July 16

TECHNO TE(A)CH INTRODUCTION: Hello guys, hope you went through enough troubles with the decade counter presented in the last issue. Did you notice, even though going through all the pains of reading the IC’s data sheets the LEDs glowed without any clock signal? This is what we call practical difficulties. So all that which is imagined (written through expressions) in theory is completely not possible in practice. But don’t give up hope the mission of engineers like us is to try and reduce the gap between theory and practice. APPLICATION: The last project that which we saw is not exactly a project because; the circuit did not have any particular use. But this time we are going to meet with an application. It’s well known to you all that (that’s what I hope) the decade counter is used as a logic switch every time a clock pulse is fed to it. The output 1 of 10 pins goes high there by switching LEDs to give a chaser circuit. Well if you think the chaser is the only application then think again because the next thing that which we are going to discuss may be far over your heads and put your minds in fire. THE LANGUAGE: It’s very essential isn’t it? Communication serves multiple purposes; similarly in order to communicate with an electronic circuit the output indicating devices are very essential (you didn’t forget the LEDs, did you?). Last time the LEDs served the purposes of indicating an increment in the clock pulse. The only thing is that the LEDs didn’t mean any thing in specific. The last project was not communication at all, it was only information. For example, a LED in your TV near the switch mearly ‘indicates’ that the TV is on. But communication is a two way process that requires the language and that fortunately happens to be binary language Remember, if at all you are good at binary language then dealing logic circuits is a problem for you. Enough of threatening and lets learn how to communicate with your logic circuit. Now let me present you with a table, study that and try to make out a relationship:

DECIMAL

BINARY

0

0000

1

0001

2

0010

3

0011

4

0100

5

0101

6

0110

7

0111

8

1000

9

1001

10

1010

11

1011

12

1100

13

1101

14

1110

15

1111

Got it, if not try reading this paragraph: the number system that we use in our day to day life to count and manipulate math is by using the decimal number system. But unfortunately the digital world didn’t find enough space and time to teach the logic gates the number system that which we know. The binary world only consists of 0s and 1s. See how 7 is represented! This way of representing is called 4 bit representation. If it were 8 bit representation then 7 is represented as 0000 0111. Each position in the binary word is called a bit. Eight bit is one byte and four bit is a nibble. 1024 byte is a kilo byte. 1024 KB is 1MB; 1024MB is one 1GB. Think how many bits your hard disk has? (GB is not the limit! search and tell me what’s next to GB)

The basics starts like this: in binary there are only two choices 0 and\or 1. With several different combinations any variety of data such as alphabets, numbers, and fractions can be given as input to any digital appliance. For example take your DVD player, it flashes ‘WELCOME’ on its screen every time you turn it on. But it really does not know what its doing. According to it it’s just outputting some binary numbers stored in its MEMORY through some conversion circuits (more on this later) but it appears as though the DVD is responding to us. Similarly our circuit also is going to respond to us in a logical fashion with out knowing what its doing. So we are going to cast a trick using circuit knowledge and fool the little devil (CD 4017 BE). I DON’T GET IT? Why the binary system has to be like this? Yeah, I have asked this question many times to my teachers none provided me with a proper response. “So what?” I asked this question to my self and started building circuits that uses this binary system and does wonders in my day to day life. One day you will also feel the same way about binary. A simple method to remember the coding is like this. “The cup won’t spill unless it’s full” try and remember this line to remember binary, what “I don’t get it?” well here is the explanation. The representation of 0 is very simple 0000 (I am talking about nibbles) so the next number is 1 so start pouring 1s in the cup of zeros like 0001. The next digit is 2 so fill the cup 0010 (now don’t ask where the first drop went? Just read and you WILL GET IT) next is 3 go on, there is one more combination allowed 0011 now the initial two spaces are filled (the cup is full careful it’s going to spill) so the next digit four is represented as 0100. Now again the cup is empty fill it with 1s. so it will be 0101 for 5 and 0110 for 6 and 0111 for 7, now the cup is full so spill the 1 and it will become 1000 that is 8 and it goes till 16 which is 1111. For some matured ones the 2 bit positions can have four combinations like 00, 01, 10 and 11. So when ever this overflow occurring just shift a carry to the next bit position like this 0000,0001,0010,0011 and 0100 got it. Now practice it in a separate sheet to master all the binary codes. It’s enough if you make a 4 bit cup, full. So the first two bits go through this cycle and they change the positions of the next two bits. This cycle continues up to 1111. THE PROJECT: This time let us handle the TTL \LS ICs, in that series we shall handle the simple 4 bit binary code generator the IC specification is called 74LS90, BINARY COUNTER. These IC increments its output according to the clock pulse fed as input. For each clock input the output increments by one binary count. Initially it is 0000 and for next transition it’s 0001. Similarly, the output keeps incrementing till the output reaches 1111. We can also wire the circuit to replay the whole order. Construct the circuit in your bread board and see the LEDs glow in a definite fashion. Please try to remember that last time in the decade counter the output just

incremented along with the clock pulse; but this time the output has a definite meaning and it’s demonstrated in the given circuit. Check it out! The LED that glows is represented as logic 1 and that which is off is logic 0.

BCD TO DECIMAL DECODER: This IC converts the input binary to decimal number system. So now let us see another IC called the 74LS42. This IC is called the BCD to decimal decoder. Note the word decoder; it is otherwise called as diversifier. It converts few inputs to many outputs. I introduce this IC because the same decade counter is related with this. Since you are most familiar with the decade counter I thought this circuit will really entertain you. You can notice that the count is only up to 9 (1001) after which the IC resets (recycling based on wiring).Try the following circuit and post your comments. The working, configuration and pin details can be downloaded from our blog spot. So what are you waiting for? Go ahead and start your work. Catch you soon.