such. All the data in the article simply represents the way in which a designer like me approaches a prototyping project and finish it quickly and successfully. Consider this only as yet another useful technique other than those already known to you and I’m sure it can take you a long way further. A representation of electronic Modular Design and Assembly is shown in the following figure.

Representation Of Electronic Modular Design And Assembly

Fig. 1: Representation Of Electronic Modular Design And Assembly

In the above figure A, B, C... up to H represents separate modules or circuit boards which are individually designed, built and tested and finally assembled in the main circuit board marked as Z. This is the whole concept of modularization or Modular Design and Assembly.

Summarizing Modularization & its Advantages

A circuit module is a piece of hardware with some specific functionality and it can be connected or interfaced as a part of other large circuits. A module is supposed to have pins for input, power and output. The pins should be available through connectors which will allow easy plug in and plug out from other circuit boards. The examples of advanced modules used in modern day circuit boards are RF modules, LCD, Zigbee etc.

The circuit modules commonly found with male burg stick board-to-board connectors, and the female connectors allow them to be connected into other circuit boards. The following diagram represents a module with male burg stick connector and can be plugged in and out to the underneath circuit board using female burg stick connector.

Module With Male Burg Stick Plugged In Circuit Board Through Female Burg Stick

Fig. 2: Module with Male Burg Stick Plugged in Circuit Board Through Female Burg Stick

In Modular Design and Assembly the first step is to collect all the significant details regarding the features that the prototype should possess. The details are analyzed and the basic functions required are figured out. Each of these functions is then represented as a functional block. The functional blocks are then interconnected in the required order suitable for the project and form the functional block diagram. If the waveforms related with the project are significant then a plot of them is also generated at this stage from the expected waveforms at the input and output of the functional blocks.  With the better understanding of the design from the block diagram and the timing diagram each functional blocks are converted into their corresponding circuit diagram and then interconnect the individual circuit diagrams as per the block diagram to finish the complete circuit diagram. Reusable modules are identified from the circuit diagram based on the block diagram and their corresponding hardware is developed as hardware modules. Test each module separately and finally assemble them together in the main circuit board.

The entire process is summarized in the following steps

1)  Analyze the required features

2)  Required features into functional blocks

3)   Functional blocks into block diagram

4)  Individual waveforms into timing diagram

5)  Design circuit for each functional block with the help of timing diagram

6)   Individual circuit diagram to final complete circuit diagram

7)   Make a Bill Of Materials (BOM)

8)  Find reusable modules from the complete circuit diagram

9)  Build and test the modules individually

10) Assemble the modules and test the assembled unit

Now let me list the advantages of the modularization technique.

1

Quick block diagram

The project requirements are analyzed and convert them into functional blocks. The functional blocks are then interconnected in the required order and thus form the functional block diagram quickly

2

Quick timing diagram

Expected waveforms at the input and output of each block are learned, integrated and thus form the complete timing diagram quickly

3

Better understanding of the working

The block diagram along with the timing diagram gives a better idea about the working of the design.

4

Quick circuit diagram

With the better understanding of the design idea, each functional blocks are converted into circuit diagram and then interconnect the circuits as per the block diagram and thus form the complete circuit diagram quickly

5

Quick modules

With the help of the circuit diagram each of the functional blocks are converted into hardware modules.

When it comes to individual blocks the circuit complexity reduces and hence the modules can be built quickly

6

Individual testing

Each modules can be tested individually before assemble them

7

Hardware reuse

It is the most important advantage. The modules build for one particular project can be used for another without making any change in the module’s hardware. Thus we can reuse the hardware.

8

Quick assembly

The modules are finally assembled in a main board where a few simple connections between the modules are requires results in quick assembly

9

Time saving

If we already have a few required hardware modules in our collection we can considerably reduce the time to prototype.

10

Efficient

Since individual testing of the modules are done before final assembly the chances of multiple errors in the hardware is minimized significantly

11

Easy to debug

Since each modules can be dissembled and tested again the process of debugging is made easy

12

Space saving

The modules are usually assembled vertically in a horizontal main board. This method save a considerable amount of space compared with the same circuit build in a single large horizontal board

 Table: advantages of the electronic Modular Design and Assembly

Note that the modules usually consist of very simple electronic circuits known to all, and those who know them can make complex circuit devices using this method within a small amount of time.

Remote Controlled DVD-DTH-TV Switch

In this article I will be discussing the details of the Modular Design and Assembly technique with the help of a reference project. I shall give you an introduction to the reference project first up. The project is named as “Remote Controlled DVD-DTH-TV Switch”.

I have an old CRT TV at my home. I have recently taken a DTH connection . As a result it requires two remote controls to operate the TV, one for the TV itself and the other for the set-top box. My parents who were not very used to the technology actually don’t know how to properly operate the TV remote itself. Two remote instead of single one made them really unhappy. But there was another big issue; the TV has only one video input pin and two video channels, one for the video and the other for the cable TV. When we were using the local cable connection the video input pin has DVD player connected to it. Whenever my parents want to watch the VCD of family events like marriage, house warming etc. they simply put the VCD in the player and change the video channel. But the introduction of the set-top box for the DTH connection made the task a lot more complex for them.

When they wish to watch the VCD, first they need to unplug the set-top box audio video pins from the TV and plug in the respective pins from the DVD player. They are really afraid to do this simple task and as a result they used to wait for me to come home which happens only once in a month. 

The government has made the digitalization of cable network mandatory from the November 2012. Now every cable TV customer needs a set-top box with their TV set. I realized that a large number of people are going to face the same issue. If I could find a solution for the same it would be helpful for others as well. This thought made me to think of a project like this.

I don’t claim that I’ve found a perfect solution or a new invention or even anything great, but I was able to find a solution and made a prototype of the same in a very short period of time, two days precisely and we are using it successfully with our TV set. In this article I will share the details of  how I designed  the circuit for this project from some of the basic known circuits and how I did  the hardware based on the same circuit along with the testing.

I decided to keep it simple and don’t want to use any microcontrollers or complex ICs. My immediate thought was to make a remote controlled device which can switch the audio video connectors of the DVD player and the set-top box. When I shared this idea with my parents they told me that it won’t be useful for them since they don’t like to memorize one more key in the remote control. I told them not to worry press any key in the remote for a period of time greater than normally does for changing channel or varying volume and the device will switch from DTH set-top box to DVD or back each time. That’s all about the idea I have in mind and now the question is how to make such a device.

We follow the steps listed in the Electronic Modular Assembly and Design and let us see how we can develop a working prototype using the method.

Steps to develop Modular Design

Step: 1 Analyze the required features

This is the first step and we must collect all the possible information regarding the project and most importantly we should have a good idea about the required features that the device should have.

I started analyzing  a way and came up the solution where I can use a remotely operated toggle switch h between the audio video cables of the DVD player and the DTH set-top box.  The device should receive IR light from the remote control for a fixed period of time and when the period exceeds it should automatically toggle the switch.

The basic idea is simply represented in the following figure.

Block Diagram of Remote Controlled DVD-DTH-TV Swich Project

Fig. 3: Block Diagram of Remote Controlled DVD-DTH-TV Switch Project

Let us think about the basic requirements for the device.

A)    We need a kind of switch which can be operated in such a way that I can have signal from either DVD player or set-top box at a time. We need at least three of them so that we can switch video, speaker left and speaker right.

B)    We need IR sensing circuit to receive signal from the TV remote.

C)  We need a timer circuit which fires after getting output from the IR sensor for a predefined period of time.  

D)  Also a circuit which toggles the switch of kind mentioned in 1) each and every time the timer fires.

Step: 2 Required features into functional blocks

At the end of this step we represent the required features into corresponding functional blocks. Each of the above requirements is actually functionalities which operate together to make our device work and hence we can consider them as the basic functional blocks of the device. Let’s represent them with a block diagram as shown below.

A)  Switch block

 Basic Functional Blocks In Project

Fig. 4: Basic Functional Blocks in Project

 B)  IR sensing block

 IR Sensing Block

Fig. 5: IR Sensing Block

 C) Timer block

Timer Block

Fig. 6: Timer Block

D) Switch operating block

 Switch Operator Block

Fig. 7: Switch Operator Block

Let’s go deep into the requirements of individual blocks and try to develop them into more meaningful blocks. Take the block A, which is the switch block. As we have already understood that we need three of them and each switch should have three conductors running into it, one from the DVD player, one from the set top box and another one from the TV itself. We can draw the diagram for such a kind of switch as shown below.

Representing Method Of Switching The Signals

Fig. 8: Representing Method Of Switching The Signals

We can easily make out from the above figure that the kind of switch should be SPDT (Single Pole Dual Throw). There is only a single pole (the lead in which the movable contact is attached) in the switch where the connector to the TV is connected. This pole can make contact with either of the other two leads in which the connector from DVD player and connector from the set top box is attached.

We need three such switches, one for the VIDEO, one for SPEAKER LEFT and another one for the SPEAKER RIGHT. Let’s represent them as shown in the following figure.

Representing Method Of Switching Audio Video Signals

Fig. 9: Representing Method Of Switching Audio Video Signals

Now the important thing, in the above diagrams we have seen the symbol for manually operated mechanical switches, but in our project we need automatic, remotely controlled switches. We already have a remote controlled unit which somehow should operate these switches. In short we need electrically operated mechanical switches. We are very familiar with a kind of electrically operated mechanical switches which are nothing but RELAYS.

Block Diagram Of Relay

                                                                              Fig. 10: Block Diagram of Relay

A SPDT relay has an electromagnetic coil inside it and a SPDT switch like mechanical arrangement with the pole will be in contact with one of the other two leads. The default lead with which the pole is in contact is called Normally Closed (NC) and we call the other lead as Normally Open (NO). The pole inside the relay will shift its contact only when current flows through the coil. When we energize the coil by applying power to it, the coil become an electromagnet and attract the pole contact towards it and it starts moving toward the coil, but before reaching the coil surface it will end up in contact with the NO contact metal. The symbolic representation of a relay is shown below.

 Symbolic Representation Of Relay

Fig. 11: Symbolic Representation Of Relay

The relay can act as an electrically operated toggle switch which can be electrically operated by applying voltage to its coil leads. We can apply this knowledge in our project since we are looking for an electrically operated toggle switch. Now it is easy for us to redraw the diagram previous diagram in which we’ve used the symbol of the manually operated mechanical switch with the symbol of electrically operated mechanical switch or simply the relay.

If you are an experienced guy you might not be pleased with the idea of using the relays for switching the high frequency video signals but the fact is that I could hardly find any signal degradation from the TV screen due to this.

 Represention Of Audio Video Switching Using Relay

Fig. 12: Representation Of Audio Video Switching Using Relay

From this point onwards I think it would be comfortable to represent the above diagram with a simple block diagram with the name RELAY as shown below.

Relay

Fig. 13: Relay Block

So far we were developing the Block A) Switch Block an during the process we found that the relays are suitable candidate for our switching purpose and we’ve planned how to use them for switching the audio and video signals and represented them using a diagram. Also we’ve integrated the three individual blocks inside Block A) named SWITCH1, SWITCH2 and SWITCH3 into a single functional block named RELAYS.

Relay Block

Fig. 14: Functional Block Of Relay

Now let’s take the block B) IR sensing block.

The TV remote transmit IR rays which should be sense by the IR sensing block. But the IR rays from the TV remote are not the only IR rays exist in the environment. There is always an existence of significant amount of IR light especially during the day time. Now the question is how you will identify the IR light that comes from the TV remote from other IR rays.

The TV remote transmit IR light in the form of pulse train with a fixed frequency. The IR sensor should be designed in such a way that it detects only IR rays of that particular frequency. Using this method we can easily identify the IR transmitted from the TV remote among other IR rays. There are lot of IC designed as IR sensor and can be operated along with the TV remote. The difficulty is that they are costly and not easily available in local electronic shops. As I mentioned previously in this article I wish to keep things simple. We are not going to use any complex or costly IC but a simple IR photodiode instead of them.

Timing Diagram and Parameters

The IR photodiode can sense IR rays falling on a wide frequency range and as a result it will produce an output for all the IR rays exist in the room conditions. We must use a technique to identify the IR rays emitted by the TV remote. The TV remote emits IR rays at a very high intensity compared to other existing IR rays in the room conditions. The output of the photo detector should be comparatively high for the IR light from the TV remote. With this knowledge if we implement a level detector at the output of the IR photodiode we can easily detect the IR rays transmitted by the TV remote from other IR rays. The level detector can also be referred to as comparator.

A level detector produces an output whenever an input signal’s voltage exceeds a particular limit. Here we should set the level detector in such a way that it produces an output only when the IR light from the TV remote falls on the IR photodiode. The working of a level detector or a comparator can be easily understood from the following diagram.

 Voltage Levels At Photo Detector And Comparator

         Fig. 15: Voltage Levels At Photo Detector And Comparator

We can read from the above diagram that whenever the IR light from the TV remote adds to the normal IR light falls on the photo detector the output of the photo detector exceeds a particular level and as a result the comparator output goes high and similarly whenever there is no IR light from the TV remote the output of the photo detector falls below the detection level and the comparator produces no output.  

Since the IR light transmitting from the TV remote is in the form of train of short period light pulses, the output from the comparator block is also in the form of short period pulses. The output from the photodiode and the comparator is shown in the following diagram.

 Output of Photodetector And Comparator

Fig. 16: Output of Photodetector and Comparator

Now we know that we must use an IR sensor along with a level detector. Also we have decided to use a simple IR photodiode as sensor and for the level detector we can use any simplest comparator IC. In case of block A) we have combined three blocks into a single one but here we are going to split a single block into two one named IR PHOTODIODE and other as COMPARATOR. Let’s represent them as shown in the following diagram.

 IR Photodiode And Comparator Block

            Fig. 17: IR Photodiode And Comparator Block

We will discuss more about the IR photodiode and comparator when we come to it later for the actual circuit designing. For the time being, let us move on to the next block, Block C) TIMER.

Think about the purpose of a timer and the kind of timing it should generate. We need a timer block with the following features

It should have a default output

The timer should start whenever there is an input

The timer should produce an output whenever the input to the timer holds itself beyond a particular time period.

 We can represent such a timer using the following diagram

Timing Diagram Of Timer Block

Fig. 18: Timing Diagram Of Timer Block

The details of the parameters shown in the above timing diagram are mentioned in the following table.

INPUT

The voltage at the input of the timer block

OUTPUT

The voltage at the output of the timer block

TIME

 

 

 

The axis shows the increasing time at which the above voltage

values are observed

T0

 

 

 

The time period from the start of observation till the time the INPUT

become high

T

 

 

 

The time period till the OUTPUT become high after the INPUT

become high and remains high

T1

The time period up to which both the INPUT and OUTPUT remains  high

T2

The time period till the input become high again

 Table: timer block waveform parameters

The different instants in the timing diagram are marked with numbers 0, 1 up to 4. The significance of these points in the time axis are mentioned in the following table:

0

No input and no output

1

Input become high and the timer starts its timing

2

The timer reached its time period and generates output high

3

The input falls and the output of the timer also becomes low

4

Input is low and the output remains low

 

Table: timer block timing diagram details

We can simply state that the OUTPUT become high after a period of time from which the INPUT become high and remains high. The period of time required for the OUTPUT to become high is called the time period of this particular timer and is represented as ‘T’ in the above timing diagram. 

One more thing to be noted is that once the OUTPUT becomes high it will remain high as long as the INPUT remains high. Whenever the input voltage falls the output falls as well.

Those who are familiar with electronics, circuits and timing diagrams might have already seen this kind of a timing diagram. There is an electronic circuit which produces this kind of timing diagram, which is nothing but a mono-stable multi-vibrator.

The mono-stable multi-vibrator has a single stable state and a quasi-stable state at the output. Once the mono-stable is enabled its output will be in the quasi-stable state. The quasi-stable state is the state in which the output will remain in the state (either high or low) only for a fixed time period and once the time period exceeds the output will automatically come to its final stable state. The common waveform for a mono-stable multi-vibrator is shown in the following diagram.

Mono-Stable Multi-Vibrator Output Waveform

Fig. 19: Mono-Stable Multi-Vibrator Output Waveform

In the above figure the time period for which the mono-stable multi-vibrator remained in its quasi-stable state is represented by ‘T’ and it is the time period of the mono-stable.

Note that in the above figure after the quasi-stable state the output of the mono-stable changes from high to low. Since the output of the mono-stable is high during its active period ‘T’ we call such a mono-stable multi-vibrator as “ACTIVE HIGH”. In our design we expect the output of the mono-stable to be maintained low during its active period and hence our mono-stable should be “ACTIVE LOW” 

Another important thing is that if you take a look at the timing diagram again the INPUT once become high it can remain high for a considerable period of time and hence triggering a high at the OUTPUT. We are expecting this INPUT from the output of the IR photodiode and comparator blocks. But as we have discussed before the IR light transmitting from the TV remote is in the form of train of short period light pulses and hence the output from the comparator block is also in the form of short period pulses. As a result the INPUT cannot remain high for a time required for the OUTPUT to trigger. The output from the comparator block is shown in the following diagram.

 Output Pulses From Comparator

Fig. 20: Output Pulses From Comparator

The details of the parameters of the above waveform are mentioned in the following table

COMPARATOR OUTPUT

The output voltage from the comparator

TIME

The increasing time at which COMPARATOR OUTPUT is observed

Ta

The time period for which the IR light from TV remote fall on the photodiode and the COMPARATOR OUTPUT is high, simply time period of a pulse

Tb

The time period for which there is no IR light from the TV remote and the COMPARATOR OUTPUT is low, simply time period between two pulses

Table: the comparator output waveform parameters

If you compare the previous two timing diagrams it can be noted that the time period of the pulse generated at the output of the comparator block is very small than the time period required for the triggering of the mono-stable multi-vibrator. From the explanation of previous two timing diagrams, simply

Ta << T                       (Equation: 1)

Now we must think about a mechanism by which we can extend the time period of Ta. This mechanism can be implemented as a new functional block between the comparator and the mono-stable multi-vibrator. The time period of Ta should be extended in such a way that the output remains high at least till the beginning of the next pulse. The first pulse will trigger the output of the block high and the following pulses will keep the output at high. The idea can be explained with the help of the following diagram.

 Timing Diagram To Extend Pulses

Fig. 21: Timing Diagram To Extend Pulses

The explanation of the above timing diagram is as follows.  The details of the parameters are mentioned in the following table

COMPARATOR OUTPUT

The output voltage from the comparator

TIME

The increasing time at which COMPARATOR OUTPUT is observed

Ta

The time period for which the IR light from TV remote fall on the photodiode and the COMPARATOR OUTPUT is high, simply time period of a pulse

Tb

The time period for which there is no IR light from the TV remote and the COMPARATOR OUTPUT is low, simply time period between two pulses

EXTENDED OUTPUT

The output of the block which is used to produce an extended output corresponding to the COMPARATOR INPUT

Tex

The time period from the end of a pulse till the output of the new functional block falls low. Simply, it is the time period for which the pulse is extended

Table: extended waveform parameters

The significant instants in the timing diagram and their details are mentioned in the following table

1

Pulse A is generated at the COMPARATOR OUTPUT and COMPARATOR OUTPUT is high and it will hold high till ‘2’

EXTENDED OUTPUT become high

2

COMPARATOR OUTPUT falls and it will remain low till the next pulse is generated

EXTENDED OUTPUT is maintained high even though the input falls low. It is supposed to be maintained high till the end of the extension period ‘Tex’ which can extend the pulse up to ‘3’

3

At this point the extension period expires and the output is supposed to fall, but it is maintained high since the input is high. The high input is due to the pulse B generated at the COMPARATOR OUTPUT

4

COMPARATOR OUTPUT falls and once again the EXTENDED OUTPUT is maintained high due to the extension period ‘Tex’ which can extend the pulse up to ‘5’

5

At this point the extension period expires and the output is supposed to fall, but it is maintained high since the input is high. The high input is due to the pulse C generated at the COMPARATOR OUTPUT

6

COMPARATOR OUTPUT falls and once again the EXTENDED OUTPUT is maintained high due to the extension period ‘Tex’ which can extend the pulse up to ‘7’

7

At this point the extension period expires and the output is supposed to fall, but it is maintained high since the input is high. The high input is due to the pulse D generated at the COMPARATOR OUTPUT

8

COMPARATOR OUTPUT falls and once again the EXTENDED OUTPUT is maintained high due to the extension period ‘Tex’ which can extend the pulse up to ‘9’

9

At this point the extension period expires and the output is supposed to fall, and it does fall since the pulse ‘E’ is missing and the COMPARATOR OUTPUT is low.

10

The COMPARATOR OUTPUT and the EXTENDED OUTPUT both remain low.

Table: pulse extending timing diagram details

Whenever the COMPARATOR OUTPUT becomes high the EXTENDED OUTPUT also becomes high. But the EXTENDED OUTPUT falls low only after a period of time ‘Tex’ after the COMPARATOR OUTPUT became low.

In the above timing diagram you can see that the EXTENEDED OUTPUT is maintained high right from the pulse named A and up to a certain period of time after pulse named C. This is because after the falling end of the pulse and a time period ‘Tex’ the EXTENDED OUTPUT is about to fall, but it cannot fall since the COMPARATOR OUTPUT has already generated another pulse keeping the COMPARATOR OUTPUT high. As you can see after the pulse C the comparator is not generating any pulse, hence the COMPARATOR OUTPUT holds low at the tentative time for the pulse D and the EXTENDED OUTPUT eventually falls.

The advantage of such a mechanism is that when we have a continuous stream of short period pulses we have a stable high output. We can use this mechanism to convert the pulsed output from the comparator block to a continuous output so that the mono-stable can perform its uninterrupted timing and trigger its output.

This can be realized by using another mono-stable multi-vibrator in between the comparator and the previously discussed mono-stable multi-vibrator. Now we have two mono-stables in the design. We have defined the timing period of the first mono-stable as ‘T’ and now from the previous timing diagram we can represent the timing period of the second mono-stable as the ‘Tex’ itself. The only thing that must be taken care of is that the ‘Tex’ should always larger than the ‘Tb’; take a look at the timing diagram.

Tex > Tb                       (Equation: 2)

 We already know that the ‘Ta’ and ‘Tb’ are very small values since they are generated due to the high speed IR pulses from the TV remote. The value of ‘Tex’ is required to be just above the ‘Tb’ and hence the ‘Tex’ is also small. But for our purpose the timing period of initially discussed mono-stable should be a comparatively large value. Thus we have a comparison of the timing period of the two mono-stables and can be represented as given below.

T >> Tex                     (Equation: 3)

Where;

‘T’ is the timing period of the first mono-stable and

‘Tex’ is the timing period of the second mono-stable

 

The relation simply means that the timing period of the first mono-stable is much larger than the timing period of the second mono-stable.  With this knowledge we can name the first mono-stable as ‘mono-stable large’ and second mono-stable as the ‘mono-stable small’. 

Small & Large Monostable

Now come back to the Block C) TIMER which we were trying to develop and came to the conclusion that the functional block can be realized by using two mono-stable multi-vibrators of different timing periods. As we did in the case of the Block B) just split the TIMER block into two namely MONOSTABLE SMALL and MONOSTABLE LARGE. Let’s redraw the Block C) TIMER block as shown in the following diagram.

 Timer Block Redrawn As Monostable Large And Small

 Fig. 22: Timer Block Redrawn as Monostable Large and Small

Now we have one more block left to develop and it is Block D) Switch operating block. The purpose of this block is to operate the Switch block which we’ve developed into the functional block named RELAYS. Simply the Block D) should have all the necessary features to drive the RELAY block properly. This block is supposed to be taking its input from the MONOSTABLE LARGE block so that whenever the mono-stable fires the bock should toggle the relays in the RELAY block.

The basic requirement for this particular block is mentioned as follows;

Each time the output of the MONOSTABLE LARGE block become high the Switch operating block should toggle its output (If the output was low change to high or if the output was high change to low).

The idea can be represented using the following diagram.

 Output Waveform at Switch Operating Block and Monostable Large Block

Fig. 23: Output Waveform At Switch Operating Block and Monostable Large Block

The details of the significant points in the above timing diagram is mentioned in the following table

1

Pulse A is generated at the MONOSTABLE LARGE OUTPUT. The MONOSTABLE LARGE OUTPUT will remain high till the end of the pulse A occurs at ‘2’.

SWITCH OPERATING BLOCK OUTPUT becomes high due to the low to high transition of MONOSTABLE LARGE OUTPUT, and it will remain high till ‘3’ where the MONOSTABLE LARGE OUTPUT makes a transition from low to high again.

2

MONOSTABLE LARGE OUTPUT falls but it has no effect on the SWITCH OPERATING BLOCK OUTPUT. SWITCH OPERATING BLOCK OUTPUT is maintained high.

3

The MONOSTABLE LARGE OUTPUT makes a transition from low to high again due to the generation of pulse B and the SWITCH OPERATING BLOCK OUTPUT becomes low. The MONOSTABLE LARGE OUTPUT will remain high till the end of the pulse B occurs at ‘4’. SWITCH OPERATING BLOCK OUTPUT will remain low till ‘5’ where the MONOSTABLE LARGE OUTPUT makes a transition from low to high again.

4

MONOSTABLE LARGE OUTPUT falls but it has no effect on the SWITCH OPERATING BLOCK OUTPUT. SWITCH OPERATING BLOCK OUTPUT is maintained low.

5

Pulse C is generated at the MONOSTABLE LARGE OUTPUT. The MONOSTABLE LARGE OUTPUT will remain high till the end of the pulse C occurs at ‘6’.

SWITCH OPERATING BLOCK OUTPUT becomes high due to the low to high transition of MONOSTABLE LARGE OUTPUT, and it will remain high till ‘7’ where the MONOSTABLE LARGE OUTPUT makes a transition from low to high again.

6

MONOSTABLE LARGE OUTPUT falls but it has no effect on the SWITCH OPERATING BLOCK OUTPUT. SWITCH OPERATING BLOCK OUTPUT is maintained high.

7

The MONOSTABLE LARGE OUTPUT makes a transition from low to high again due to the generation of pulse D and the SWITCH OPERATING BLOCK OUTPUT becomes low. The MONOSTABLE LARGE OUTPUT will remain high till the end of the pulse D occurs at ‘8’. SWITCH OPERATING BLOCK OUTPUT will remain low till the MONOSTABLE LARGE OUTPUT again makes a transition from low to high.

8

MONOSTABLE LARGE OUTPUT falls but it has no effect on the SWITCH OPERATING BLOCK OUTPUT. SWITCH OPERATING BLOCK OUTPUT is maintained low.

Table: switch operating block timing diagram details

From the above diagram we can read that the switch operating block output was low initially and as soon as the 1st pulse generated at the output of the mono-stable large, the output of the switch operating block become high. The output of the switch operating block is maintained high till the 2nd pulse arrives. As soon as the 2nd pulse is generated at the mono-stable large output the output of the switch operating block again changes its state, this time from the high to low. The output is maintained at low itself till the 3rd pulse is generated at the output of the mono-stable large. On the arrival of the 3rd pulse the switch operating block output changes its state from low to high. Thus the change of output state occurs at the switch operating block each and every time a pulse generated at the mono-stable large output. Note that the time period of the mono-stable large output pulse has nothing to do with the output of the switch operating block. The pulse generated by the mono-stable is simply used as triggering pulses for the toggling of the switch operating block’s output.

A device with the previously given waveform and the above mentioned peculiarities can be easily realized by using a bi-stable multi-vibrator. A bi-stable multi-vibrator is an electronic circuit with two stable output states. It can remain in any of its stable state for a very long time until it is triggered for a change of state. A generic output waveform for the bi-stable multi-vibrator with triggering pulses is shown in the following diagram.

Waveform Of Bi-Stable Multi-vibrator With Triggering Pulses

           Fig. 24: Waveform Of Bi-Stable Multi-Vibrator With Triggering Pulses

We have thus assumed a bi-stable multi-vibrator in our design for the switch operating block. But the experienced guys know that the bi-stable cannot simply operate the switches since the switches are realized using relays. The relays require comparatively high amount of current and hence it is safe to operate them only with the suitable driving circuit.

A relay driving device is a circuit which can provide comparatively large amount of current so that the relays can operate normally and without affecting other circuits. They take low current input voltage and convert them to high current output voltage. Simply the relay driving block performs a current amplification. We can design the relay driving block in such a way that it can do small amount of voltage amplification as well.

We were discussing about how to develop the Block D) switch operating block and we’ve learned that the block should have two key functionalities, a bi-stable multi-vibrator and a relay driving circuit. Let’s represent the entire block with two separate blocks one for the bi-stable function and the other for relay driving function. We can call the first block simply as BI-STABLE (D1) and the second one as DRIVER (D2).

Block Representation Of Relay Driving Circuit As Switch As Bi-Stable And Driver Block

     Fig. 25: Block Representation of Relay Driving Circuit as Switch as Bi-Stable And Driver Block

With that we have successfully represented the required features for the working of the device with their corresponding functional blocks. It can give a quick idea about what all circuits to be designed for the project. Now it is time to make this individual blocks to a connected complete block diagram so that we can have an idea about how to design the circuit for the device.