Most of us have heard of CCTV security systems at some point, however there are those who are not quite certain as to the benefits, or for that matter, what CCTV actually stands for. CCTV means "closed circuit television" which, as the name implies, is a camera that has been hooked up to a TV screen on a closed circuit type system. Today the most commonly used CCTV camera is in black and white and for several reasons. The first is cost, as they are much cheaper to produce. Secondly, maintenance costs are much lower than the colour versions, and finally the black and white system transmits a much sharper and clearer image.

The very first systems were installed way back in the early 1940's in New York. They are now used worldwide for monitoring; in fact just about every major retail business is utilizing this technology. Other institutions using CCTV will of course include just about every bank in the world, for obvious reasons. They are mainly used to deter crime and also employee theft. However, in many cases they also record crimes in progress. CCTV technology has put many a crook behind bars as this type of evidence is hard to refute.

Some companies that are quite large use these systems to observe what is going on inside their organization and for monitoring staff members. Many also install the cameras outside the premises as additional security. But it is not just businesses that have CCTV installed. They are also used by homeowners as protection, as well as city governments and in some instances, by law enforcement. Many sting operations have been carried out via this technology, as many types of these cameras have become very tiny and easy to hide.

There are also models that will work regardless of whether it is day or night, and with today's technology many are operated as wireless devices. Because of the fact that you can get miniature versions, they are commonly used to survey and monitor care workers or babysitters who look after children or the elderly. Some daycare centers utilize CCTV and give parents the ability to access the rooms via the internet.

Another really big advantage over other types of security systems is the fact that CCTV is live coverage, yet at the same time it can be recording everything, depending on the type that is used. In the UK for example, this technology and its usage has greatly proliferated over the past ten years. In fact governments in UK have the distinction of being one of the biggest users of CCTV surveillance in the world. Studies have clearly shown that they are a deterrent to criminal activity.

We have all seen the live shows of high speed chases involving law enforcement on television from the point of view of the officer involved. This is a perfect example of CCTV at work, steadfastly recording in real time. Another prime example would be a casino, the cameras are commonly referred to as the "eye in the sky". And for good reason, as these units can focus in and observe even the slightest movements that could be deemed as suspicious. Many a casino cheat has been busted thinking that they were going to outsmart the cameras that stealthily are beaming down on them and taping everything.

As for home usage, you can purchase units that are very reasonably priced and many come with outstanding features. Depending on what type of setup you want, there are systems that can monitor one room or an entire household. Some higher end models give you the ability to view in very low light, different zooms and audio. If desired you can also get CCTV setup as a network of IP cameras, broadcasting over the internet in varying resolutions dependent on the model and price range.

This technology is here to stay, and with crime rates the way they are we will see more of these installed in public. Many taxis, buses, civic centers, public and private parking lots, airports, hospitals and many others are taking advantage of the protection these CCTV cameras offer. It should also be noted that CCTV is used more frequently to monitor school property so as to discourage unruly or violent behavior. Because of this technology, students are less likely to commit acts that are deemed against the rules for fear of being recorded.

High spec CCTV images are sharp and this type of advanced security is there to give governments, businesses, homeowners and even private citizens the benefits of full camera coverage 24 hours a day, 7 days a week, in the fight against crime, to protect properties and ensure public safety. From the smallest inexpensive to the most sophisticated, CCTV security system is available to suit every needs.
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Security is your top issues within this living. A new Dvr (Digital dvr) home basic safety photographic camera technique is employed to give protection. A dvr (Dvr) can be any gadget which allows analogue signals within the safety alarm photographic camera, changes that in electric formatting, along using suppliers a item within this hard drive (or in other sorts connected with marketing).

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A new Digital recording device security technique information this recording indicators transmitted which contains a Closed-circuit television digital camera. Some sort of lots of these signs will be next a new good choice with regard to data as well as diagnosis methods.

Dvr video movie security cameras monitor digital digital camera signals which includes a hard disk and also maybe a Digital recording device security photographic camera recorder. Digital recording device training video movie protection cameras own a large amount of characteristics like movement recognition, evening perspective, remote computer repair gain access to, simplicity, although any very important an example may be very simple navigation. This characteristic allows you to be able to bounce to be able to any type regarding training video without rewinding while well as forwarding. Around the actual past movie security cameras accustomed to transfer analog signs with your movie recorded argument recorder (VCR).

Digital video camera creating along with hard disk undoubtedly provides advantages more than analog signals. A new Digital recording device home safety measures digicam method features pursuing gains: these guys Throughout Dvr home stability digicam systems, creating good quality is great. these guys Producing may be accomplished around your hard disk consequently you don't possess to buy plus modify your mp3 typically while hdd can readily retail store far more files in comparison to be able to your record. before i forget- Intelligent creating amazing .

Permits effortless homework through your movie based with period of time, occurrence as well while working day. Using a until now reviewed positive aspects, a Safety measures Dvr technique includes a several pull back also. This specific bring buttocks tend to be: before i forget- As any result associated with high facts compresion amount, electronic video burns lots of room pertaining to the actual hard disk drive in addition to then the hard disk is merely chock-full. There are couple of Dvr video security cameras systems, utilizing motion discovery function to resolve that obstacle a great level. Motions detectors notify you got the item for determining the results compresion fee through the photo (a picture with great data compresion is required so when with decreased data compresion).
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This application of the Basic Serial Interface circuit illustrats how easy it is to develop a sophisticated home security system.


The security system application and program offers a simple demonstration of the BASIC Serial Interface. By adding only a few door and window switches, a transistor, a siren, (see schematic) and a few lines of BASIC program (see program listing) the interface can become a multi-function security system. Please note, however, that it is a "barebones" program. It is left to the reader to fancy it up to their liking.

Normally closed door and window switches can be attached to the interface "in" ports as shown in the schematic(all unused ports should be grounded). In this configuration with all the switches closed the "in" port is held "low". When any switch opens the port goes "high". The program recognizes this as an alarm condition for the zone associated with that port. If the program "detects" a high on "in" port number 1 it will delay sounding the alarm for a user defined length of time. This is done to allow the owner time to enter the secured area and reset the alarm before the siren is activated. If a "high" condition is detected on any of the other ports, 2 to 7, an alarm will be sounded immediately.

The alarm is sounded by bringing "out" port number 1 high. Connected to "out" port 1 is a NPN transistor which switches a 12 volt supply to a security siren or bell (figure 3) . The.alarm remains on until the system is reset or it reaches it's time out period.

In order for the BSI to transmit the status of it's "in" ports Data Strobe (pin. 23 of IC1) must be toggled. This toggling of the Data Strobe is done by program control. In this application Data Strobe is connected to "out" port 8 by a jumper. In order to trigger a transmission of the port conditions the program turns "out" port8 "on" then "off". This causes IC1 to transmit the status of it's "in" ports.

20 '
40 '
50 ' setup
60 KEY OFF:CLS:CLOSE'......................................... turn key off, clear screen, close
70 OPEN "COM1:1200,N,8,2" AS #1' .......................all files, open the serial port
80 PRINT#1,CHR$(NUL);' com port #1, and transmit "0".
90 GOTO 310
100 '
110 FOR X=1 TO 8'.................................................. Subroutine to convert decimal number
120 B=C MOD 2:C=INT(C/2):R(X)=B'...................... received from the UART to binary
130 NEXT X'............................................................. and set array variables to represent
140 RETURN'.............................................................UART port conditions,R(1) to R(8)
150 REM
160 IF T(HP)=1 THEN 210'.......................................Subroutine to turn one UART port on
170 FOR X=1 TO 8'..................................................without changing the condition of
180 IF HP=X THEN OT=OT+2^(X-1):T(X)=1'...............any other UART port.---
190 NEXT X
200 PRINT #1,CHR$(OT);
220 '
230 IF T(HP)=0 THEN 280'........................................Subroutine to turn one UART port off
240 FOR X=1 TO 8'...................................................without changing the condition of
250 IF HP=X THEN OT=OT-2^(X-1):T(X)=0'................any other port.---
260 NEXT X
270 PRINT #1,CHR$(OT);
290 '********************* SECURITY SYSTEM MAIN PROGRAM *******
300 '
310 PRINT" Security System Program
320 '
330 PRINT:PRINT"Note:'OUT' port 8 of the UART (pin 5) must be connected to Data Strobe (pin 23)before running this program.":PRINT
360 '
370 CLS:PRINT#1,CHR$(128);'.................................clear screen and turn UART port 8 on
380 PRINT "Ctrl E to reset"
390 HP=8 :GOSUB 220:HP=8:GOSUB 150'................Ask UART for 'in' port status.
400 IF LOC(1)=0 THEN 470'.......................................If transmission not received,skip.
410 IN$=INPUT$(1,#1):C=ASC(IN$):GOSUB 100'... read transmission and convert to
420 FOR X=1 TO 8'......................................................binary,assign each bit to array R(X)
430 LOCATE X+9,10
440 IF R(X)=1 THEN PRINT X;" ALARM !!!!!"' Print UART port status conditions
450 IF R(X)=0 THEN PRINT X;" ZONE SECURE"' as either 'alarm' or 'secure'
460 NEXT X
470 IF R(1)=1 THEN TIME=TIME+'.............................If zone 1 is high start delay time.
480 IF TIME=DELAY THEN ALARM=1'.......................if delay time is up set alarm.
490 FOR X=2 TO 8
500 IF R(X)=1 THEN ALARM=1'..................................if any zone,2-8,is high,set alarm.
510 NEXT X
520 IF ALARM=1 THEN HP=1:GOSUB 150'............... if alarm set,turn port 1 on.
530 IF ALARM=1 THEN RESETT=RESETT+1'............ if alarm is set start timeout.
540 IF RESETT=TIMEOUT THEN GOTO 580'............ If timeout is up then shutdown.
550 A$=INKEY$:IF A$="" THEN 570'........................ Check to see if Ctrl E was entered,
560 IF ASC(A$)=5 THEN 50'...................................... if it was then reset program.
570 GOTO 390
580 PRINT#1,CHR$(NUL);'.......................................Turn alarm off
600 END

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Extended Abstract

When we think of programmable speech recognition, we think of calling FedEx customer service call center with automated voice recognition response systems. We also think of PC-based speech recognition Dragon NaturallySpeaking. Now we took that a step further. We are talking about speech recognition in a tiny Mega32 microcontroller. We are talking about real-time speech processing which means there is no need to store the samples in an external memory at all. This was made possible by implementing bandpass filters in assembly language with fixed-point format onto the microcontroller. In this filter design, not only the output of the filter is calculated, but its square and accumulation also obtained. Thus much time is saved so that each speech sample can be processed to get its frequency spectrum before next new sample comes. In addition, the analysis of the voice is made using correlation and regression method to compare the voiceprint of different words. These techniques provide stronger ability to recognize the same word. Training procedure is also used to reduce the random changes due to one word is spoken different times. The training procedure can get the more accurate frequency spectrum for one word. The experimental results demonstrate high accuracy for this real-time speech recognition system.


Description of Project

The function of this speech recognition security system is to have a system that will only unlock upon recognizing a voice password spoken by the administrator or password holder.


Firstly, we looked at the speech recognition algorithm to understand the implementation. We then prepared the microphone circuit, and then proceeded to start sampling and generate the digital data for the speech. Once we have the data, we started writing the code based on Tor's speech recognition algorithm. We also wrote the digital filters in assembly code to save the number of cycles necessary for the sampling rate of the speech, which is at 4K/second. Afterwards, we analyzed the output of the filters to recognize which word was spoken. Finally, we added an LCD for better user interface to signal if the password spoken is correct or not.

High Level Design

Rationale and Sources of Project Idea

We are inspired by the lab 3 where we did a 'Security system'. We would like to add on to that using a speech recognition feature. This eliminates the need to type in a security code. Instead, you just have to speak a password to unlock the system. We are also interested in exploring and implementing the speech recognition algorithm and DSP.

Background Math

What we need to know in this project is how to calculate the frequency to sample speech based on the Nyquist Rate Theorem. Secondly, we also need to know how to calculate filter cutoff frequency to build the high and low pass RC filter for human speech. Thirdly, we need to know how to calculate the gain of differential op-amp. We had to learn about Chebychev filters to determine the cutoff frequencies to build the digital filters for human voice. As for the analysis part of the speech, we need to know how to calculate euclidean, correlation and simple linear regression. Lastly, we need to know how the Fourier Transform works, because we need to understand and analyze the outputs of the digital filters.

Logical Structure

The structure is very simple. The microphone circuit goes to the ADC of the MCU. The digitized sampling of the word is passed through the digital filters (flash programmed onto the MCU). The analysis is done on the MCU as well. Once that is done, the LCD which is connected to the MCU displays if the word spoken matches the password or not.

Hardware/Software Tradeoffs

The software tradeoff in this project is between the number of filters we can implement and the maximum number of cycles we have to adhere to. The more filters there are, the more accurate the speech recognition will be. However,because each filter takes about 320 cycles and we could not implement more than 2000 cycles, we had to trade off the accuracy of the system and limit the number of filters to 7.

Standards applicable to design

There should be no standards that would affect this project.

Program/Hardware Design

Program Design

Because there is not enough memory (SRAM) on the STK500, we have to deal with speech analysis during each sample interval. The key point of this project is to how to design filters and how to implement them. There are two major difficulties we need to solve: First reduce the running time of each filter in order to get all the finger prints before next new sample comes. So we have to use fixed-point algorithm. Secondly, set the reasonable cutoff rate for each filter and number of stages of the filters.
  1. Speech spectrum analysis

    Generally the human speech spectrum is less than 4000Hz. According to Nyquist theory, the minimum sampling rate for speech should be 8000samples/second. Due to our system is voice-controlled safety system; it is very helpful to analyze the speaker's voice before our actual design.

    Our design is based on the recorder program installed in Windows XP and FFT function in Matlab. After we speak one word, the recorder program will store the word in a .wav file. Notice this file is sampled at 16000 samples/second, 16bit/sample, so we need to convert it into 8000samples/second, 8bits/sample. The whole analysis procedure is as the following figure.

    The following figures are tester's "hello" and "left" frequency spectrum.
    Fig.2 The frequency spectrum of "HELLO" Fig.3 The frequency spectrum of "LEFT"

    From the above analysis result, we select the sampling rate in our system is 4000sample/second, 8bits/sample. The cutoff frequency for LPF and HPT is 50Hz, 1500Hz respectively. In order to get the accurate fingerprint of the code, we use seven filters, their working range are:

    • LPF: [0-50Hz]
    • BPF_1: [50-350Hz]
    • BPF_2: [350-500Hz]
    • BPF_3: [500-750Hz]
    • BPF_4: [750-1000Hz]
    • BPF_5: [1000-1500Hz]
    • HPF: [> 1500Hz]

  2. Filter Design

    From previous analysis, we know the frequency range of each filter. So first we use Matlab to generate their coefficients. Here we use ChebychevII filter.

    • Fs=4000; %Hz
    • Fnaq=Fs/2; % Nyquist
    • [B0, A0]=cheby2 (2, 20, f0); % LPF
    • [B6, A6]=cheby2 (2,20, f6, 'high'); % HPF
    • [B1, A1]=cheby2 (2, 20, [f0 f1]); % BPF

    For LPF and HPF, we just use second order filter. For BPF filters, we use fourth-order filter. In implementation, the fourth-order filter actually is cascaded by two second-order filters. The coefficients of these two second-order filters are obtained by the following Matlab command:

    [sos1, g1]=ft2sos (B1, A1,'up', 'inf');

    For the LPF and HPF filter, we take Bruce's sample code as a reference. However, we made a little change. The fingerprint of the speech is the accumulation of the square of the output of each filter. So we combine the calculation the square and accumulation in one filter function. For the fourth-order BPF, we duplicate the second-order filter but using different coefficients. After finishing our code, we tested the filter based on two cases.

    First, using an Impulse sequence to test it and compare the result with Matlab. This case is to test whether our filter correction is correct. Here we used sample impulse sequence xn=[16 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ]

    Second, using source generator to generate different frequency sine waves and send them to the filter. The results are also comparing with Matlab's result. This case is to test whether our filter's frequency setup is correctly or not. The following figure is our test result of the second case. We print the sine wave's fingerprint to the hyper terminal and use Matlab to plot its figure.

    Figure. 4 Fingerprints of sine wave (f=355Hz)

    From the above plots, the output from BPF (350-500Hz) has a maximum value, which exactly matches the testing sine wave (355Hz).

    We also use 800Hz sine wave to test our filter arrays. Figure.5 shows the result which also proves our filter design is correct.

    Below is the block diagram of the filters. 4th order filter is made up of two 2nd order filters cascaded.

    Finally, we tested the running time of our filter. Table1 shows the result.

    The sample interval is 1/4000*16M=4000cycles, which is much longer than processing time of all filters. So our filter design can meet the real time requirement of speech recognition.

  3. Fingerprint analysis

    Each sample of the speech will pass all eight filters and gets its corresponding outputs. The fingerprint of each filter is an accumulation of 250 consecutive outputs square of this filter. Basically, different words has different frequency spectrum, then it has different fingerprint. Same words have same fingerprints. However, even one person speaks the same word for several times, its fingerprints are a little different. So we need to calculation the difference of different words and compare to the difference of same words to test whether system can recognize it.

    Because LPF and HPF have some frequency components which we are not interested, in actual process, we disable these two filters and just use other five filter to analysis the speech.

    The first method we used in this project is to calculate the Euclidian distance. That is the accumulation of the square of each difference. The formula is as following

    Because all codes are written using fixed-point algorithm, the maximum value of integer is only 256. So if we use Euclidian distance, the result will overflow. To solve this problem, we first convert output of ADC, ADCH from the unsigned char to integer and send it to filter without using int2fix () function. That is, the input of the filter is always less than one. There is no overflowing anymore.

    During the test, when we plot the fingerprint of the same word as figure 7 shows, we found their shapes are similar but they have different amplitude. At that time, Euclidian distance will judge them as different words. So here we used correlation to tell the similarities of the same word's fingerprint.

    From mathematics view, the correlation is to detect the linear relationship between two vectors. Suppose Y and X and two vectors, if Y=aX+b, where a and b are constants, we say Y and X are closely correlated.

    Actually, in our project, we combine correlation and Euclidian distance together. The system first detects the correlation of dictionary and testing code. If they are distinct, the system thinks it already recognize the word. If more than two correlation results are close, and then calculate the Euclidian distance of these similar words. Pick up the minimum of the Euclidian distance as its recognition result.


    There are two hardest parts in our speech recognition project. One is for filter design, the other is fingerprints analysis. The shortcoming for filter is its frequency spectrum resolution is coarse and can't tell the difference in its band. So we have to select some distinct words as our codes. FFT is a good candidate for filter design. For fingerprints analysis, because the outputs of each band filter are largely different, so we tried to modify the gain to equalize them. However, the improvement seems little for us.

    Another problem is when a tester spoke the same word, even if there is a tiny difference when he spoke, the fingerprint changed a lot. We didn�t solve this problem until now. But we think if we increase frequency resolutions, maybe it will be helpful.

Hardware Design

The main hardware of this project is the microphone circuit. There are three stages for the microphone analog circuit which is shown in the schematics in the appendix. The sampling frequency will be 4000Hz. It will need a high pass filter, an amplifier and a low pass filter. The first stage, an RC high pass filter uses a 1uF capacitor and a 1K resistor. The cutoff frequency is 1/(2pi*R*C) which is 159.2Hz. It is near 150 Hz, the lower limit of the human voice spectrum. This will also cut off the 60Hz surrounding noise. The next stage of the circuit is the amplifier. A three stage amplifier is needed to obtain gain desired. The gain of each stage is 10K/1K which is 10. Because there are three stages, the total gain is 1000. This is because the input voice is around 0-50mV, and the range of the ADC is 0-5V. Therefore a 1000 gain is necessary. The third stage of the circuit is an RC low pass filter. It uses a 2.2nF capacitor and a 25K resistor. The cutoff frequency of this filter is 1(2pi*R*C) which is 2895.2 Hz. It is near the upper limit of the human voice spectrum.

Another hardware is the LCD circuit is simply connected to the MCU's Port C as output. To switch between recording the password to, the switches 0,1 and 7 on the MCU are used. Switch 0 is for recording the password, up to three codes. Switch 1 is for testing a word against the passwords stored. Switch 7 is for debugging. When pressed, it will output the outputs of the digital filters.

Results of the Design

  1. The dictionary has three words

    In this case, we first record three words and store them in the dictionary. Then record a new word. The system will recognize which word. The following is each word's recognition probability.

  2. The dictionary has five words
  3. The dictionary has eight words
  4. The result when using training

    Actually, we have a big problem during the testing. We found the fingerprint of the same word will change a lot even if his pronunciation changes a little. So tried to record the same word for 20 times and get the average of the fingerprints. But we can't calculate their average value directly because their amplitude is quite different. So we use linear regression method [1][2], i.e., try to normalize the every training sample to equivalent level then get their arithmetic average. The linear regression algorithm is as following

    The best fit line associated with the n points (x1, y1), (x2, y2), . . . , (xn, yn) has the form




Conclusions drawn of the project

The project has not met our expectations fully, as we initially specified that the system would be able to recognize a sentence as a password. But we are more than happy that it is able to recognize a word as the password by more than 80%-90% of the time, depending on the choice of passwords. There is a training procedure that need to be implemented, which is an added feature to increase the accuracy of the security system. However, the system can still be used without training. In this case, there is a maximum of 5 words only.

Currently we can recognize at most eight words if we do the training. But we desperately pick up distinct words, such as [ai],[i:],[u]. We still need to make our system be robust for regular words. In the future, we are going to use SPI interface to store the speech samples to data flash, then use mel-frequency cepstral coefficients (MCC) and Vector quantization (VQ) to process the data. This will be more accurate than using digital filters.

Intellectual Property Considerations and Ethical Considerations

Referring to the IEEE Code of Ethics, we accept full responsibility for the decisions we have made in this project. We believe that the decisions that we have made so far have no potential harm on the public's safety and health, and is consistent with the public's welfare.

In our opinion, the third code, which says that IEEE members have to be honest and realistic in stating claims based on the availabe data, is especially important and applicable to our project. For the results and conclusions of our project, we have declared the accuracy of our speech recognition system. In this aspect, we have been honest and have not 'exaggerated" our numbers to make our report look good. The percentage of accuracies have been charted and calculated using Microsoft Excel and we have strived to make sure the samplings have been done fairly.

Another code of ethics that is highly applicable to our project is the seventh code, which states that members of IEEE should seek, accept and offer criticism of technical work, to acknowldge and correct errors, and to credit properly the contributions of others. With regards to the second part of the code mostly, we have credited the use of two authors whose codes we have used and modified for our project. We have also credited the group whose past final project have inspired and influenced the design of our microphone circuit. We have taken care to ensure their work have been credited rightfully throughout the report. This is also in accordance with intellectual property considerations.

Lastly, the tenth code says to asssist colleagues and co-workers in their professional development. In our case, it is about sharing with another group, Andrew and Chirag, whom we must mention here,Their group is also doing speech recognition, and (but with a robot),in this aspect, we have helped each other a lot. Neither group have been selfish despite the "competition" for grade due to the way the projects are ranked in this course.


Commented Program Listing

Code without flash
Code for training to build flash
Code with flash built in


Microphone Analog Circuit

Hardware Block Diagram

Parts Cost List

  Part                                          Unit          Cost $
  --------                                      ----         --------
Digi_Key Part Number: 423-1019-ND                 1            1.46
Manufacturer Part Number: MD6030NSZ-0
Description: MIC CONDENSER ELECT NC -39+-4DB
Vendor: Knowles Acoustics
STK500                                            1           15.00
White board                                       2           12.00
Mega32                                            1            8.00 
Op amp LMC7111                                    3            0.00
Resistors 10K                                     9            0.00
           1K                                     4            0.00                                  
          25K                                     1            0.00                                   
Capacitor 1 uF                                    1            0.00 
          2.2nF                                   1            0.00
                                             Total cost:     $36.46

List of Specific Tasks

  1. Speech Recognition Design - Xiaowen
  2. Matlab Simulation of Digital Filter for Checking/Comparison - Xiaowen
  3. Hardware Design and Preparation - Shihjia
  4. Digital Filter in Assembly Language - Shihjia
  5. System Integration and Testing - shared


The microphone circuit The STK and MCU
The LCD circuit Both of us working on our project, picture taken by Prof Bruce
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Wireless Home Security

Introduction to PCB Design

 You've 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 circuit 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 your circuit does.

 PCB Packages

 There are many PCB design packages available on the market, a few of which are freeware, shareware, or limited component full versions. Protel is the defacto industry standard package in Australia. Professionals use the expensive high end Windows based packages such as 99SE and DXP. Hobbyists use the excellent freeware DOS based Protel AutoTrax program, which was, once upon a time, the high-end package of choice in Australia. Confusingly, there is now another Windows based package also called AutoTrax EDA. This is in no way related to the Protel software. This article does not focus on the use of any one package, so the information can be applied to almost any PCB package available. There is however, one distinct exception. Using a PCB only package, which does not have schematic capability, greatly limits what you can do with the package in the professional sense.

 The Schematic

 Before you even begin to layout your PCB, you MUST have a complete and accurate schematic diagram. Many people jump straight into the PCB design with nothing more than the circuit in their head, or the schematic drawn on loose post-it notes with no pin numbers and no order. This just isn’t good enough, if you don’t have an accurate schematic then your PCB will most likely end up a mess, and take you twice as long as it should. “Garbage-in, garbage-out” is an often-used quote, and it can apply equally well to PCB design. A PCB design is a manufactured version of your schematic, so it is natural for the PCB design to be influenced by the original schematic. If your schematic is neat, logical and clearly laid out, then it really does make your PCB design job a lot easier. Good practice will have signals flowing from inputs at the left to outputs on the right. With electrically important sections drawn correctly, the way the designer would like them to be laid out on the PCB. Like putting bypass capacitors next to the component they are meant for. Little notes on the schematic that aid in the layout are very useful. For instance, “this pin requires a guard track to signal ground”, makes it clear to the person laying out the board what precautions must be taken. Even if it is you who designed the circuit and drew the schematic, notes not only remind yourself when it comes to laying out the board, but also they are useful for people reviewing the design.

Your schematic really should be drawn with the PCB design in mind.

 Good Grounding


Grounding is fundamental to the operation of many circuits. Good or bad grounding techniques can make or break your design. There are several grounding techniques which are always good practices to incorporate into any design.

·        Use copper, and lots of it. The more copper you have in your ground path, the lower the impedance. This is highly desirable for many electrical reasons. Use polygon fills and planes where possible.

·        Always dedicate one of your planes to ground on multi-layer boards. Make it the layer closest to the top layer.

·        Run separate ground paths for critical parts of your circuit, back to the main filter capacitor(s). This is known as “star” grounding, because the ground tracks all run out from a central point, often looking like a star. In fact, try and do this as matter of course, even if your components aren’t critical. Separate ground lines keep current and noise from one component from affecting other components.

·        If using a ground plane, utilize “split” plane techniques to give effective star grounding.

·        “Stitch” required points straight through to your ground plane, don’t use any more track length than you need.

·        Use multiple vias to decrease your trace impedance to ground.


Good Bypassing


Active components and points in your circuit that draw significant switching current should always be “bypassed”. This is to “smooth” out your power rail going to a particular device. “Bypassing” is using a capacitor across your power rails as physically and electrically close to the desired component or point in your circuit as possible. A typical bypass capacitor value is 100nF, although other values such as 1uF, 10nF and 1nF are often used to bypass different frequencies. You can even have two or three different value capacitors in parallel. When bypassing, you cannot replace multiple capacitors with one single capacitor, it defeats the entire purpose of bypassing! It is not uncommon for a large design to have hundreds of bypass capacitors.

As a general rule, you should use at least one bypass capacitor per IC or other switching component if possible. Common values of bypass capacitors are 100nF for general purpose use, 10nF or 1nF for higher frequencies, and 1uF or 10uF for low frequencies.

Special low Equivalent Series Resistance (ESR) capacitors are sometimes used on critical designs such as switch mode power supplies.

Basic PCB Manufacture


A PCB usually consists of a blank fiberglass substrate (“the board”), which is usually 1.6mm thick. Other common thicknesses are 0.8mm and 2.4mm. There is many types PCB substrate material, but by far the most common is a standard woven epoxy glass material known as FR4. This material has standard known properties, typical values of which are shown in the accompanying table. The most often used parameter is probably the dielectric constant. This figure is important for calculating high-speed transmission line parameters and other effects. An FR4 PCB is made up of glass and resin. Glass has a dielectric constant of approximately 6, and the resin has a dielectric constant of approximately 3. So an FR4 PCB can typically have a figure ranging from under 4, to almost 5. If you need an exact figure you will have to consult with your PCB manufacturer.

Typical FR4 Properties:

Dielectric Constant 3.9 to 4.8

Dielectric Breakdown 39kV/mm

Water Absorption <1.3%

Dissipation Factor 0.022

Thermal Expansion 16-19ppm/degC

Posted by Aqar

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Posted by Aqar

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